Search

We provide a physical interpretation and explanation of the morphology-density relation for galaxies, drawing on stellar masses, star formation rates, axis ratios, and group halo masses from the Sloan Digital Sky Survey. We first re-cast the classical morphology-density relation in more quantitative terms, using low star formation rate (quiescence) as a proxy for early-type morphology and dark matter halo mass from a group catalog as a proxy for environmental density: for galaxies of a given stellar mass the quiescent fraction is found to increase with increasing dark matter halo mass. Our novel result is that--at a given stellar mass--quiescent galaxies are significantly flatter in dense environments, implying a higher fraction of disk galaxies. Supposing that the denser environments differ simply by a higher incidence of quiescent disk galaxies that are structurally similar to star-forming disk galaxies of similar mass, explains simultaneously and quantitatively these quiescence-environment and shape-environment relations. Our findings add considerable weight to the slow removal of gas as the main physical driver of the morphology-density relation, at the expense of other explanations.

We present a new and simple technique for selecting extensive, complete, and pure quasar samples, based on their intrinsic variability. We parameterize the single-band variability by a power-law model for the light-curve structure function, with amplitude A and power-law index g. We show that quasars can be efficiently separated from other non-variable and variable sources by the location of the individual sources in the A-g plane. We use [?]60 epochs of imaging data, taken over [?]5 years, from the SDSS stripe 82 (S82) survey, where extensive spectroscopy provides a reference sample of quasars, to demonstrate the power of variability as a quasar classifier in multi-epoch surveys. For UV-excess selected objects, variability performs just as well as the standard SDSS color selection, identifying quasars with a completeness of 90% and a purity of 95%. In the redshift range 2.5 < z < 3, where color selection is known to be problematic, variability can select quasars with a completeness of 90% and a purity of 96%. This is a factor of 5-10 times more pure than existing color selection of quasars in this redshift range. Selecting objects from a broad griz color box without u-band information, variability selection in S82 can afford completeness and purity of 92%, despite a factor of 30 more contaminants than quasars in the color-selected feeder sample. This confirms that the fraction of quasars hidden in the "stellar locus" of color space is small. To test variability selection in the context of Pan-STARRS 1 (PS1) we created mock PS1 data by down-sampling the S82 data to just six epochs over 3 years. Even with this much sparser time sampling, variability is an encouragingly efficient classifier. For instance, a 92% pure and 44% complete quasar candidate sample is attainable from the above griz-selected catalog. Finally, we show that the presented A-g technique, besides selecting clean and pure samples of quasars (which are stochastically varying objects), is also efficient at selecting (periodic) variable objects such as RR Lyrae.

We map the stellar structure of the Galactic thick disk and halo by applying color-magnitude diagram (CMD) fitting to photometric data from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. The SEGUE imaging scans allow, for the first time, a comprehensive analysis of Milky Way structure at both high and low latitudes using uniform Sloan Digital Sky Survey photometry. Incorporating photometry of all relevant stars simultaneously, CMD fitting bypasses the need to choose single tracer populations. Using old stellar populations of differing metallicities as templates, we obtain a sparse three-dimensional map of the stellar mass distribution at |Z|>1 kpc. Fitting a smooth Milky Way model comprising exponential thin and thick disks and an axisymmetric power-law halo allows us to constrain the structural parameters of the thick disk and halo. The thick-disk scale height and length are well constrained at 0.75 +- 0.07 kpc and 4.1 +- 0.4 kpc, respectively. We find a stellar halo flattening within [?]25 kpc of c/a = 0.88 +- 0.03 and a power-law index of 2.75 +- 0.07 (for 7 kpc [?]RGC [?] 30 kpc). The model fits yield thick-disk and stellar halo densities at the solar location of rthick, = 10-2.3+-0.1 M pc-3 and rhalo, = 10-4.20+-0.05 M pc-3, averaging over any substructures. Our analysis provides the first clear in situ evidence for a radial metallicity gradient in the Milky Way's stellar halo: within R [?] 15 kpc the stellar halo has a mean metallicity of [Fe/H] [?] -1.6, which shifts to [Fe/H] [?] -2.2 at larger radii, in line with the two-component halo deduced by Carollo et al. from a local kinematic analysis. Subtraction of the best-fit smooth and symmetric model from the overall density maps reveals a wealth of substructures at all latitudes, some attributable to known streams and overdensities, and some new. A simple warp cannot account for the low latitude substructure, as overdensities occur simultaneously above and below the Galactic plane.

The narrow GD-1 stream of stars, spanning 60deg on the sky at a distance of [?]10 kpc from the Sun and [?]15 kpc from the Galactic center, is presumed to be debris from a tidally disrupted star cluster that traces out a test-particle orbit in the Milky Way halo. We combine Sloan Digital Sky Survey (SDSS) photometry, USNO-B astrometry, and SDSS and Calar Alto spectroscopy to construct a complete, empirical six-dimensional (6D) phase-space map of the stream. We find that an eccentric orbit in a flattened isothermal potential describes this phase-space map well. Even after marginalizing over the stream orbital parameters and the distance from the Sun to the Galactic center, the orbital fit to GD-1 places strong constraints on the circular velocity at the Sun's radius Vc = 224 +- 13 km s-1 and total potential flattening q Ph = 0.87+0.07 -0.04. When we drop any informative priors on Vc , the GD-1 constraint becomes Vc = 221 +- 18 km s-1. Our 6D map of GD-1, therefore, yields the best current constraint on Vc and the only strong constraint on q Ph at Galactocentric radii near R [?] 15 kpc. Much, if not all, of the total potential flattening may be attributed to the mass in the stellar disk, so the GD-1 constraints on the flattening of the halo itself are weak: q Ph,halo > 0.89 at 90% confidence. The greatest uncertainty in the 6D map and the orbital analysis stems from the photometric distances, which will be obviated by GAIA.

We have obtained deep photometry in two 1deg x 1deg fields covering the close pair of dwarf spheroidal galaxies Leo IV and Leo V and part of the area in between. From the distribution of likely red giant branch (RGB) and horizontal-branch (HB) stars in the data set, we find that both Leo IV and Leo V are significantly larger than indicated by previous measurements based on shallower data. With a half-light radius of rh = 4.'6 +- 0.'8 (206 +- 36 pc) and rh = 2.'6 +- 0.'6 (133 +- 31 pc), respectively, both systems are now well within the physical size bracket of typical dwarf spheroidal Milky Way satellites. Both are also found to be significantly elongated with an ellipticity of [?] [?] 0.5, a characteristic shared by many of the fainter (MV > - 8) Milky Way dwarf spheroidals. The large spatial extent of our survey allows us to search for extra-tidal features in the area between the two dwarf galaxies with unprecedented sensitivity. The spatial distribution of candidate RGB and HB stars in this region is found to be non-uniform at the [?]3s level. Interestingly, this substructure is aligned along the direction connecting the two systems, indicative of a possible "bridge" of extra-tidal material. Fitting the stellar distribution with a linear Gaussian model yields a significance of 4s for this overdensity, a most likely FWHM of [?]16 arcmin, and a central surface brightness of [?]32 mag arcsec-2. We investigate different scenarios to explain the close proximity of Leo IV and Leo V, and the possible tidal bridge between them. Orbit calculations demonstrate that the two systems cannot share the exact same orbit, while a compromise orbit does not approach the Galactic center more than [?]160 kpc, rendering it unlikely that they are remnants of a single disrupted progenitor. A comparison with cosmological simulations shows that a chance collision between unrelated subhalos is negligibly small. Given their relative distance and velocity, Leo IV and Leo V could be a bound "tumbling pair," if their combined mass exceeds (8 +- 4) x 109 M . The scenario of an internally interacting pair that fell into the Milky Way together appears to be the most viable explanation for this close celestial companionship.

Masers found in massive star-forming regions can be located precisely in six-dimensional phase space and therefore serve as a tool for studying Milky Way dynamics. The non-random orbital phases at which the masers are found and the sparseness of current samples require modeling. Here, we model the phase-space distribution function of 18 precisely measured Galactic masers, permitting a mean velocity offset and a general velocity dispersion tensor relative to their local standards of rest, and accounting for different pieces of prior information. With priors only on the Sun's distance from the Galactic Center and on its motion with respect to the local standard of rest, the maser data provide a weak constraint on the circular velocity at the Sun of Vc = 246 +- 30 km s-1. Including prior information on the proper motion of Sgr A* leads to Vc = 244 +- 13 km s-1. We do not confirm the value of Vc [?] 254 km s-1 found in more restrictive models. This analysis shows that there is no conflict between recent determinations of Vc from Galactic Center analyses, orbital fitting of the GD-1 stellar stream, and the kinematics of Galactic masers; a combined estimate is Vc = 236 +- 11 km s-1. Apart from the dynamical parameters, we find that masers tend to occur at post-apocenter, circular-velocity-lagging phases of their orbits.

Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses, and UV SFRs from COMBO-17, 24 mm SFRs from Spitzer, and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically selected very close pairs (separation <2'') and merger remnants from the HST imaging. Our analysis confirms that the most intensely star-forming systems are indeed interacting or merging. Yet, for massive (M * [?] 1010 M ) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios [?] 1:4 and separations [?] 40 kpc) are only 1.80 +- 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. Our results also agree with hydrodynamical simulations of galaxy interactions, which produce some mergers with large bursts of star formation on [?]100 Myr timescales, but only a modest SFR enhancement when averaged over the entire merger timescale. We demonstrate that these results imply that only [?]10% of star formation at 0.4 [?] z [?] 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z = 1.

We present a simple, empirically motivated model that simultaneously predicts the evolution of the mean size and the comoving mass density of massive (>1011 M ) early-type galaxies from z = 2 to the present. First, we demonstrate that some size evolution of the population can be expected simply due to the continuous emergence of early-type galaxies. The Sloan Digital Sky Survey (SDSS) data reveal that in the present-day universe more compact early-type galaxies with a given dynamical mass have older stellar populations. This implies that with increasing look-back time, the more extended galaxies will be more and more absent from the population. In contrast, at a given stellar velocity dispersion, SDSS data show that there is no relation between size and age, which implies that the velocity dispersion can be used to estimate the epoch at which galaxies stopped forming stars, turning into early-type galaxies. Based on this, we define an empirically motivated, redshift-dependent velocity dispersion threshold above which galaxies do not form stars at a significant rate, which we associate with the transformation into early-type galaxies. Applying this "formation" criterion to a large sample of nearby early-type galaxies, we predict the redshift evolution in the size distribution and the comoving mass density. The resulting evolution in the mean size is roughly half of the observed evolution. Then we include a prescription for the merger histories of galaxies between the "formation" redshift and the present, based on cosmological simulations of the assembly of dark matter halos. Such mergers after the transformation into an early-type galaxy are presumably dissipationless ("dry"), where the increase in size is expected to be approximately proportional to the increase in mass. This model successfully reproduces the observed evolution since z [?] 2 in the mean size and in the comoving mass density of early-type galaxies with mass M > 1011 M . We conclude that the recently measured, substantial size evolution of early-type galaxies can be explained by the combined effect of the continuous emergence of galaxies as early types and their subsequent growth through dry merging.

We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z[?] 0.24-0.80 (lookback time T b[?] 3-7 Gyr) using [?]3600 (M[?] 1 x 109 M ) galaxies with GEMS Hubble Space Telescope, COMBO-17, and Spitzer data. Our results are as follows. (1) Among [?]790 high-mass (M[?] 2.5 x 1010 M ) galaxies, the visually based merger fraction over z[?] 0.24-0.80, ranges from 9% +- 5% to 8% +- 2%. Lower limits on the major merger and minor merger fraction over this interval range from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first, albeit approximate, empirical estimate of the frequency of minor mergers over the last 7 Gyr. Assuming a visibility timescale of [?]0.5 Gyr, it follows that over T b[?] 3-7 Gyr, [?]68% of high-mass systems have undergone a merger of mass ratio >1/10, with [?]16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous "major or minor" mergers. The average merger rate is [?] a few x10-4 galaxies Gyr-1 Mpc-3. Among [?]2840 blue-cloud galaxies of mass M[?] 1.0 x 109 M , similar results hold. (2) We compare the empirical merger fraction and merger rate for high-mass galaxies to three L cold dark matter-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of 5 dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among [?]3698 M[?] 1.0 x 109 M galaxies, we find that the mean star formation rate (SFR) of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z[?] 0.24-0.80. Visibly merging systems only account for a small fraction (<30%) of the cosmic SFR density over T b[?] 3-7 Gyr. This complements the results of Wolf et al. over a shorter time interval of T b[?] 6.2-6.8 Gyr, and suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.

We revisit the well known discrepancy between the observed number of Milky Way (MW) dwarf satellite companions and the predicted population of cold dark matter (CDM) subhalos, in light of the dozen new low-luminosity satellites found in imaging data from the Sloan Digital Sky Survey (SDSS) and our recent calibration of the SDSS satellite detection efficiency, which implies a total satellite population far larger than these dozen discoveries. We combine a detailed dynamical model for the CDM subhalo population with simple, physically motivated prescriptions for assigning a stellar content to each subhalo, then apply observational selection effects and compare to the current observational census. Reconciling the observed satellite population with CDM predictions still requires strong mass-dependent suppression of star formation in low-mass subhalos: models in which the stellar mass is a constant fraction F *(O b /O m ) of the subhalo mass M sat at the time it becomes a satellite fail for any choice of F *. However, previously advocated models that invoke suppression of gas accretion after reionization in halos with circular velocity V circ [?] V crit [?] 35 km s-1 can reproduce the observed satellite counts for -15 [?] MV [?] 0. Successful models require F * [?] 10-3 in halos with V circ>V crit and strong suppression of star formation before reionization in halos with V circ [?] 10 km s-1; models without pre-reionization suppression predict far too many satellites with -5 [?] MV [?] 0. In this successful model, the dominant fraction of stars formed after reionization at all luminosities. Models that match the satellite luminosity distribution also match the observed heliocentric radius distribution, and they reproduce the observed characteristic stellar velocity dispersion s* [?] 5-10 km s-1 of the SDSS dwarfs given the observed sizes ([?]50-200 pc) of their stellar distributions. The model satellites have M(<300 pc) [?] 107 M as observed even though their present-day total halo masses span more than two orders of magnitude; the constancy of central masses mainly reflects the profiles of CDM halos. Our modeling shows that natural physical mechanisms acting within the CDM framework can quantitatively explain the properties of the MW satellite population as it is presently known, thus providing a convincing solution to the "missing satellite" problem.

We present the color-magnitude and color-stellar mass diagrams for galaxies with z phot [?] 2, based on a K (AB) < 22 catalog of the $\frac{1}{2} \times \frac{1}{2} \square ^{\circ }$ Extended Chandra Deep Field South from the MUltiwavelength Survey by Yale-Chile. Our main sample of 7840 galaxies contains 1297 M *>1011 M galaxies in the range 0.2 < z phot < 1.8. We show empirically that this catalog is approximately complete for M *>1011 M galaxies for z phot < 1.8. For this mass-limited sample, we show that the locus of the red sequence color-stellar mass relation evolves as D(u - r) [?] (-0.44 +- 0.02)z phot for z phot [?] 1.2. For z phot [?] 1.3, however, we are no longer able to reliably distinguish red and blue subpopulations from the observed color distribution; we show that this would require much deeper near-infrared data. At 1.5 < z phot < 1.8, the comoving number density of M *>1011 M galaxies is [?]50% of the local value, with a red fraction of [?]33%. Making a parametric fit to the observed evolution, we find n tot(z) [?] (1 + z phot)-0.52+-0.12(+-0.20). We find stronger evolution in the red fraction: f red(z) [?] (1 + z phot)-1.17+-0.18(+-0.21). Through a series of sensitivity analyses, we show that the most important sources of systematic error are (1) systematic differences in the analysis of the z [?] 0 and z [?] 0 samples; (2) systematic effects associated with details of the photometric redshift calculation; and (3) uncertainties in the photometric calibration. With this in mind, we show that our results based on photometric redshifts are consistent with a completely independent analysis which does not require redshift information for individual galaxies. Our results suggest that, at most, 1/5 of local red sequence galaxies with M *>1011 M were already in place at z [?] 2.

To examine the role of the host galaxy structure in fueling nuclear activity, we estimated gas flow rates from several kpc down to the inner few 10 pc for seven nearby spiral galaxies, selected from the NUclei of GAlaxies sample. We calculated gravitational torques from near-infrared images and determined gas in/out-flow rates as a function of radius and location within the galactic disks, based on high angular resolution interferometric observations of molecular (CO using Plateau de Bure interferometer) and atomic (H I using the Very Large Array) gas. The results are compared with kinematic evidence for radial gas flows and the dynamical state of the galaxies (via resonances) derived from several different methods. We show that gravitational torques are very efficient at transporting gas from the outer disk all the way into the galaxies centers at [?]100 pc; previously assumed dynamical barriers to gas transport, such as the corotation resonance of stellar bars, seem to be overcome by gravitational torque induced gas flows from other nonaxisymmetric structures. The resulting rates of gas mass inflow range from 0.01 to 50 M yr-1 and are larger for the galaxy center than for the outer disk. Our gas flow maps show the action of nested bars within larger bars for three galaxies. Noncircular streaming motions found in the kinematic maps are larger in the center than in the outer disk and appear to correlate only loosely with the in/out-flow rates as a function of radius. We demonstrate that spiral gas disks are very dynamic systems that undergo strong radial evolution on timescales of a few rotation periods (e.g., 5 x 108 yrs at a radius of 5 kpc), due to the effectiveness of gravitational torques in redistributing the cold galactic gas.

Close to its center, the bulgeless galaxy NGC 2139 hosts a star cluster that is younger and less massive than any nuclear star cluster (NC) studied so far. We have measured the Ha velocity field around the photometric center of this galaxy using the VLT ARGUS integral field unit and the GIRAFFE spectrograph in order to constrain different proposed theories of NC formation. We observe that the best-fit kinematic center and the candidate NC appear to be separated by 2.8 '' (320 pc). Indeed, the kinematic center is also offset from the galaxy's photometric center and a possible bar or extended region of star formation in which the young cluster resides, implying that this galaxy is not in dynamic equilibrium. The Ha flux map also reveals other regions of strong star formation in the possible bar. These observations suggest that a nascent NC is forming away from the kinematic center of NGC 2139, which may come to rest there on a timescale of a few 100 Myr.

We derive the structural parameters of the recently discovered very low luminosity Milky Way satellites through a maximum likelihood algorithm applied to SDSS data. For each satellite, even when only a few tens of stars are available down to the SDSS flux limit, the algorithm yields robust estimates and errors for the centroid, position angle, ellipticity, exponential half-light radius and number of member stars (within the SDSS). This latter parameter is then used in conjunction with stellar population models of the satellites to derive their absolute magnitudes and stellar masses, accounting for color-magnitude diagram shot noise. Most parameters are in good agreement with previous determinations, but we now properly account for parameter covariances. However, we find that faint satellites are somewhat more elliptical than initially thought, and ascribe this effect to the previous use of smoothed maps, which can be dominated by the smoothing (round) kernel. As a result, the faintest half of the Milky Way dwarf galaxies ( MV [?] [?]7.5) is significantly (4 s) flatter ( =0.47\pm 0.03"/> [?][?][?] = 0.47 +- 0.03) than its brightest half ( MV[?] [?] 7.5, =0.32\pm 0.02"/> [?][?][?] = 0.32 +- 0.02). From our best models, we also investigate whether the seemingly distorted shape of the satellites, often taken to be a sign of tidal distortion, can be quantified. We find that, except for tentative evidence of distortion in Canes Venatici I and Ursa Major II, these can be completely accounted for by Poisson scatter in the sparsely sampled systems. We consider three scenarios that could explain the rather elongated shape of faint satellites: rotation supported systems, stars following the shape of more triaxial dark matter subhalos, or elongation due to tidal interaction with the Milky Way. Although none of these is entirely satisfactory, the last one appears the least problematic, but obviously warrants much deeper observations to track evidence of such tidal interaction.

We present spectroscopic evidence for a low-luminosity, low-excitation active galactic nucleus (AGN) in NGC 1042, powered by an intermediate-mass black hole. These findings are significant in that the AGN is coincident with a compact star cluster known to reside in the nucleus, thus providing an example where the two types of central mass concentration coexist. The existence of a central black hole is additionally remarkable in that NGC 1042 lacks a stellar bulge. Objects such as NGC 1042 may have an important role in testing theories for the genesis of massive black holes in galaxy nuclei, and the extent to which they are in symbiosis with the larger stellar host.

We have used data from the Sloan Digital Sky Survey (SDSS) Data Release 5 to explore the overall structure and substructure of the stellar halo of the Milky Way using ~4 million color-selected main-sequence turnoff stars with 0.2 < g [?] r < 0.4 and 18.5 [?] r < 22.5. We fit oblate and triaxial broken power law models to the data, and found a "best-fit" oblateness of the stellar halo 0.5 < c/a < 0.8, and halo stellar masses between galactocentric radii of 1 and 40 kpc of 3.7 +- 1.2 x 108 M. The density profile of the stellar halo is approximately r [?] r[?]a, where - \alpha > -4"/> 2 > a > [?] 4. Yet, we found that all smooth and symmetric models were very poor fits to the distribution of stellar halo stars because the data exhibit a great deal of spatial substructure. We quantified deviations from a smooth oblate/triaxial model using the rms of the data around the model profile on scales [?]100 pc, after accounting for the (known) contribution of Poisson uncertainties. Within the DR5 area of the SDSS, the fractional rms deviation s/total of the actual stellar distribution from any smooth, parameterized halo model is [?]40%: hence, the stellar halo is highly structured. We compared the observations with simulations of galactic stellar halos formed entirely from the accretion of satellites in a cosmological context by analyzing the simulations in the same way as the SDSS data. While the masses, overall profiles, and degree of substructure in the simulated stellar halos show considerable scatter, the properties and degree of substructure in the Milky Way's halo match well the properties of a "typical" stellar halo built exclusively out of the debris from disrupted satellite galaxies. Our results therefore point toward a picture in which an important fraction of the stellar halo of the Milky Way has been accreted from satellite galaxies.

Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that there is only mild evolution in the relationship between radial size and stellar mass for galactic disks from z [?] 1 to the present day. Using a sample of nearby disk-dominated galaxies from the Sloan Digital Sky Survey (SDSS) and high-redshift data from the GEMS (Galaxy Evolution from Morphology and SEDs) survey, we investigate whether this result is consistent with theoretical expectations within the hierarchical paradigm of structure formation. The relationship between virial radius and mass for dark matter halos in the LCDM model evolves by about a factor of 2 over this interval. However, N-body simulations have shown that halos of a given mass have less centrally concentrated mass profiles at high redshift. When we compute the expected disk size-stellar mass distribution, accounting for this evolution in the internal structure of dark matter halos and the adiabatic contraction of the dark matter by the self-gravity of the collapsing baryons, we find that the predicted evolution in the mean size at fixed stellar mass since z [?] 1 is about 15%-20%, in good agreement with the observational constraints from GEMS. At redshift z [?] 2, the model predicts that disks at fixed stellar mass were on average only 60% as large as they are today. Similarly, we predict that the rotation velocity at a given stellar mass (essentially the zero point of the Tully-Fisher relation) is only about 10% larger at z [?] 1 (20% at z [?] 2) than at the present day.

We investigate structural properties of dark matter halos of disk galaxies, using a well-defined sample of 81 disk-dominated galaxies from the SDSS redshift survey. We model the mass-velocity (TF) and fundamental plane (FP) relations of these galaxies, using the observed stellar mass M*, disk scale length Rd, and optical Ha rotation velocity at 2.2 Rd. We calculate model galaxy populations, defined by the distribution of the stellar disk-to-total mass fraction, md, and include the effect of adiabatic contraction. We find that models with constant md underpredict the intrinsic scatter of the TF and FP relations and predict an (unobserved) strong correlation between TF residuals, even with the full range of halo concentration scatter. Introducing a scatter of md and allowing the mean value \overline{m}_{d} $&overline;{m}_d$ $&overline;{m}t SUBgt t Igt dt/Igt t/SUBgt $ to scale with the stellar surface density significantly improves observational match and reduces the predicted residual correlation enough to be consistent with the data. The distribution of angular momentum parameters required to match the observed scale lengths is an output of the models and is narrower than that predicted for halo spin parameters. However, our best-fit models with a Kroupa stellar IMF overproduce the galaxy stellar mass function and predict the virial mass-to-light ratios lower than those inferred from galaxy-galaxy weak lensing and satellite dynamics. We suggest three possible solutions to these problems: (1) ignoring the effects of adiabatic contraction, (2) adopting a "light" stellar IMF with M*/L lower by 0.15 dex, or (3) considering the lower halo concentrations expected for a low power spectrum normalization s8 [?] 0.74. In combination with our proposed correlation of \overline{m}_{d} $&overline;{m}_d$ $&overline;{m}t SUBgt t Igt dt/Igt t/SUBgt $ with surface density, any of these solutions yields acceptable residual correlations and relieves most of the observational tension between the TF relation and the galaxy stellar mass function.

We analyze the infrared (IR) spectral energy distributions (SEDs) for 10 mm < lrest < 100 mm for ~600 galaxies at z ~ 0.7 in the extended Chandra Deep Field South by stacking their Spitzer 24, 70, and 160 mm images. We place interesting constraints on the average IR SED shape in two bins: the brightest 25% of z ~ 0.7 galaxies detected at 24 mm, and the remaining 75% of individually detected galaxies. Galaxies without individual detections at 24 mm were not well detected at 70 and 160 mm even through stacking. We find that the average IR SEDs of z ~ 0.7 star-forming galaxies fall within the diversity of z ~ 0 templates. While dust obscuration LIR/LUV seems to be only a function of star formation rate (SFR; ~LIR + LUV), not of redshift, the dust temperature of star-forming galaxies (with SFR ~ 10 M yr-1 ) at a given IR luminosity was lower at z ~ 0.7 than today. We suggest an interpretation of this phenomenology in terms of dust geometry: intensely star-forming galaxies at z ~ 0 are typically interacting, and host dense centrally concentrated bursts of star formation and warm dust temperatures. At z ~ 0.7, the bulk of intensely star-forming galaxies are relatively undisturbed spirals and irregulars, and we postulate that they have large amounts of widespread lower density star formation, yielding lower dust temperatures for a given IR luminosity. We recommend which IR SEDs are most suitable for modeling intermediate-redshift galaxies with different SFRs.

We present sensitive near-infrared spectroscopic observations for a sample of five z ~ 6 quasars. These quasars are among the most distant, currently known quasars in the universe. The spectra have been obtained using ISAAC at the VLT and include the C IV, Mg II, and Fe II lines. We measure the Fe II/Mg II line ratio, as an observational proxy for the Fe/a-element ratio. We derive a ratio of 2.7 +- 0.8 for our sample, which is similar to that found for lower redshift quasars; i.e., we provide additional evidence for the lack of evolution in the Fe II/Mg II line ratio of quasars up to the highest redshifts. This result demonstrates that star formation must have commenced at z [?] 8 in the quasar hosts. The line widths of the Mg II and C IV lines give two estimates for the black hole masses. A third estimate is given by assuming that the quasars emit at their Eddington luminosity. The derived masses using these three methods agree well, implying that the quasars are not likely to be strongly lensed. We derive central black hole masses of (0.3-5.2) x 109 M. We use the difference between the redshift of Mg II (a proxy for the systemic redshift of the quasar) and the onset of the Gunn-Peterson trough to derive the extent of the ionized Stromgren spheres around our target quasars. The derived physical radii are about 5 Mpc. Using a simple ionization model, the emission of the central quasars would need of order 106-108 yr to create these cavities. As the e-folding timescale for the central accreting black hole is on the order of a few times 107 yr, it can grow by one e-folding or less within this time span.

Using deep NIR VLT ISAAC and optical HST WFPC2 imaging in the HDF-S and MS 1054-03, we study the rest-frame UV-to-optical colors and magnitudes of galaxies to z ~ 3. Unlike the present day, there is no evidence for a red sequence at z ~ 3, but there does appear to be a well-defined CMR for blue galaxies at all redshifts, with more luminous galaxies having redder U - V colors. The slope of the blue CMR is independent of redshift d(U - V)/dMV = -0.09 +- 0.01 and can be explained by a correlation of dust reddening with luminosity. The average color at fixed luminosity reddens strongly D(U - V) [?] 0.75 from z ~ 3 to z = 0, much of which can be attributed to aging of stars. The color scatter of the blue sequence is relatively small s(U - V) = 0.25 +- 0.03 and constant to z ~ 3, but notably asymmetrical, having a sharp blue ridge and a wing toward redder colors. We explore sets of SFHs to study the constraints placed by the shape of the scatter at z = 2-3. One particular set of models, episodic star formation, reproduces the detailed properties very well. For a two-state model with high and low star formation, the duty cycle is constrained to be >40% and the contrast in SFR between the states must be a factor >5 [or a scatter in log(SFR) of >0.35 dex around the mean]. Nevertheless, episodic models do not explain the observed tail of very red galaxies, primarily DRGs, which may have ceased star formation altogether or are more heavily obscured. Finally, the relative number density of red, luminous MV < -20.5 galaxies increases by a factor of ~6 from z = 2.7 to 0.5, as does their contribution to the total rest-frame V-band luminosity density. We are likely viewing the progressive formation of red, passively evolving galaxies.

We analyze very deep HST, VLT, and Spitzer photometry of galaxies at 2 < z < 3.5 in the Hubble Deep Field-South. The sample is selected from the deepest public K-band imaging currently available. We show that the rest-frame U - V versus V - J color-color diagram is a powerful diagnostic of the stellar populations of distant galaxies. Galaxies with red rest-frame U - V colors are generally red in rest-frame V - J as well. However, at a given U - V color a range in V - J colors exists, and we show that this allows us to distinguish young, dusty galaxies from old, passively evolving galaxies. We quantify the effects of IRAC photometry on estimates of masses, ages, and the dust content of z > 2 galaxies. The estimated distributions of these properties do not change significantly when IRAC data are added to the UBVIJHK photometry. However, for individual galaxies the addition of IRAC can improve the constraints on the stellar populations, especially for red galaxies: uncertainties in stellar mass decrease by a factor of 2.7 for red [(U [?] V)rest > 1] galaxies, but only by a factor of 1.3 for blue [(U [?] V)rest < 1] galaxies. We find a similar color dependence of the improvement for estimates of age and dust extinction. In addition, the improvement from adding IRAC depends on the availability of full NIR JHK coverage; if only K band were available, the mass uncertainties of blue galaxies would decrease by a more substantial factor of 1.9. Finally, we find that a trend of galaxy color with stellar mass is already present at z > 2. The most massive galaxies at high redshift have red rest-frame U - V colors compared to lower mass galaxies, even when allowing for complex star formation histories.

We present results from a program of optical spectroscopy for 23 nearby galaxies with emission-line nuclei. This investigation takes advantage of the spatial resolution of the Hubble Space Telescope to study the structure and energetics of the central ~10-20 pc, and the resulting data have value for quantifying central black hole masses, star formation histories, and nebular properties. This paper provides a description of the experimental design, and new findings from the study of emission lines. The sample targets span a range of nebular spectroscopic class, from H II to Seyfert nuclei. This data set and the resulting measurements are unique in terms of the sample size, the range of nebular class, and the investigation of physical scales extending down to parsecs. The line ratios indicative of nebular ionization show only modest variations over order-of-magnitude differences in radius, and demonstrate in a systematic way that geometrical dilution of the radiation field from a central source cannot be assumed as a primary driver of ionization structure. Comparisons between large- and small-aperture measurements for the H II/LINER transition objects provide a new test that challenges conventional wisdom concerning the composite nature of these systems. We also list a number of other quantitative results that are of interest for understanding galaxy nuclei, including (1) the spatial distribution/degree of concentration of Ha emission as a function of nebular type; (2) the radial variation in electron density as a function of nebular type; and (3) quantitative broad Ha estimates obtained at a second epoch for these low-luminosity nuclei. The resulting measurements provide a new basis for comparing the nuclei of other galaxies with that of the Milky Way. We find that the Galactic center is representative across a wide span of properties as a low-luminosity emission-line nucleus.

We present the evolution of the volume-averaged properties of the rest-frame optically luminous (LV > 3 x 1010 himg1.gif L) galaxy population to z ~ 3, determined from four disjoint deep fields. We characterize their rest-frame UV through optical properties via the mean SED. To measure evolution, we apply the selection criteria to a sample of galaxies from the SDSS and COMBO-17 survey. The mean rest-frame 2200 A through V-band SED becomes steadily bluer with increasing redshift, but at all redshifts z < 3 the mean SED falls within the range defined by ``normal'' galaxies in the nearby universe. We measure the mean stellar mass-to-light ratios ([?]/L) and stellar mass densities (r[?]) by fitting models to the mean rest-frame UV-optical SEDs. The r[?] in galaxies selected at a fixed luminosity has increased by a factor of 3.5-7.9 from z = 3 to 0.1. If we instead use our observed [?]/LV evolution to select galaxies at a fixed mass, r[?] evolves by a factor of 5.3-16.7. After correcting to total, the measured r[?] at z < 2 lie below the integral of the star formation rate density as a function of redshift as derived from UV-selected samples after a standard correction for extinction. We find large discrepancies between recent model predictions for the evolution of r[?] and our results, even when our observational selection is applied to the models. Finally, we determine that distant red galaxies (selected to have Js - Ks > 2.3) in our Limg2.gif-selected samples contribute 30% and 64% of the stellar mass budget at z ~ 2 and z ~ 2.8, respectively. These galaxies are largely absent from UV surveys, and this result highlights the need for mass selection of high-redshift galaxies.

We present the evolution of the luminosity-size and stellar mass-size relations of luminous (LV [?] 3.4 x 1010 himg1.gif L) and massive (M* [?] 3 x 1010 himg1.gif M) galaxies in the last ~11 Gyr. We use very deep near-infrared images of the Hubble Deep Field-South and the MS 1054-03 field in the Js, H, and Ks bands from FIRES to retrieve the sizes in the optical rest frame for galaxies with z > 1. We combine our results with those from GEMS at 0.2 < z < 1 and SDSS at z ~ 0.1 to achieve a comprehensive picture of the optical rest-frame size evolution from z = 0 to 3. Galaxies are differentiated according to their light concentration using the Sersic index n. For less concentrated objects, the galaxies at a given luminosity were typically ~3 +- 0.5 (+-2 s) times smaller at z ~ 2.5 than those we see today. The stellar mass-size relation has evolved less: the mean size at a given stellar mass was ~2 +- 0.5 times smaller at z ~ 2.5, evolving proportionally to (1 + z)-0.40+-0.06. Simple scaling relations between dark matter halos and baryons in a hierarchical cosmogony predict a stronger (although consistent within the error bars) than observed evolution of the stellar mass-size relation. The observed luminosity-size evolution out to z ~ 2.5 matches well recent infall model predictions for Milky Way-type objects. For low-n galaxies, the evolution of the stellar mass-size relation would follow naturally if the individual galaxies grow inside out. For highly concentrated objects, the situation is as follows: at a given luminosity, these galaxies were ~2.7 +- 1.1 times smaller at z ~ 2.5 (or, put differently, were typically ~2.2 +- 0.7 mag brighter at a given size than they are today), and at a given stellar mass the size has evolved proportionally to (1 + z)-0.45+-0.10.

In the present-day universe, supermassive black hole masses (BH) appear to be strongly correlated with their galaxy's bulge luminosity, among other properties. In this study we explore the analogous relationship between BH, derived using the virial method, and the stellar R-band bulge luminosity (LR) or stellar bulge mass (*) at epochs of 1 [?] z [?] 4.5, using a sample of 31 gravitationally lensed AGNs and 20 nonlensed AGNs. At redshifts z > 1.7 (10-12 Gyr ago), we find that the observed BH-LR relation is nearly the same (to within ~0.3 mag) as it is today. When the observed LR are corrected for luminosity evolution, this means that the black holes grew in mass faster than their hosts, with the BH/* mass ratio being a factor of [?]4img1.gif times larger at z > 1.7 than it is today. By the redshift range 1 [?] z [?] 1.7 (8-10 Gyr ago), the BH/* ratio is at most 2 times higher than today, but it may be consistent with no evolution. Combining the results, we conclude that the ratio BH/* rises with look-back time, although it may saturate at [?]6 times the local value. Scenarios in which moderately luminous quasar hosts at z [?] 1.7 were fully formed bulges that passively faded to the present epoch are ruled out.

We use near-infrared (NIR) spectroscopy obtained with the GNIRS on Gemini, NIRSPEC on the Keck, and ISAAC on the VLT to study the rest-frame optical continua of three "distant red galaxies" (with Js - Ks > 2.3) at z > 2. All three galaxy spectra show the Balmer/4000 A break in the rest-frame optical. The spectra allow us to determine spectroscopic redshifts from the continuum with an estimated accuracy of Dz/(1 + z) ~ 0.001-0.04. These redshifts agree well with the emission-line redshifts for the two galaxies with Ha emission. This technique is particularly important for galaxies that are faint in the rest-frame UV, as they are underrepresented in high-redshift samples selected in optical surveys and are too faint for optical spectroscopy. Furthermore, we use the break, continuum shape, and equivalent width of Ha, together with evolutionary synthesis models, to constrain the age, star formation timescale, dust content, stellar mass, and star formation rate of the galaxies. Inclusion of the NIR spectra in the stellar population fits greatly reduces the range of possible solutions for stellar population properties. We find that the stellar populations differ greatly among the three galaxies, ranging from a young dusty starburst with a small break and strong emission lines to an evolved galaxy with a strong break and no detected line emission. The dusty starburst galaxy has an age of 0.3 Gyr and a stellar mass of 1 x 1011 M. The spectra of the two most evolved galaxies imply ages of 1.3-1.4 Gyr and stellar masses of 4 x 1011 M. This large range of properties strengthens our previous, more uncertain results from broadband photometry. Larger samples are required to determine the relative frequency of dusty starbursts and (nearly) passively evolving galaxies at z ~ 2.5.

We present a study of the colors, structural properties, and star formation histories for a sample of ~1600 dwarfs over look-back times of ~3 Gyr (z = 0.002-0.25). The sample consists of 401 distant dwarfs drawn from the Galaxy Evolution from Morphologies and SEDs (GEMS) survey, which provides high-resolution Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images and accurate redshifts, and also 1291 dwarfs at 10-90 Mpc compiled from the Sloan Digital Sky Survey (SDSS). The sample is complete down to an effective surface brightness of 22 mag arcsec-2 in z and includes dwarfs with Mg = -18.5 to -14 mag. Rest-frame luminosities in Johnson UBV and SDSS ugr filters are provided by the COMBO-17 survey, and structural parameters have been determined by Sersic fits. We find that the GEMS dwarfs are bluer than the SDSS dwarfs by ~0.13 mag in g - r, which is consistent with the color evolution over ~2 Gyr of star formation histories involving moderate starbursts and long periods of continuous star formation. The full color range of the samples cannot be reproduced by single starbursts of different masses or long periods of continuous star formation alone. Furthermore, an estimate of the mechanical luminosities needed for the gas in the GEMS dwarfs to be completely removed from the galaxies shows that a significant number of low-luminosity dwarfs are susceptible to such a complete gas loss, if they would experience a starburst. On the other hand, a large fraction of more luminous dwarfs is likely to retain their gas. We also estimate the star formation rates per unit area for the GEMS dwarfs and find good agreement with the values for local dwarfs.

The use of gravitationally lensed quasars for cosmological applications and as probes of galaxy structure and evolution requires knowledge about the lens redshift, which is unknown for a large fraction of lenses. We report Very Large Telescope observations of 11 lensed quasars, designed to measure the redshifts of their lens galaxies. We successfully determined the redshifts for seven systems, five of which were previously unknown. The securely measured redshifts for the lensing galaxies are: HE 0047-1756 z = 0.408; PMN J0134-0931 z = 0.766; HE 0230-2130 z = 0.522; HE 0435-1223 z = 0.455; SDSS 0924+021 z = 0.393; LBQS 1009-025 z = 0.871; and WFI J2033-472 z = 0.658, all with uncertainties of 0.001. For four additional systems (BR I0952-0115, Q1017-207, Q1355-2257, and PMN J1632-003) we estimate tentative redshifts based on some features in their spectra.

We stack Spitzer 24 mm images for ~7000 galaxies with 0.1 [?] z < 1 in the Chandra Deep Field South to probe the thermal dust emission in low-luminosity galaxies over this redshift range. Through stacking, we can detect mean 24 mm fluxes that are more than an order of magnitude below the individual detection limit. We find that the correlations for low- and moderate-luminosity galaxies between the average LIR/LUV and rest-frame B-band luminosity, and between the star formation rate (SFR) and LIR/LUV, are similar to those in the local universe. This verifies that oft-used assumption in deep UV/optical surveys that the dust obscuration-SFR relation for galaxies with SFR [?] 20 M yr-1 varies little with epoch. We have used this relation to derive the cosmic IR luminosity density from z = 1 to z = 0.1. The results also demonstrate directly that little of the bolometric luminosity of the galaxy population arises from the faint end of the luminosity function, indicating a relatively flat faint-end slope of the IR luminosity function with a power-law index of 1.2 +- 0.3.

At low redshift, there are fundamental correlations between the mass of supermassive black holes (BH) and the mass (bulge) and luminosity of the host galaxy bulge. We investigate the same relation at z [?] 1. Using virial mass estimates for 11 quasars at z [?] 2 to measure their black hole mass, we find that black holes at high z fall nearly on the same BH versus R-band magnitude (MR) relation (to ~0.3 mag) as low-redshift active and inactive galaxies, without making any correction for luminosity evolution. Using a set of conservative assumptions about the host galaxy stellar population, we show that at z [?] 2 (10 Gyr ago), the ratio of BH/bulge was 3-6 times larger than today. Barring unknown systematic errors on the measurement of BH, we also rule out scenarios in which moderately luminous quasar hosts at z [?] 2 were fully formed bulges that passively faded to the present epoch. On the other hand, five quasar hosts at z [?] 1 are consistent with the current-day BH-MR relationship after taking into account evolution that is appropriate for E/S0 galaxies. Therefore, z [?] 1 host galaxies appear to fit the hypothesis that they are fully formed early-type galaxies. We also find that most quasar hosts with absolute magnitudes brighter than MR = -23 cannot fade below L* galaxies today, regardless of their stellar population makeup, because their black hole masses are too high and they must arrive at the local BH-MR relationship by z = 0.

We have used the 28' x 28' Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric redshift survey to constrain the incidence of major mergers between spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A set of N-body merger simulations was used to explore the morphological signatures of such interactions: they are recognizable either as <5 kpc separation close pairs or because of broad, low surface brightness tidal features and asymmetries. Data with the depth and resolution of GEMS are sensitive to dry mergers between galaxies with MV [?] -20.5 for z [?] 0.7; dry mergers at higher redshifts are not easily recovered in single-orbit HST imaging. Six dry mergers (12 galaxies) with luminosity ratios between 1 : 1 and 4 : 1 were found from a sample of 379 red early-type galaxies with MV < -20.5 and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of dry merging are visible for ~150 Myr, and we use this timescale to convert the observed merger incidence into a rate. On this basis we find that present-day spheroidal galaxies with MV < -20.5 on average have undergone between 0.5 and 2 major dry mergers since z ~ 0.7. We have compared this result with the predictions of a cold dark matter-based semianalytic galaxy formation model. The model reproduces the observed declining major merger fraction of bright galaxies and the space density of luminous early-type galaxies reasonably well. The predicted dry merger fraction is consistent with our observational result. Hence, hierarchical models predict and observations now show that major dry mergers are an important driver of the evolution of massive early-type galaxies in recent epochs.

We combine HST imaging from the GEMS (Galaxy Evolution from Morphologies and SEDs) survey with photometric redshifts from COMBO-17 to explore the evolution of disk-dominated galaxies since z [?] 1.1. The sample is composed of all GEMS galaxies with Sersic indices n < 2.5, derived from fits to the galaxy images. We account fully for selection effects through careful analysis of image simulations; we are limited by the depth of the redshift and HST data to the study of galaxies with MV [?] -20, or equivalently, log img1.gif [?] 10. We find strong evolution in the magnitude-size scaling relation for galaxies with MV [?] -20, corresponding to a brightening of ~1 mag arcsec-2 in rest-frame V band by z ~ 1. Yet disks at a given absolute magnitude are bluer and have lower stellar mass-to-light ratios at z ~ 1 than at the present day. As a result, our findings indicate weak or no evolution in the relation between stellar mass and effective disk size for galaxies with log img1.gif [?] 10 over the same time interval. This is strongly inconsistent with the most naive theoretical expectation, in which disk size scales in proportion to the halo virial radius, which would predict that disks are a factor of 2 denser at fixed mass at z ~ 1. The lack of evolution in the stellar mass-size relation is consistent with an "inside-out" growth of galaxy disks on average (galaxies increasing in size as they grow more massive), although we cannot rule out more complex evolutionary scenarios.

We investigate the correlations among stellar mass (M*), disk scale length (Rd), and rotation velocity at 2.2 disk scale lengths (V2.2) for a sample of 81 disk-dominated galaxies (disk/total [?] 0.9) selected from the SDSS. We measure V2.2 from long-slit Ha rotation curves and infer M* from galaxy i-band luminosities (Li) and g - r colors. We find logarithmic slopes of 2.60 +- 0.13 and 3.05 +- 0.12 for the (forward fit) Li-V2.2 and M*-V2.2 relations, somewhat shallower than most previous studies, with intrinsic scatter of 0.13 and 0.16 dex, respectively. Our direct estimates of the total-to-stellar mass ratio within 2.2Rd, assuming a Kroupa IMF, yield a median ratio of 2.4 for M* > 1010 M and 4.4 for M* = 109-1010 M, with large scatter at a given M* and Rd. The typical ratio of the rotation speed predicted for the stellar disk alone to the observed rotation speed at 2.2Rd is ~0.65. The distribution of scale lengths at fixed M* is broad, but we find no correlation between disk size and the residual from the M*-V2.2 relation, implying that the M*-V2.2 relation is an approximately edge-on view of the disk galaxy fundamental plane. Independent of the assumed IMF, this result implies that stellar disks do not, on average, dominate the mass within 2.2Rd. We discuss our results in the context of infall models where disks form in adiabatically contracted cold dark matter halos. A model with a disk-to-halo mass ratio md = 0.05 provides a reasonable match to the Rd-M* distribution for spin parameters l ranging from ~0.04 to 0.08, and it yields a reasonable match to the mean M*-V2.2 relation. A model with md = 0.1 predicts overly strong correlations between disk size and M*-V2.2 residual. Explaining the wide range of halo-to-disk mass ratios within 2.2Rd requires significant scatter in md values, with systematically lower md for galaxies with lower M* or lower stellar surface density S*.

We compare the statistical properties of giant gravitationally lensed arcs produced in matched simulated and observed cluster samples. The observed sample consists of 10 X-ray-selected clusters at redshifts zc ~ 0.2 imaged with HST by Smith et al. The simulated data set is produced by lensing the Hubble Deep Field, which serves as a background source image, with 150 realizations (different projections and shifts) of five simulated zc = 0.2 clusters from a LCDM N-body simulation. The real and simulated clusters have similar masses, the real photometric redshift is used for each background source, and all the observational effects influencing arc detection in the real data set, including light from cluster galaxies, are simulated in the artificial data set. We develop, and apply to both data sets, an objective automatic arc-finding algorithm. We find consistent arc statistics in the real and in the simulated sample, with an average of ~1 detected giant (length-to-width ratio l/w [?] 10) arc per cluster and ~0.2 giant luminous (RST < 22.3 mag) arcs per cluster. Thus, taking into account a realistic source population and observational effects, the clusters predicted by LCDM have the same arc production efficiency as the observed clusters. If, as suggested by other studies, there is a discrepancy between the predicted and the observed total number of arcs on the sky, it must be the result of differences between the redshift-dependent cluster mass functions and not due to differences in the lensing efficiency of the most massive clusters.

We present the first deep color-magnitude diagram of the putative central region (0.deg5 x 0.deg5) of the Canis Major stellar overdensity (l, b) = (240, - 8) found recently by Martin and coworkers, which has been proposed as the remnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial orbit. We find a narrow (in apparent magnitude) main sequence extending 6 mag below the turnoff to our limiting magnitude of B ~ 24.5 mag. This main sequence has very high contrast (>3) with respect to the thin/thick disk/halo background; its narrowness at brighter magnitudes clearly implies the presence of a distinct and possibly still bound stellar system. We derived the line-of-sight size (r1/2) of this system based on the B-band width of the lower main sequence, obtaining 0.94 +- 0.18 (random) +- 0.18 (systematic) kpc. That size matches a model prediction for the main body of the parent galaxy of the Monoceros tidal stream. The high-density contrast and limited spatial extent in the radial direction are very hard to reconcile with the alternative explanation put forward to explain the Canis Major stellar overdensity: a flared or warped Galactic disk viewed in projection, as found in the recent work of Momany and coworkers. We also derived a central surface brightness of mV,0 = 23.3 +- 0.1 mag arcsec-2 and an absolute magnitude of MV = -14.5 +- 0.1 mag. These values place the Canis Major object in the category of dwarf galaxy, considering the LV-size and MV - mV planes for such objects. However, like the Sagittarius dwarf, it is an outlier in the [Fe/H] - MV plane in the sense that it is too metal-rich for its estimated absolute magnitude. This suggests that the main mechanism driving its recent and current star formation history (possibly tidal stripping) is different from that of isolated dwarfs.

We combine imaging from the Hubble Space Telescope Advanced Camera for Surveys, as part of the GEMS (Galaxy Evolution from Morphologies and SEDs) survey, with redshifts and rest-frame quantities from COMBO-17 to study the evolution of morphologically early-type galaxies with red colors since z = 1. From 0.deg5 x 0.deg5 imaging, we draw a large sample of 728 galaxies with centrally concentrated radial profiles (i.e., n [?] 2.5 from Sersic fits) and rest-frame (U - V) colors on the red sequence. We explore how the correlations of rest-frame V-band luminosity and of stellar mass with intrinsic half-light size change over the last half of cosmic time. By appropriate comparison with the well-defined local relations from the Sloan Digital Sky Survey, we find that the luminosity-size and stellar mass-size relations evolve in a manner that is consistent with the passive aging of ancient stellar populations. By itself, this result is consistent with a completely passive evolution of the red early-type galaxy population. If instead, as demonstrated by a number of recent surveys, the early-type galaxy population builds up in mass by roughly a factor of 2 since z ~ 1, our results imply that new additions to the early-type galaxy population follow similar luminosity-size and stellar mass-size correlations, compared to the older subset of early-type galaxies. Adding early-type galaxies to the red sequence through the fading of previously prominent disks appears to be consistent with the data. Through comparison with models, the role of dissipationless merging is limited to <1 major merger on average since z = 1 for the most massive galaxies. Predictions from models of gas-rich mergers are not yet mature enough to allow a detailed comparison to our observations. We find tentative evidence that the amount of luminosity evolution depends on galaxy stellar mass, such that the least massive galaxies show stronger luminosity evolution compared to more massive early types. This could reflect a different origin of low-mass early-type galaxies and/or younger stellar populations; the present data are insufficient to discriminate between these possibilities.

We present Hubble Space Telescope blue spectra at intermediate spectral resolution for the nuclei of 23 nearby disk galaxies. These objects were selected to have nebular emission in their nuclei and span a range of emission-line classifications, as well as Hubble types. In this paper we focus on the stellar population as revealed by the continuum spectral energy distribution measured within the central 0.''13 (~8 pc) of these galaxies. The data were modeled with linear combinations of single-age stellar population synthesis models. The large majority (~80%) of the surveyed nuclei have spectra whose features are consistent with a predominantly old ([?]5 x 109 yr) stellar population. Approximately 25% of these nuclei show evidence of a component with age younger than 1 Gyr, with the incidence of these stars related to the nebular classification. Successful model fits imply an average reddening corresponding to AV [?] 0.4 mag and a stellar metallicity of 1-2.5 Z. We discuss the implications of these results for understanding the star formation history in the environment of quiescent and active supermassive black holes. Our findings reinforce the picture wherein Seyfert nuclei and the majority of low-ionization nuclear emission-line regions are predominantly accretion-powered and suggest that much of the central star formation in H II nuclei is actually circumnuclear.

We present a first analysis of deep 24 mm observations with the Spitzer Space Telescope of a sample of nearly 1500 galaxies in a thin redshift slice, 0.65 [?] z < 0.75. We combine the infrared data with redshifts, rest-frame luminosities, and colors from COMBO-17 and with morphologies from Hubble Space Telescope images collected by the Galaxy Evolution from Morphology and SEDs (GEMS) and Great Observatories Origins Deep Survey (GOODS) projects. To characterize the decline in star formation rate (SFR) since z ~ 0.7, we estimate the total thermal IR luminosities, SFRs, and stellar masses for the galaxies in this sample. At z ~ 0.7, nearly 40% of intermediate- and high-mass galaxies (with stellar masses [?]2 x 1010 M) are undergoing a period of intense star formation above their past-averaged SFR. In contrast, less than 1% of equally massive galaxies in the local universe have similarly intense star formation activity. Morphologically undisturbed galaxies dominate the total infrared luminosity density and SFR density: at z ~ 0.7, more than half of the intensely star-forming galaxies have spiral morphologies, whereas less than ~30% are strongly interacting. Thus, a decline in major merger rate is not the underlying cause of the rapid decline in cosmic SFR since z ~ 0.7. Physical properties that do not strongly affect galaxy morphology--for example, gas consumption and weak interactions with small satellite galaxies--appear to be responsible.

We report the discovery of two doubly imaged quasars, SDSS J100128.61+502756.9 and SDSS J120629.65+433217.6, at redshifts of 1.838 and 1.789, and with image separations of 2.''86 and 2.''90, respectively. The objects were selected as lens candidates from the Sloan Digital Sky Survey (SDSS). Based on the identical nature of the spectra of the two quasars in each pair and the identification of the lens galaxies, we conclude that the objects are gravitational lenses. The lenses are complicated; in both systems there are several galaxies in the fields very close to the quasars, in addition to the lens galaxies themselves. The lens modeling implies that these nearby galaxies contribute significantly to the lens potentials. On larger scales, we have detected an enhancement in the galaxy density near SDSS J100128.61+502756.9. The number of lenses with image separation of ~3'' in the SDSS already exceeds the prediction of simple theoretical models based on the standard L-dominated cosmology and observed velocity function of galaxies.

We present a detailed (U - V) color-magnitude relation (CMR) analysis for three local (z < 0.06) clusters of galaxies. From square-degree imaging of the Abell clusters A85, A496, and A754, we select 637 galaxies down to L ~ 0.1Limg1.gif with spectroscopic membership, which minimizes uncertain field contamination corrections. To characterize the degree of CMR uniformity among nearby clusters, we use a maximum likelihood technique to quantify the CMR properties of the red galaxies in each cluster. We find that these clusters have similar CMRs with a mean color of (U - V) = 1.40 at MV = -20 + 5img2.gif h and narrow limits of intrinsic color scatter [0.047, 0.079] and slope [-0.094, -0.075]. If we restrict our analysis to the core cluster population of red galaxies, the resulting CMRs are in close agreement with that of the Coma Cluster, the only local cluster with a rest-frame (U - V) CMR determination of comparable precision. Therefore, the CMR uniformity of present-day clusters spans a fairly wide range of cluster masses. We test how sensitive the CMR uniformity is to variations in color aperture size and sample selection, and we find at most slightly wider limits of scatter [0.047, 0.112] and slope [-0.104, -0.054]. This upper limit for (U - V) scatter is consistent with the bulk of the stellar populations of red cluster galaxies forming before z = 1.2, with a maximum age spread of 5.2 Gyr. In addition, we note that using colors from apertures containing equal fractions of galactic light does not remove the CMR slope and that none of the slopes exhibit a break as claimed by Metcalf and coworkers. Our findings expand the single Coma data point and provide a much needed z = 0 baseline for comparisons to high-redshift cluster CMRs at the same rest-frame wavelengths. The range in CMR scatter that we find among nearby clusters is consistent with the values reported for clusters at higher redshifts, further suggesting that there has been little evolution in the stellar populations of red-sequence cluster galaxies out to at least z ~ 1. To identify the most recently accreted cluster galaxies, we divide each cluster's membership into three galaxy populations based on (U - V) color relative to the well-defined CMR. Blue and moderately blue galaxies make up 18%-23% by number of each cluster population more luminous than 0.1L*. Our color-magnitude division should represent a rough time since cluster accretion. In testing this hypothesis, we find that blue galaxies are spatially, kinematically, and morphologically distinct from red cluster galaxies. Even in projection, the blue galaxies reside toward the outskirts of the cluster and appear to avoid the inner half-megaparsec, in contrast with the increasing density of red-sequence galaxies toward the cluster center. In addition, the blue galaxies have velocity distributions relative to the cluster rest frame that are flatter, and some have different means, compared to the roughly Gaussian distribution of red member velocities. Members with the bluest colors tend to be disklike or irregular in appearance compared to the red galaxies, which have mostly early-type (E/S0) morphologies. Moderately blue cluster galaxies may be an intermediate mix with a fraction of small bulge-to-disk ratio S0s, yet these require closer scrutiny. The spatial, kinematic, and morphological distinctions between blue and red cluster galaxies provide further evidence that CMR-relative color is related to time since cluster infall and that bluer members are indeed the most recently accreted field spirals as expected in a hierarchical universe.

We recently identified a substantial population of galaxies at z > 2 with comparatively red rest-frame optical colors. These distant red galaxies (DRGs) are efficiently selected by the simple observed color criterion Js - Ks > 2.3. In this paper we present near-infrared spectroscopy with Keck NIRSPEC of six DRGs with previously measured redshifts 2.4 < z < 3.2, two of which were known to host an active nucleus. We detect continuum emission and emission lines of all observed galaxies. Equivalent widths of Ha in the nonactive galaxies are 20-30 A, smaller than measured for Lyman break galaxies (LBGs) and nearby luminous infrared galaxies and comparable to normal nearby galaxies. The modest equivalent widths imply that the galaxies either have a decreasing star formation rate or are very dusty. Fitting both the photometry and the Ha lines, we find continuum extinction AV = 1-2 mag, ages 1-2.5 Gyr, star formation rates 200-400 M yr-1, and stellar masses (1-5) x 1011 M for models with constant star formation rates. Models with a declining star formation lead to significantly lower extinction, star formation rates, and ages but similar stellar masses. From [N II]/Ha ratios we infer that the metallicities are high, 1-1.5 times solar. For four galaxies we can determine line widths from the optical emission lines. The widths are high, ranging from 130 to 240 km s-1, and by combining data for LBGs and DRGs, we find significant correlations between line width and rest-frame U - V color and between line width and stellar mass. The latter correlation has a similar slope and offset as the "baryonic Tully-Fisher relation" for nearby galaxies. From the line widths and effective radii we infer dynamical masses and mass-to-light (M/L) ratios. The median dynamical mass is ~2 x 1011 M, supporting the high stellar masses inferred from the photometry. We find that the median M/LV [?] 0.8(M/L), a factor of ~5 higher than measured for LBGs. We infer from our small sample that DRGs are dustier, more metal-rich, and more massive and have higher ages than z [?] 3 LBGs of the same rest-frame V-band luminosity. Although their volume density is still uncertain, their high M/L ratios imply that they contribute significantly to the stellar mass density at z [?] 2.5. As their stellar masses are comparable to those of early-type galaxies, they may have already assembled most of their final mass.

We present a detailed comparison of structural properties in the rest-frame V band of cluster and field galaxies, selected and analyzed in the same manner, to test the hypothesis that much of the current cluster galaxy membership resulted from the fairly rapid (1-2 Gyr) transformation of infalling, field spirals into red, cluster early types. Specifically, we have selected ~140 galaxies from three nearby Abell clusters (A85, A496, and A754) that have colors significantly bluer than the red sequence population and compared them to ~80 field galaxies with similar colors and luminosities from the Nearby Field Galaxy Survey. The comparison is based on the hypothesis that recent (1-4 Gyr) cluster arrivals were originally blue and star-forming, then stopped forming stars to dim and redden in a few gigayears. For the comparison we quantify galaxy internal structure and morphology from two-dimensional bulge/disk decompositions using GIM2D. We observe structural differences between blue galaxies in local (z < 0.06) clusters, compared to field environments. All cluster galaxies have spectroscopic membership. The majority of blue cluster members, presumably recent additions, are physically smaller and fainter than their equally colored field counterparts. At a matched size and luminosity, the newer cluster arrivals are quantifiably smoother in appearance, yet their total light is as disk dominated as in normal field spirals. Moreover, half of the blue cluster members appear to have blue cores or globally blue color profiles, in contrast with field spirals, which typically exhibit red inward color gradients. Blue cores suggest enhanced nuclear star formation, possibly a starburst, while uniformly blue profiles are consistent with an episode of fairly strong global star formation in the past few gigayears. Our previous work shows that the blue membership of local clusters is a recently infalling population that has yet to encounter the dense core. In a universe without environmentally dependent evolution outside of cluster cores, we would expect blue disk galaxies inhabiting field and cluster regions to have similar morphology, size, and color gradient distributions. Our findings show conclusively not only that the abundance of red and blue galaxies depends on environment, but also that fundamental structural and morphological galaxy properties do indeed reflect the environment in which the galaxy is found. Moreover, the data show that the transformation of accreted galaxies is not confined to the dense cluster core. The overall properties of bluer cluster members are best explained by environment-driven transformation of accreted field spirals, and our results suggest that the processes that govern color and morphological evolution occur separately.

We present the rest-frame colors and luminosities of ~25,000 mR [?] 24 galaxies in the redshift range 0.2 < z [?] 1.1 drawn from 0.78 deg2 of the COMBO-17 survey (Classifying Objects by Medium-Band Observations in 17 Filters). We find that the rest-frame color distribution of these galaxies is bimodal at all redshifts out to z ~ 1. This bimodality permits a model-independent definition of red early-type galaxies and blue late-type galaxies at any given redshift. The colors of the blue peak become redder toward the present day, and the number density of blue luminous galaxies has dropped strongly since z ~ 1. Focusing on the red galaxies, we find that they populate a color-magnitude relation. Such red sequences have been identified in galaxy cluster environments, but our data show that such a sequence exists over this redshift range even when averaging over all environments. The mean color of the red galaxy sequence evolves with redshift in a way that is consistent with the aging of an ancient stellar population. The rest-frame B-band luminosity density in red galaxies evolves only mildly with redshift in a L-dominated cold dark matter universe. When we account for the change in stellar mass-to-light ratio implied by the redshift evolution in red galaxy colors, the COMBO-17 data indicate an increase in stellar mass on the red sequence by a factor of 2 since z ~ 1. The largest source of uncertainty is large-scale structure, implying that considerably larger surveys are necessary to further refine this result. We explore mechanisms that may drive this evolution in the red galaxy population, finding that both galaxy merging and truncation of star formation in some fraction of the blue star-forming population are required to fully explain the properties of these galaxies.

We study the recently discovered gravitational lens SDSS J1004+4112, the first quasar lensed by a cluster of galaxies. It consists of four images with a maximum separation of 14.''62. The system was selected from the photometric data of the Sloan Digital Sky Survey (SDSS) and has been confirmed as a lensed quasar at z = 1.734 on the basis of deep imaging and spectroscopic follow-up observations. We present color-magnitude relations for galaxies near the lens plus spectroscopy of three central cluster members, which unambiguously confirm that a cluster at z = 0.68 is responsible for the large image separation. We find a wide range of lens models consistent with the data, and despite considerable diversity they suggest four general conclusions: (1) the brightest cluster galaxy and the center of the cluster potential well appear to be offset by several kiloparsecs; (2) the cluster mass distribution must be elongated in the north-south direction, which is consistent with the observed distribution of cluster galaxies; (3) the inference of a large tidal shear (~0.2) suggests significant substructure in the cluster; and (4) enormous uncertainty in the predicted time delays between the images means that measuring the delays would greatly improve constraints on the models. We also compute the probability of such large-separation lensing in the SDSS quasar sample on the basis of the cold dark matter model. The lack of large-separation lenses in previous surveys and the discovery of one in SDSS together imply a mass fluctuation normalization s8 = 1.0img1.gif (95% confidence) if cluster dark matter halos have an inner density profile r [?] r-1.5. Shallower profiles would require higher values of s8. Although the statistical conclusion might be somewhat dependent on the degree of the complexity of the lens potential, the discovery of SDSS J1004+4112 is consistent with the predictions of the abundance of cluster-scale halos in the cold dark matter scenario.

The luminosity-size and mass-size distributions of galaxies out to z ~ 3 are presented. We use very deep near-infrared images of the Hubble Deep Field-South in the Js, H, and Ks bands, taken as part of FIRES at the VLT, to follow the evolution of the optical rest-frame sizes of galaxies. For a total of 168 galaxies with Ks,AB [?] 23.5, we find that the rest-frame V-band sizes re,V of luminous galaxies (img1.gif ~ 2 x 1010 h-2 L) at 2 < z < 3 are 3 times smaller than for equally luminous galaxies today. In contrast, the mass-size relation has evolved relatively little: the size at mass img2.gif ~ 2 x 1010 h-2 M has changed by 20% (+-20%) since z ~ 2.5. Both results can be reconciled by the fact that the stellar M/L ratio is lower in the luminous high-z galaxies than in nearby ones because they have young stellar populations. The lower incidence of large galaxies at z ~ 3 seems to reflect the rarity of galaxies with high stellar mass.

We present the evolution of the rest-frame optical luminosity density jimg1.gif, the integrated rest-frame optical color, and the stellar mass density, r*, for a sample of Ks band-selected galaxies in the Hubble Deep Field-South (HDF-S). We derived jimg1.gif in the rest-frame U, B, and V bands and found that jimg1.gif increases by a factor of 1.9 +- 0.4, 2.9 +- 0.6, and 4.9 +- 1.0 in the V, B, and U rest-frame bands, respectively, between redshifts of 0.1 and 3.2. We derived the luminosity-weighted mean cosmic (U-B)rest and (B-V)rest colors as a function of redshift. The colors bluen almost monotonically with increasing redshift; at z = 0.1, the (U-B)rest and (B-V)rest colors are 0.16 and 0.75, respectively, while at z = 2.8 they are -0.39 and 0.29, respectively. We derived the luminosity-weighted mean M/Limg2.gif, using the correlation between (U-V)rest and log M/Limg2.gif that exists for a range in smooth star formation histories (SFHs) and moderate extinctions. We have shown that the mean of individual M/Limg2.gif estimates can overpredict the true value by ~70%, while our method overpredicts the true value by only ~35%. We find that the universe at z ~ 3 had ~10 times lower stellar mass density than it does today in galaxies with Limg3.gif > 1.4 x 1010 himg4.gif L. Half of the stellar mass of the universe was formed by z ~ 1-1.5. The rate of increase in r* with decreasing redshift is similar to but above that for independent estimates from the HDF-N, but it is slightly less than that predicted by the integral of the SFR(z) curve.

We present new data exploring the scaling relations, such as the Tully-Fisher relation (TFR), of bright barred and unbarred galaxies. A primary motivation for this study is to establish whether barredness correlates with, and is a consequence of, virial properties of galaxies. Various lines of evidence suggest that dark matter is dominant in disks of bright unbarred galaxies at 2.2 disk scale lengths, the point of peak rotation for a pure exponential disk. We test the hypothesis that the Tully-Fisher (TF) plane of barred high surface brightness galaxies is offset from the mean TFR of unbarred galaxies, as might be expected if barred galaxies are "maximal" in their inner parts. We use existing and new TF data to search for basic structural differences between barred and unbarred galaxies. Our new data consist of two-dimensional Ha velocity fields derived from SparsePak integral field spectroscopy and V- and I-band CCD images collected at the WIYN Observatory for 14 strongly barred galaxies. Differences may exist between kinematic and photometric inclination angles of barred versus unbarred galaxies. These findings lead us to restrict our analysis to barred galaxies with i > 50deg. We use WIYN/SparsePak (two-dimensional) velocity fields to show that long-slit (one-dimensional) spectra yield reliable circular speed measurements at or beyond 2.2 disk scale lengths, far from any influence of the bar. This enables us to consider line width measurements from extensive TF surveys that include barred and nonbarred disks and derive detailed scaling relation comparisons. We find that for a given luminosity, barred and unbarred galaxies have comparable structural and dynamical parameters, such as peak velocities, scale lengths, and colors. In particular, the location of a galaxy in the TF plane is independent of barredness. In a global dynamical sense, barred and unbarred galaxies behave similarly and are likely to have, on average, comparable fractions of luminous and dark matter at a given radius.