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Using the Horizon-AGN simulation data, we study the relative role of mergers and environmental effects in shaping the spin of early-type galaxies (ETGs) after $z \simeq 1$. We follow the spin evolution of 10,037 color-selected ETGs more massive than 10$^{10} \rm \, M_{\odot}$ that are divided into four groups: cluster centrals (3%), cluster satellites (33%), group centrals (5%), and field ETGs (59%). We find a strong mass dependence of the slow rotator fraction, $f_{\rm SR}$, and the mean spin of massive ETGs. Although we do not find a clear environmental dependence of $f_{\rm SR}$, a weak trend is seen in the mean value of spin parameter driven by the satellite ETGs as they gradually lose their spin as their environment becomes denser. Galaxy mergers appear to be the main cause of total spin changes in 94% of central ETGs of halos with $M_{vir} > 10^{12.5}\rm M_{\odot}$, but only 22% of satellite and field ETGs. We find that non-merger induced tidal perturbations better correlate with the galaxy spin-down in satellite ETGs than mergers. Given that the majority of ETGs are not central in dense environments, we conclude that non-merger tidal perturbation effects played a key role in the spin evolution of ETGs observed in the local ($z < 1$) universe., Accepted for publication in ApJ

We observed two fields near M32 with the Advanced Camera for Surveys/High Resolution Channel (ACS/HRC) on board the Hubble Space Telescope (HST). The main field, F1, is 1.8 arcmin from the center of M32; the second field, F2, constrains the M31 background, and is 5.4 arcmin distant. Each field was observed for 16-orbits in each of the F435W (narrow B) and F555W (narrow V) filters. The duration of the observations allowed RR Lyrae stars and other short-period variables to be detected. A population of RR Lyrae stars determined to belong to M32 would prove the existence of an ancient population in that galaxy, a subject of some debate. We detected 17 RR Lyrae variables in F1 and 14 in F2. A 1-sigma upper limit of 6 RR Lyrae variables belonging to M32 is inferred from these two fields alone. Use of our two ACS/WFC parallel fields provides better constraints on the M31 background, however, and implies that $7_{-3}^{+4}$ (68 % confidence interval) RR Lyrae variables in F1 belong to M32. We have therefore found evidence for an ancient population in M32. It seems to be nearly indistinguishable from the ancient population of M31. The RR Lyrae stars in the F1 and F2 fields have indistinguishable mean V-band magnitudes, mean periods, distributions in the Bailey diagram and ratios of RRc to RR(tot) types. However, the color distributions in the two fields are different, with a population of red RRab variables in F1 not seen in F2. We suggest that these might be identified with the detected M32 RR Lyrae population, but the small number of stars rules out a definitive claim., 19 pages, 18 figures, accepted ApJ

We use a sample of 53 massive early-type strong gravitational lens galaxies with well-measured redshifts (ranging from z = 0.06 to 0.36) and stellar velocity dispersions (between 175 and 400 km s(-1)) from the Sloan Lens ACS (SLACS) Survey to derive numerous empirical scaling relations. The ratio between central stellar velocity dispersion and isothermal lens-model velocity dispersion is nearly unity within errors. The SLACS lenses define a fundamental plane (FP) that is consistent with the FP of the general population of early-type galaxies. We measure the relationship between strong-lensing mass M-lens within one-half effective radius (R-e/2) and the dimensional mass variable M-dim equivalent to G(-1) sigma(2)(e2)(R-e/2) to be log (M-lens/10(11) M-circle dot) = (1.03 +/- 0.04) log (M-dim/10(11) M-circle dot)+ (0.54 +/- 0.02) (where sigma(e2) is the projected stellar velocity dispersion within R-e/2). The near-unity slope indicates that the mass-dynamical structure of massive elliptical galaxies is independent of mass and that the "tilt'' of the SLACS FP is due entirely to variation in total (luminous plus dark) mass- to-light ratio with mass. Our results imply that dynamical masses serve as a good proxies for true masses in massive elliptical galaxies. Regarding the SLACS lenses as a homologous population, we find that the average enclosed two-dimensional (2D) mass profile goes as log[M(

We present the definitive data for the full sample of 131 strong gravitational lens candidates observed with the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope by the Sloan Lens ACS (SLACS) Survey. All targets were selected for higher redshift emission lines and lower redshift continuum in a single Sloan Digital Sky Survey (SDSS) spectrum. The foreground galaxies are primarily of early-type morphology, with redshifts from z similar or equal to 0.05 to 0.5 and velocity dispersions from sigma similar or equal to 160 to 400 km s(-1); the faint background emission-line galaxies have redshifts ranging from z similar or equal to 0.2 to 1.2. We confirm 70 systems showing clear evidence of multiple imaging of the background galaxy by the foreground galaxy, as well as an additional 19 systems with probable multiple imaging. For 63 clear lensing systems, we present singular isothermal ellipsoid and light-traces-mass gravitational lens models fitted to the ACS imaging data. These strong-lensing mass measurements are supplemented by magnitudes and effective radii measured from ACS surface brightness photometry and redshifts and velocity dispersions measured from SDSS spectroscopy. These data constitute a unique resource for the quantitative study of the interrelations between mass, light, and kinematics in massive early-type galaxies. We show that the SLACS lens sample is statistically consistent with being drawn at random from a parent sample of SDSS galaxies with comparable spectroscopic parameters and effective radii, suggesting that the results of SLACS analyses can be generalized to the massive early-type population.

We report the discovery of two concentric Einstein rings around the gravitational lens SDSS J0946+ 1006. The main lens is at redshift z(l) = 0.222, while the inner ring ( 1) is at redshift z(s1) 0.609 (R-Ein1 = 1.43 '' +/- 0.01 ''). The wider image separation ( R-Ein2 = 2.07 '' +/- 0.02 '') of the outer ring (2) implies a higher redshift than that of ring 1; the detection of ring 2 in the F814WACS filter implies an upper limit of z(s2) less than or similar to 6.9. The main lens can be described by a power- law total mass density profile rho(tot) proportional to r(-gamma ') with gamma ' = 2.00 +/- 0.03 and velocity dispersion sigma(SIE) = 287 +/- 5 km s(-1) ( the stellar velocity dispersion is sigma(v,*) = 284 +/- 24 km s(-1)). The strong lensing configuration is inconsistent with light traces mass. Adopting a prior on the stellar mass- to- light ratio from previous SLACS work, we infer a 73% +/- 9% dark matter fraction within the cylinder of radius equal to the effective radius of the lens. We find that, for the case of SDSS J0946+ 1006, the geometry of the two rings does not place interesting constraints on cosmography because of the suboptimal redshifts of lens and sources. We then consider the perturbing effect of the mass associated with ring 1 building a compound lens model. This introduces minor changes to the mass of the main lens and provides an estimate of z(s2) = 3.1(-1.0)(+2.0) and of the mass of the source responsible for ring 1 (sigma(SIE,s1) = 94(-47)(+27) km s(-1)). We conclude by examining the prospects of doing cosmography with a sample of 50 double rings, expected from future space- based surveys. Accounting for uncertainties in the mass profile of the lens and the effects of the perturber, we find that such a sample would constrain Omega(m) and within 10%, assuming flatness.

We present a weak-lensing analysis of 22 early-type (strong) lens galaxies, based on deep HST images obtained as part of the Sloan Lens ACS Survey. Using advanced techniques to control systematic uncertainties, we show that weak-lensing signal is detected out to similar to 300 h(-1) kpc (at the mean lens redshift z = 0.2). We analyze blank control fields from COSMOS in the same manner, inferring that the residual systematic uncertainty in the tangential shear is less than 0.3%. A joint strong- and weak-lensing analysis shows that the average total mass density profile is consistent with isothermal (i.e., rho proportional to r(-2)) over two decades in radius (3-300 h(-1) kpc, approximately 1-100 effective radii). This finding extends by over an order of magnitude in radius previous results, based on strong lensing and/or stellar dynamics, that luminous and dark components "conspire" to form an isothermal mass distribution. In order to disentangle the contributions of luminous and dark matter, we fit a two-component mass model (deVaucouleurs+NFW) to the weak- and strong-lensing constraints. It provides a good fit to the datawith only two free parameters: (1) the average stellar mass-to-light ratio M-*/L-V =4.48 +/- 0.46 h M-circle dot L-circle dot(-1) (at z = 0.2), in agreement with that expected for an old stellar population; (2) the average virialmass-to-light ratio M-vir/L-V = 246(-87)(+101) h M-circle dot L-circle dot(-1) Taking into account the scatter in the mass-luminosity relation, the latter result is in good agreement with semianalytical models of massive galaxy formation. The dark matter fraction inside the sphere of radius, the effective radius, is found to be 27% +/- 4%. Our results are consistent with galaxy-galaxy lensing studies of early-type galaxies that are not strong lenses, in the 30- 300 h(-1) kpc radius range. Thus, within the uncertainties, our results are representative of early- type galaxies in general.

Gravitational lensing and stellar dynamics are two independent methods, based solely on gravity, to study the mass distributions of galaxies. Both methods suffer from degeneracies, however, that are difficult to break. In this paper we present a new framework that self-consistently unifies gravitational lensing and stellar dynamics, breaking some classical degeneracies that have limited their individual usage, particularly in the study of high-redshift galaxies. For any given galaxy potential, the mapping of both the unknown lensed source brightness distribution and the stellar phase-space distribution function onto the photometric and kinematic observables can be cast as a single set of coupled linear equations, which are solved by maximizing the likelihood penalty function. The Bayesian evidence penalty function subsequently allows one to find the best potential-model parameters and to quantitatively rank potential-model families or other model assumptions ( e. g., PSF). We have implemented a fast algorithm that solves for the maximum-likelihood pixelized lensed source brightness distribution and the two-integral stellar phase- space distribution function f ( E; L(z)), assuming axisymmetric potentials. To make the method practical, we have devised a new Monte Carlo approach to Schwarzschild's orbital superposition method, based on the superposition of twointegral ( E and Lz) toroidal components, to find the maximum-likelihood two-integral distribution function in a matter of seconds in any axisymmetric potential. The nonlinear parameters of the potential are subsequently found through a hybrid MCMC and Simplex optimization of the evidence. Illustrated by the power-law potential models of Evans, we show that the inclusion of stellar kinematic constraints allows the correct linear and nonlinear model parameters to be recovered, including the potential strength, oblateness, and inclination, which, in the case of gravitational-lensing constraints only, would otherwise be fully degenerate.

We combine strong-lensing masses with SDSS stellar velocity dispersions and HST ACS effective (half-light) radii for 36 lens galaxies from the Sloan Lens ACS (SLACS) Survey to study the mass dependence of mass-dynamical structure in early-type galaxies. We find that over a 180-390 km s(-1) range in velocity dispersion, structure is independent of lensing mass to within 5%. This result suggests a systematic variation in the total (i.e., luminous plus dark matter) mass-to-light ratio as the origin of the "tilt" of the fundamental plane ( FP) scaling relationship between galaxy size, velocity dispersion, and surface brightness. We construct the FP of the lens sample, which we find to be consistent with the FP of the parent SDSS early-type galaxy population. We also present the first observational correlation between the mass-to-light ratio and residuals about the FP. Finally, we reformulate the FP in terms of surface mass density rather than surface brightness. By removing the complexities of stellar-population effects, this mass-plane formulation will facilitate comparison to numerical simulations and possible use as a cosmological distance indicator.

We present results from Planetary Nebula Spectrograph (PN. S) observations of the elliptical galaxy NGC 3379 and a description of the data reduction pipeline. We detected 214 planetary nebulae, of which 191 are ascribed to NGC 3379 and 23 to the companion galaxy NGC 3384. Comparison with data from the literature shows that the PN. S velocities have an internal error of

We present a joint gravitational lensing and stellar-dynamical analysis of 15 massive field early-type galaxies selected from the Sloan Lens ACS (SLACS) Survey. The following numerical results are found: (1) A joint likelihood gives an average logarithmic density slope for the total mass density of = 2.01(-0.03)(+0.02) (68% CL; rho(tot) proportional to r(-gamma')) = 4.2 +/- 0.4 kpc (rms of 1.6 kpc) for isotropic models. The inferred intrinsic rms spread in logarithmic density slopes is sigma(gamma') = 0.12. (2) The average position-angle difference between the light distribution and the total mass distribution is found to be = 0 degrees +/- 3 degrees (rms of 10 degrees), setting an upper limit of less than or similar to 0.035 on the average external shear. The total mass has an average ellipticity = 0.78 +/- 0.03 (rms of 0.12), which correlates extremely well with the stellar ellipticity, q(*), resulting in = 0.99 +/- 0.03 (rms of 0.11) for sigma greater than or similar to 225 km s(-1) omitting three S0 lens galaxies. (3) The average projected dark matter mass fraction is inferred to be = 0.25 +/- 0.06 (rms of 0.22) inside , using the stellar mass-to-light ratios derived from the fundamental plane as priors. (4) Combined with results from the Lenses Structure and Dynamics (LSD) Survey at z greater than or similar to 0.3, we find no significant evolution of the total density slope inside one effective radius for galaxies with sigma(ap) >= 200 km s(-1): a linear fit gives alpha(gamma') equivalent to d / dz = 0.23 +/- 0.16 (1 sigma) for the range z = 0.08-1.01. The small scatter and absence of significant evolution in the inner density slopes suggest a collisional scenario in which gas and dark matter strongly couple during galaxy formation, leading to a total mass distribution that rapidly converges to dynamical isothermality.

We use HST images to derive effective radii and effective surface brightnesses of 15 early-type (E+S0) lens galaxies identified by the SLACS Survey. Our measurements are combined with stellar velocity dispersions from the SDSS database to investigate for the first time the distribution of lens galaxies in the fundamental plane (FP) space. Accounting for selection effects (top priority to the largest Einstein radii and thus approximately to the largest velocity dispersions, σ>~240 km s-1) and for passive evolution, the distribution of the lens galaxies inside the FP is indistinguishable from that of the parent sample of SDSS galaxies. We conclude that SLACS lenses are a fair sample of high velocity dispersion E+S0s. By comparing the central stellar velocity dispersion (σ) with the velocity dispersion that best fits the lensing models (σSIE) we find ≡=1.01+/-0.02 with 0.065 rms scatter. We infer that within the Einstein radii (typically Re/2) the SLACS lenses are very well approximated by isothermal ellipsoids, requiring a fine tuning of the stellar and dark matter distribution (the bulge-halo ``conspiracy''). Interpreting the offset from the local FP in terms of evolution of the stellar mass-to-light ratio, we find dlog(M/LB)/dz=-0.69+/-0.08 (rms 0.11) consistent with the rate found for field E+S0s and with most of the stars being old (zf>2) and less than ~10% of the stellar mass having formed below z=1. We discuss our results in the context of formation mechanisms such as collisionless (``dry'') mergers. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555. These observations are associated with program 10174. Support for program 10174 was provided by NASA through a grant from STScI, which is operated by the AURA, Inc., under NASA contract NAS5-26555.

The Sloan Lens ACS (SLACS) Survey is an efficient Hubble Space Telescope (HST) Snapshot imaging survey for new galaxy-scale strong gravitational lenses. The targeted lens candidates are selected spectroscopically from the Sloan Digital Sky Survey (SDSS) database of galaxy spectra for having multiple nebular emission lines at a redshift significantly higher than that of the SDSS target galaxy. The SLACS survey is optimized to detect bright early-type lens galaxies with faint lensed sources in order to increase the sample of known gravitational lenses suitable for detailed lensing, photometric, and dynamical modeling. In this paper, the first in a series on the current results of our HST Cycle 13 imaging survey, we present a catalog of 19 newly discovered gravitational lenses, along with nine other observed candidate systems that are either possible lenses, nonlenses, or nondetections. The survey efficiency is thus >= 68%. We also present Gemini 8 m and Magellan 6.5 m integral-field spectroscopic data for nine of the SLACS targets, which further support the lensing interpretation. A new method for the effective subtraction of foreground galaxy images to reveal faint background features is presented. We show that the SLACS lens galaxies have colors and ellipticities typical of the spectroscopic parent sample from which they are drawn (SDSS luminous red galaxies and quiescent MAIN sample galaxies), but are somewhat brighter and more centrally concentrated. Several explanations for the latter bias are suggested. The SLACS survey provides the first statistically significant and homogeneously selected sample of bright early-type lens galaxies, furnishing a powerful probe of the structure of early-type galaxies within the half-light radius. The high confirmation rate of lenses in the SLACS survey suggests consideration of spectroscopic lens discovery as an explicit science goal of future spectroscopic galaxy surveys.

We have determined spectroscopic ages of elliptical galaxies in the Virgo Cluster using spectra of very high signal-to-noise ratio (S/N > 100 angstrom(-1)). We observed eight galaxies with the Subaru Telescope and have combined this sample with six galaxies previously observed with the WHT. To determine their ages, we have used a new method based on the H gamma(sigma) age indicator, which is virtually independent of the effects of metallicity. Apart from ages we have estimated abundances of various elements. In this paper we present the observations, the data reduction, and the reliability of the H gamma(sigma) method. The results of this investigation are presented in a companion paper.

We present Gemini integral-field unit spectroscopy and Hubble Space Telescope (HST) F435W- and F814W-band images of a newly discovered four-image gravitational lens, SDSS J140228.22 + 632133.3 ( hereafter SDSSJ1402). The system is the first of 49 spectroscopically selected gravitational lens candidates to be imaged with the Advanced Camera for Surveys Wide-Field Channel on board HST as part of a Snapshot Survey program designed to expand the sample of known gravitational lenses amenable to detailed photometric, lensing, and dynamical studies. The lens is an r = 17.00 +/- 0.05 elliptical galaxy at a redshift of z(l) = 0.2046 +/- 0.0001, with a luminosity-weighted stellar velocity dispersion of 267 +/- 17 km s(-1) within a 3 ''-diameter aperture. Multiple emission lines place the faint lensed source galaxy at a redshift of z(s) = 0.4814 +/- 0.0001. The best-fitting singular isothermal ellipsoid lens model has an Einstein radius b = 1.'' 35 +/- 0.'' 025 (or 4.9 +/- 0.2 h(65)(-1) kpc), corresponding to an enclosed mass of (30.9 +/- 1.1) x 10(10) h(65)(-1) M-., and a rest-frame B-band mass-to-light ratio of 8.1 +/- 0.5 h(65) times solar within the same region. By calculating an expected stellar mass-to-light ratio for SDSSJ1402 using a local universe value of 7.3 +/- 2.1 h(65) and the measured evolution of the fundamental plane, we estimate the fraction of luminous matter within the Einstein radius to be 0.64 +/- 0.22: stellar mass is dominant, but some dark matter appears to be required even at this small scale of roughly one-half effective radius.

We examine the dynamics of the stars and globular clusters in the nearby giant elliptical galaxy M87 and constrain the mass distribution, using all the available data over a large range of radii, including higher order moments of the stellar line-of-sight velocity distributions and the discrete velocities of over 200 globular clusters. We introduce an extension of spherical orbit modeling methods that makes full use of all the information in the data and provides very robust constraints on the mass models. We conclusively rule out a constant mass-to-light ratio model and infer that the radial density profile of the galaxy's dark halo falls off more slowly than r(-2), suggesting that the potential of the Virgo Cluster is already dominant at r similar to 300" similar to 20 kpc.

Most elliptical galaxies contain central black holes, whose masses scale proportionally to the observed central velocity dispersions of the host galaxies according to the so-called M-BH-sigma (c) relation. Here we discuss some consequences that can be derived by combining the M-BH-sigma (c) relation with the scaling relation describing the fundamental plane of elliptical galaxies. In particular, the possibility of substantial dissipationless merging in the formation and evolution of elliptical galaxies is discussed. We enforce the merger end products to satisfy the two scaling relations mentioned above, and a major role of dissipation in galaxy formation is strongly suggested by our analysis. Moreover, we show that existing observational data may shed some light on the complex process of black hole merging.

We present a morphological study of the galaxy population of the luminous X-ray cluster MS 1054-03 at z = 0.83. The sample consists of 81 spectroscopically confirmed cluster members in a 3 x 2 h(50)(-1) Mpc area imaged in F606W and F814W with WFPC2. We find 13 ongoing mergers in MS 1054-03, comprising 17% of the L greater than or similar to L-* cluster population. Most of these mergers will likely evolve into luminous (similar to 2L(*)) elliptical galaxies, and some may evolve into SO galaxies. Assuming the galaxy population in MS 1054-03 is typical for its redshift, it is estimated that similar to 50% of present-day cluster elliptical galaxies experienced a major merger at z

We present deep Keck telescope spectroscopy of eight galaxies in the luminous X-ray cluster MS 1054-03 at z = 0.83, The data are combined with imaging observations from the Hubble Space Telescope (HST). The spectroscopic data are used to measure the internal kinematics of the galaxies, and the HST data are used to measure their structural parameters. Six galaxies have early-type spectra, and two have "E + A" spectra. The galaxies with early-type spectra define a tight fundamental plane (FP) relation. The evolution of the mass-to-light ratio is derived from the FP. The M/L ratio evolves as Delta log M/L-B proportional to -0.40z (Omega(m) = 0.3, Omega(Lambda) = 0). The observed evolution of the MIL ratio provides a combined constraint on the formation redshift of the stars, the initial mass function (IMF), and cosmological parameters. For a Salpeter IMF (x = 2.35), we find that z(form) > 2.8 and Omega(m) 0: z(form) > 1.7 if Omega(m) = 0.3 and Omega(Lambda) = 0.7. At present, the limiting factor in constraining z(form) and Omega from the observed luminosity evolution of early-type galaxies is the poor understanding of the IMF We find that if Omega(m) = 1, the IMF must be significantly steerer than the Salpeter IMF (x > 2.6).

We have analyzed the ROSAT PSPC observations of the central region of the Fornax cluster, a relatively poor group of galaxies at a distance of about 24 Mpc. The brightest X-ray and optical galaxy in the group is NGC 1399, an E1 galaxy located near the center of the Fornax cluster. We characterize the hot gas around the galaxy, derived from a 2' to 18' annulus around NGC 1399, as having a mean temperature of 1.30 +/- 0.05 keV and a heavy element abundance of 0.6 +/- 0.1 with respect to solar abundance (Fe/H = 4.68 x 10(-5) by number). Spatially resolved spectral data provide both gas temperature and gas abundance profiles extending to 125 kpc (18') from the galaxy. The temperature distribution, combined with the X-ray surface brightness profile, yields an accurate determination of the gravitating mass within 125 kpc, which falls in the range (4.3 - 8.1) x 10(12) M. (95% confidence range, including systematic uncertainties). If we include the extended optical halo around NGC 1399, the mass-to-light ratio increases with radius from 33 +/- 8 M./L. at 18 kpc to 70 +/- 22 M./L. at 110 kpc. We compare the heavy element abundance distribution measured around NGC 1399 with that measured around the Virgo galaxy NGC 4472, as well as to models for hot coronae. We find that the abundance distribution is in good agreement with that previously measured for NGC 4472 by Forman et al. in 1993. For both galaxies, the observed abundance profiles require both a weak evolution of the type Ia supernova rate with time and a present epoch rate which agrees with that of Cappellaro et al. We compare mass measurements in NGC 1399 to those for M87. The similarity of the optical masses in these systems and their differences in gas masses and gravitating masses lead us to suggest that the optical galaxies formed at an early stage when the central potentials of these two systems were similar. Subsequent infall of gas and dark matter into the larger, deeper Virgo potential resulted in the greater mass of the Virgo cluster compared to Fornax. We also report on X-ray properties of thirteen other Fornax galaxies. Eight of these were detected in ROSAT images with luminosities in the 0.2 to 2 keV energy band from 1 x 10(39) to 1.6 x 10(41) ergs s(-1) Five galaxies were sufficiently bright to permit spectral analyses and all but one (NGC 1380) had spectra consistent with thermal emission. Two (NGC 1404 and NGC 1387) of the four galaxies with well-constrained spectral parameters have hot coronae with characteristic gas temperatures of about 0.5 keV and iron abundances less than that found around NGC 1399 and other bright ellipticals. To maintain these hot coronae, the absolute magnitudes of these galaxies must be brighter than -19. Thus the distance to Fornax must be at least 18 Mpc, and, if there are no large peculiar velocities, the Hubble constant should be less than 75 km s(-1) Mpc(-1). Since these galaxies are all members of Fornax, distance uncertainties do not affect the relationship between their optical magnitude and X-ray luminosity. Analysis of the Fornax galaxies supports the contention that the scatter in the X-ray and optical relationship is intrinsic and does not arise solely from distance uncertainties. For the elliptical galaxy NGC 1404, the X-ray images show that the hot corona is distorted and likely is being stripped, indicating infall of the galaxy toward NGC 1399 and the cluster center.

Line strengths and their gradients in Mg, Fe, and H beta have been determined for a sample of 20 SO galaxies in order to study the stellar populations of their bulges and disks and to investigate their relationship to elliptical galaxies. Data are also presented for the elliptical galaxy NGC 1700 and the E/S0 NGC 3585. We find that S0 galaxies generally follow a similar, though possibly steeper, relationship between central Mg-2 line strength and central velocity dispersion as found for elliptical galaxies. The S0 galaxies show no dependence between Fe line strengths and central velocity dispersion-similar to the behavior observed in elliptical galaxies. The central Mg/Fe ratios in the luminous S0 galaxies show an overabundance of Mg to Fe with respect to solar element ratios. The disks (at 1 h) and bulges (at 1 r(e)) show Mg/Fe ratios lower than those found in the central regions. The magnitudes of the Mg and Fe line-strength gradients within the galaxies are found to be correlated, i.e., objects with steep Mg gradients have correspondingly large Fe gradients. We infer bulge and disk gradients for the nine most edge-on galaxies for which we have both major and minor-axis profiles. The metal line strengths decrease with radius along the major and minor axes in the bulge-dominated central regions. At larger radii, however, the major-axis metal line strength profiles flatten while the minor-axis bulge profiles fall to lower values. Representative color maps in B-R are presented that display a separation between bulge and disk colors corresponding to the metal line strength profiles. Based on our Mg-2 profiles, the average metal gradient found in the disks of our subsample is Delta[Fe/H]/Delta(r/h) = -0.08 +/- 0.06, which corresponds to a reduction in the mean metallicity of the disk stellar population by less than or similar to 15% per disk scale length (h). These shallow metallicity gradients are approximately a factor of 2-3 smaller than those derived for the disks of late-type spiral galaxies from H II regions and are consistent with previous investigations that showed a trend for disk metallicity gradients to decrease toward earlier Hubble types. As inferred from our Mg and Fe line strengths, the mean size of the bulge metallicity gradients is Delta[Fe/H]/Delta log r = -0.7 +/- 0.4, which is steeper than typical elliptical galaxy gradients. Our findings do not support formation scenarios in which bulges formed either from heated disk material at late times after disk formation or through dissipationless stellar merging, as neither process includes mechanisms for producing the observed metallicity gradients. Our observations are better explained in terms of formation via dissipative collapse (or merging) at early times.

We provide a new calibration of the metallicity dependence of the Mg-2 index in stellar systems. It is based on new isochrones by the Padova group and the latest stellar spectral libraries. Since the Mg-2 line is one of the strongest absorption lines in the optical spectrum, it is often used to determine metallicities of composite stellar systems assuming a certain age. This paper allows one to do this, with an uncertainty of about 0.1 in log Z, for an age range between 8 and 17 Gyr. We estimate that the uncertainties of the current Mg, calibrations in the literature amount to another 0.15 in log Z. Using our calibration we find, under conservative assumptions, that the large majority of elliptical galaxies, and also many bulges of spiral galaxies, must have Mg abundances larger than solar; this conclusion depends very little on their star formation history. If [Mg/Fe] = 0, the same can be said for total metallicities. If we assume that [Mg/Fe] = 0.45, only about 38% of the ellipticals must have metallicities (Z) larger than solar.

Line strengths and their gradients in Mg, Fe, and H beta have been determined for seven elliptical and nine brightest cluster galaxies (BCGs) in order to study their stellar populations and investigate their relationship to one another. We find that BCGs follow the same relationship between central Mg b line strength and central velocity dispersion found for elliptical galaxies. Brightest cluster galaxies are in agreement with the known trend toward more massive elliptical galaxies having larger [Mg/Fe] ratios, while the internal gradients within our BCG and E galaxies are consistent with a roughly constant [Mg/Fe] ratio. We find that a correlation exists between the central [Mg/Fe] ratio and average H beta line strength in the sense that both BCG and elliptical galaxies with larger [Mg/Fe] ratios have lower [H beta] strengths. For our sample, H beta is the best predictor of [Mg/Fe] ratio. If the dominant contribution to the H beta feature is from turnoff stars then this relation predicts that more massive elliptical galaxies are older than less massive ones. If, however, the main source of the H beta index is from horizontal-branch stars, then the observed {[Mg/Fe],H beta} relation could be the result of more massive elliptical galaxies having flatter IMFs for high-mass stars than less massive elliptical galaxies. The [H beta] line strengths of the objects in our sample span a range of values. The BCGs generally have low global H beta line strengths, which, under the assumption that the H beta feature can be used as an age discriminant, indicates that the bulk of these systems underwent their last major episode of star formation greater than or similar to 8-10 Gyr ago. For both the elliptical galaxies and BCGs we find that within a galaxy, the H beta profile is flat for objects whose H beta absorption can be reliably measured. In the presence of a declining metallicity gradient this suggests that the centers of elliptical galaxies and BCGs are similar to 1-3 Gyr younger than their outer regions. The metal line strength gradients for the elliptical galaxies and BCGs are similar and consistent with an average gradient of Delta[Fe/H]/Delta log r = -0.25 +/- 0.1, corresponding to a reduction in the mean metallicity of the stellar population by a factor of similar to 2 over a factor of 10 in radius. No strong correlations are found between the metallicity gradient sizes and either kinematic or line strength parameters of the galaxies. For the galaxy NGC 4073, the brightest member of the poor cluster MKW 4, we find enhanced H beta absorption in the region coincident with the counterrotating stellar core. These observations are consistent with a scenario in which the accreted object was a younger system and/or a recent episode of star formation was triggered by an interaction.

We report the discovery of neutral hydrogen in a second elliptical galaxy with shells, NGC 2865. Very Large Array images reveal an association between the neutral hydrogen (H I) and the fine structure (shells, tails, and loops) in the galaxy. Similar to what we previously observed in NGC 5128 (Centaurus A), most of the 6 x 10(8) h(-2) M. of cold gas is found in a broken ring in the outer regions of NGC 2865 (beyond 1/2D(25))I and is displaced to the outside of the shells and loops. The measured velocities cover a range of 500 km s(-1) around the systemic velocity. The velocity field of the outer H I has the same sense and magnitude (and line of nodes) as that of the stars in the elliptical body. Although NGC 2865 appears to be a relaxed elliptical galaxy, deep images, photometry, and spectroscopy suggest that the galaxy might be the recent (less than 7 Gyr) product of a major disk-disk merger-a ''dynamically young elliptical.'' Our H I data support this hypothesis. Nevertheless, the association between gas and stellar fine structure, with gas displaced outward from the stars in projected position, implies gas motions not predicted by any of the current merger scenarios. Using the H I ring and assuming nearly circular motion, we measure M/L(B) at large radii (4 x 1/2D(25)). We find M/L(B) = 33 +/- 4 h, a factor of 5 greater than the value of M/L(B) found for the central regions, indicating the presence of a dark halo.

Velocity dispersion profiles and rotation curves have been determined for a sample of 13 brightest duster galaxies (BCGs) in order to study their internal stellar kinematics and investigate their relationship to ellipticals. We find that BCGs generally display velocity dispersion profiles with gradients similar to those found in normal bright ellipticals. A rising velocity dispersion profile is found for the BCG IC 1101, the dominant member of Abell 2029, confirming Dressier's (1979) result. No other rising velocity dispersion profiles are found in our sample suggesting that they are not a feature common to brightest duster galaxies. Significant rotation is found in only two of the galaxies, a result in agreement with previous studies showing the declining importance of rotation with increasing luminosity for elliptical galaxies. Our data suggest that environment plays a role in the kinematic characteristics of BCGs. We find a correlation between the rotation of BCGs and the velocity dispersion of their parent clusters in the sense that very slowly rotating galaxies (V-m/(sigma))(not asymptotic to) 650 km s(-1)) clusters. This relation between the internal stellar kinematics of BCGs and the cluster velocity dispersion is in the sense expected if ellipticals formed by merging. Brightest duster galaxy mass-to-light ratios derived from photometric and kinematic modeling are found to be similar to normal elliptical M/L values. For the galaxy NGC 4073, the dominant galaxy in the poor cluster MKW 4, we find that it contains a counterrotating stellar core suggestive of the occurrence of an accretion event(s).

Stars and dark matter account for most of the mass of early-type galaxies, but uncertainties in the stellar population and the dark matter profile make it challenging to distinguish between the two components. Nevertheless, precise observations of stellar and dark matter are extremely valuable for testing the many models of structure formation and evolution. We present a measurement of the stellar mass and inner slope of the dark matter halo of a massive early-type galaxy at z = 0.222. The galaxy is the foreground deflector of the double Einstein ring gravitational lens system SDSSJ0946+ 1006, also known as the "Jackpot." By combining the tools of lensing and dynamics we first constrain the mean slope of the total mass density profile (rho(tot) proportional to r(-gamma)') within the radius of the outer ring to be gamma' = 1.98 +/- 0.02 +/- 0.01. Then we obtain a bulge-halo decomposition, assuming a power-law form for the dark matter halo. Our analysis yields gamma(DM) = 1.7 +/- 0.2 for the inner slope of the dark matter profile, in agreement with theoretical findings on the distribution of dark matter in ellipticals, and a stellar mass from lensing and dynamics M-*(LD) = 5.5(-1.3)(+0.4) x 10(11) M-circle dot. By comparing this measurement with stellar masses inferred from stellar population synthesis fitting we find that a Salpeter initial mass function (IMF) provides a good description of the stellar population of the lens while the probability of the IMF being heavier than Chabrier is 95%. Our data suggest that growth by accretion of small systems from a compact red nugget is a plausible formation scenario for this object.

Differences and similarities of E and SO galaxies have been investigated on basis of new CCD surface photometry in Gunn r for 171 galaxies within the central square degree of the Coma Cluster; 146 of the galaxies are classified as E or SO. The galaxies form a magnitude-limited sample with Gunn r less-than-or-equal-to 15.3 mag. Together with the sample of Virgo galaxies compiled by Caon et al. (1993), this sample is one of the first volume-limited samples. The isophotal shapes of the galaxies have been studied in terms of ellipticity, c4 and c6. The sample is deficit of apparently round SO galaxies. The distribution of the apparent ellipticities for the SO galaxies cannot be fitted satisfactorily with any simple distribution of the intrinsic ellipticities. This suggests that face-on SO galaxies have been classified as ellipticals. It is shown that the ellipticity-c4 diagram can be used for determination of relative disk luminosities, L(D)/L(tot). The distribution of L(D)/L(tot) for galaxies fainter than M(BT) = -22 mag is very broad (H-0 = 50 km s-1 Mpc-1 is used). The distribution is best fitted by a model where 10% of the galaxies are diskless and 90% of the galaxies are drawn from a distribution which is uniform from zero to one. This model also predicts distributions of apparent ellipticities, c4, and inclinations which are in agreement with the data. E galaxies brighter than M(BT) = -22 mag do not show any evidence of disks and may form another class. The sample contains no SO galaxies brighter than M(BT) = -22 mag. Our analysis implies that the E and SO galaxies fainter than M(BT) = -22 mag form one class of galaxies with a continuous change in L(D)/L(tot). Face-on members of this class are mostly classified as E galaxies while more edge-on members are classified as SO galaxies. Further, it is found that only 12% of all the galaxies in the sample are truly diskless systems. The diskless systems contribute almost-equal-to 30% to the total luminosity of the sample, most of this originates from the six brightest galaxies. It is estimated that almost-equal-to 60% of the total luminosity of the sample originates from bulges.