Search

Recent deep Hubble Space Telescope WFC3 imaging suggests that a majority of compact quiescent massive galaxies at z~2 may contain disks. To investigate this claim, we have compared the ellipticity distribution of 31 carefully selected high-redshift massive quiescent compact galaxies to a set of mass-selected ellipticity and Sersic index distributions obtained from 2D structural fits to ~40,000$ nearby galaxies from the Sloan Digital Sky Survey. A Kolmogorov-Smirnov test shows that the distribution of ellipticities for the high-redshift galaxies is consistent with the ellipticity distribution of a similarly chosen sample of massive early-type galaxies. However the distribution of Sersic indices for the high-redshift sample is inconsistent with that of local early-type galaxies, and instead resembles that of local disk-dominated populations. The mismatch between the properties of high-redshift compact galaxies and those of both local early-type and disk-dominated systems leads us to conclude that the basic structures of high-redshift compact galaxies probably do not closely resemble those of any single local galaxy population. Any galaxy population analog to the high-redshift compact galaxies that exists at the current epoch is either a mix of different types of galaxies, or possibly a unique class of objects on their own., Comment: 9 pages, 3 figure, accepted for publication in ApJL

For more than two decades we have known that galaxy morphological segregation is present in the Local Universe. It is important to see how this relation evolves with cosmic time. To investigate how galaxy assembly took place with cosmic time, we explore the evolution of the morphology-density relation up to redshift z~1 using about 10000 galaxies drawn from the zCOSMOS Galaxy Redshift Survey. Taking advantage of accurate HST/ACS morphologies from the COSMOS survey, of the well-characterised zCOSMOS 3D environment, and of a large sample of galaxies with spectroscopic redshift, we want to study here the evolution of the morphology-density relation up to z~1 and its dependence on galaxy luminosity and stellar mass. The multi-wavelength coverage of the field also allows a first study of the galaxy morphological segregation dependence on colour. We further attempt to disentangle between processes that occurred early in the history of the Universe or late in the life of galaxies. The zCOSMOS field benefits of high-resolution imaging in the F814W filter from the Advanced Camera for Survey (ACS). We use standard morphology classifiers, optimised for being robust against band-shifting and surface brightness dimming, and a new, objective, and automated method to convert morphological parameters into early, spiral, and irregular types. We use about 10000 galaxies down to I_AB=22.5 with a spectroscopic sampling rate of 33% to characterise the environment of galaxies up to z~1 from the 100 kpc scales of galaxy groups up to the 100 Mpc scales of the cosmic web. ABRIDGED, Comment: 23 pages, 12 figures, accepted for publication in Astronomy and Astrophysics

A broad continuum excess in the near-infrared, peaking in the rest-frame at 2-5 micron, is detected in a spectroscopic sample of 88 galaxies at 0.5

We outline a strategy to select faint (i<24.5) type 1 AGN candidates down to the Seyfert/QSO boundary for spectroscopic targeting in the COSMOS field, picking candidates by their nonstellar colors in broadband ground-based photometry and morphological properties extracted from HST-ACS. AGN optical color selection has not been applied to such faint magnitudes in such a large continuous part of the sky. Hot stars are known to be the dominant contaminant for bright AGN candidate selection at z<2, but we anticipate the highest color contamination at all redshifts to be from faint starburst and compact galaxies. Morphological selection via the Gini Coefficient separates most potential AGN from these faint blue galaxies. Recent models of the quasar luminosity function are used to estimate quasar surface densities, and studies of stellar populations in the COSMOS field infer stellar contamination. We use 292 spectroscopically confirmed type 1 AGN and quasar templates to predict AGN colors with redshift, and contrast those predictions with the colors of known contaminating populations. The motivation of this study and subsequent spectroscopic follow-up is to populate and refine the faint end of the QLF where the population of type 1 AGN is presently not well known. The anticipated AGN observations will add to the ~300 already known AGN in the COSMOS field, making COSMOS a densely packed field of quasars to be used to understand supermassive black holes and probe the structure of the intergalactic medium in the intervening volume., Comment: 15 pages, 18 figures; accepted for publication in the Astrophysical Journal

We explore the evolution of the morphology density relation using the COSMOS-ACS survey and previous cluster studies. The Gini parameter measured in a Petrosian aperture is found to be an effective way of selecting early-type galaxies free from systematic effects with redshift. We find galaxies are transformed from late (spiral and irregular) to early (E+S0) type galaxies more rapidly in dense than sparse regions. At a given density, the early-type fraction grows constantly with cosmic time, but the growth rate increases with density as a power law of index $0.29\pm0.02$. However, at densities below 100 galaxies per Mpc$^{2}$ no evolution is found at $z>0.4$. In contrast the star-formation-density relation shows strong evolution at all densities and redshifts, suggesting different physical mechanisms are responsible for the morphological and star formation transformation. We show photometric redshifts can measure local galaxy environment, but the present results are limited by photometric redshift error to densities above $\Sigma=3$ galaxies per Mpc$^{2}$., Comment: 12 pages, 13 figures, to appear in the COSMOS special issue of the ApJS

We describe a compact cluster of massive red galaxies at z=1.51 discovered in one of the Gemini Deep Deep Survey (GDDS) fields. Deep imaging with the Near Infrared Camera and Multi Object Spectrometer (NICMOS) on the Hubble Space Telescope reveals a high density of galaxies with red optical to near-IR colors surrounding a galaxy with a spectroscopic redshift of 1.51. Mid-IR imaging with Infrared Array Camera (IRAC) on the Spitzer Space telescope shows that these galaxies have spectral energy distributions that peak between 3.6 and 4.5 microns. Fits to 12-band photometry reveal 12 or more galaxies with spectral shapes consistent with z = 1.51. Most are within ~170 co-moving kpc of the GDDS galaxy. Deep F814W images with the Advanced Camera for Surveys (ACS) on HST reveal that these galaxies are a mix of early-type galaxies, disk galaxies and close pairs. The total stellar mass enclosed within a sphere of 170 kpc in radius is > 8E+11 solar masses. The colors of the most massive galaxies are close to those expected from passive evolution of simple stellar populations (SSP) formed at much higher redshifts. We suggest that several of these galaxies will merge to form a single, very massive galaxy by the present day. This system may represent an example of a short-lived dense group or cluster core typical of the progenitors of massive clusters in the present day and suggests the red sequence was in place in over-dense regions at early times., Comment: 14 pages, 6 figures, submitted to ApJ Letters

We present 39 nights of optical photometry, 34 nights of infrared photometry, and 4 nights of optical spectroscopy of the Type Ia SN 1999ac. This supernova was discovered two weeks before maximum light, and observations were begun shortly thereafter. At early times its spectra resembled the unusual SN 1999aa and were characterized by very high velocities in the Ca II H and K lines, but very low velocities in the Si II 6355 A line. The optical photometry showed a slow rise to peak brightness but, quite peculiarly, was followed by a more rapid decline from maximum. Thus, the B- and V-band light curves cannot be characterized by a single stretch factor. We argue that the best measure of the nature of this object is not the decline rate parameter Delta m_15 (B). The B-V colors were unusual from 30 to 90 days after maximum light in that they evolved to bluer values at a much slower rate than normal Type Ia supernovae. The spectra and bolometric light curve indicate that this event was similar to the spectroscopically peculiar slow decliner SN 1999aa., Comment: 42 pages, 14 figures, accepted for publication in the Astronomical Journal (January 28, 2006)

We examine the cosmic star formation rate (SFR) and its dependence on galaxy stellar mass over the redshift range 0.8 < z < 2 using data from the Gemini Deep Deep Survey (GDDS). The SFR in the most massive galaxies (M > 10^{10.8} M_sun) was six times higher at z = 2 than it is today. It drops steeply from z = 2, reaching the present day value at z ~ 1. In contrast, the SFR density of intermediate mass galaxies (10^{10.2} < M < 10^{10.8} M_sun) declines more slowly and may peak or plateau at z ~ 1.5. We use the characteristic growth time t_SFR = rho_M / rho_SFR to provide evidence of an associated transition in massive galaxies from a burst to a quiescent star formation mode at z ~ 2. Intermediate mass systems transit from burst to quiescent mode at z ~ 1, while the lowest mass objects undergo bursts throughout our redshift range. Our results show unambiguously that the formation era for galaxies was extended and proceeded from high to low mass systems. The most massive galaxies formed most of their stars in the first ~3 Gyr of cosmic history. Intermediate mass objects continued to form their dominant stellar mass for an additional ~2 Gyr, while the lowest mass systems have been forming over the whole cosmic epoch spanned by the GDDS. This view of galaxy formation clearly supports `downsizing' in the SFR where the most massive galaxies form first and galaxy formation proceeds from larger to smaller mass scales., Comment: Accepted for publication in ApJL

In this letter we present the first 3D spectroscopic study of H-alpha emission in a z ~ 1 field galaxy with an integral field unit. Using the CIRPASS spectrograph on Gemini-South we map the spatial and velocity distribution of H-alpha emission in the z=0.819 galaxy CFRS 22.1313. We detect two H-alpha emitting regions with a velocity separation of 220+/-10 km/s. Combining the 2D map of H-alpha emission with HST F814W imaging, we determine a lower limit of 180+/-20 km/s for the rotation velocity of this M_B(rest) ~ -21 galaxy. We note that our value is significantly higher than the rotation velocity of 120+/-10km/s reported by Barden et al. (2003) for their long-slit spectroscopic study of this galaxy. Our lower limit on the rotation velocity is entirely consistent with no evolution of the rest B-band Tully-Fisher relation. The position of this galaxy relative to the mean rest B-band Tully-Fisher relation of Tully & Pierce (2000) is consistent with brightening of no more than ~ 1mag at z=0.8. A larger integral field unit sample, without the uncertainties inherent to long-slit samples, is needed to accurately determine the evolution of the Tully-Fisher relation out to z ~ 1., Comment: Accepted for publication in MNRAS (Letters). 5 pages

The goal of the Gemini Deep Deep Survey (GDDS) is to study an unbiased sample of K<20.6 galaxies in the redshift range 0.81.3. The selected objects have colors typical of irregular and Sbc galaxies. Strong [OII] emission indicates high star formation activity in the HII regions (SFR~13-106 M_sun/yr). The high S/N composite spectrum shows strong ISM MgII and FeII absorption, together with weak MnII and MgI lines. The FeII column density, derived using the curve of growth analysis, is logN_FeII = 15.54^{+0.23}_{-0.13}. This is considerably larger than typical values found in damped Ly-alpha systems (DLAs) along QSO sight lines, where only 10 out of 87 (~11%) have logN_FeII > 15.2. High FeII column densities are observed in the z=2.72 Lyman break galaxy cB58 (logN_FeII ~ 15.25) and in gamma-ray burst host galaxies (logN_FeII ~ 14.8-15.9). Given our measured FeII column density and assuming a moderate iron dust depletion (delta_Fe ~ 1 dex), we derive an optical dust extinction A_V ~ 0.6. If the HI column density is log N(HI)<21.7 (as in 98% of DLAs), then the mean metallicity is Z/Z_sun > 0.2. The high completeness of the GDDS sample implies that these results are typical of star-forming galaxies in the 1

(Abridged) We present rest-frame R-band galaxy luminosity function measurements for three different redshift ranges: 0.5<=z<=0.75, 0.75<=z<=1.0, and 1.0<=z<=1.5. Our measurements are based on photometric redshifts for ~3000 H-band selected galaxies with apparent magnitudes 17<=H<=20 from the Las Campanas Infrared Survey. We show that our photometric redshifts are accurate with an RMS dispersion between the photometric and spectroscopic redshifts of \sigma_z/(1+z)~0.08. Using galaxies identified in the Hubble Deep Field South and Chandra Deep Field South regions, we find, respectively, that (7.3\pm 0.2) % and (16.7\pm 0.4)% of the H<=20 galaxies are at z>=1. We first demonstrate that the systematic uncertainty inherent in the luminosity function measurements due to uncertainties in photometric redshifts is non-negligible and therefore must be accounted for. We then develop a technique to correct for this systematic error by incorporating the redshift error functions of individual galaxies in the luminosity function analysis. The redshift error functions account for the non-gaussian characteristics of photometric redshift uncertainties. They are the products of a convolution between the corresponding redshift likelihood functions of individual galaxies and a Gaussian distribution function that characterizes template-mismatch variance. We demonstrate, based on a Monte Carlo simulation, that we are able to completely recover the bright end of the intrinsic galaxy luminosity function using this technique. Finally, we calculate the luminosity function separately for the total H-band selected sample and for a sub-sample of early-type galaxies that have a best-fit spectral type of E/S0 or Sab from the photometric redshift analysis., Comment: 42 pages and 12 figures; Accepted for publication in the Astrophysical Journal

To quantify the distribution of bar shapes in spiral galaxies, we have analysed 113 H-band and 89 B-band galaxy images from the Ohio State University Bright Spiral Galaxy Survey. Parameters measuring bar shape and position along the Hubble sequence were obtained in each waveband. Evidence was found for a bimodality in the distribution of bar shape, implying that barred and unbarred galaxies are not just the extrema of a single distribution, and that any evolution between these two states must occur on a rapid timescale. Objective bar shapes measured in the H-band were found to be more closely related to visual classifications than B-band bar strengths, as the B-band images are somewhat compromised by localised star formation, especially in later-type systems. Galaxies were found to be more centrally concentrated in the infrared. Later type galaxies showed greater asymmetry in the optical than the infrared, presumably again due to localised star formation, but on average the bar strengths in the two bands were found to be the same., Comment: 6 pages, 5 Figures. in press, 2002 MNRAS

We present results from the Las Campanas Infrared Survey, designed to identify a statistically significant sample of z>=1 galaxies using photometric redshift techniques. Here we summarize the design and strategies of the survey and present the first estimate of the galaxy luminosity function at z>=1 based on H-band selected galaxies identified in our survey. Results of number count studies and luminosity function measurements indicate that most early-type galaxies were already in place by z~1.2 with a modest space density evolution and a mild luminosity evolution over that expected from passive evolution., Comment: 5 pages, 6 figures, using mrs2001.sty; to appear in the Proceedings of the Marseille 2001 Conference: Where's the matter? Tracing dark and and bright matter with the new generation of large scale surveys, Treyer & Tresse, eds

The LCIR Survey, using the Cambridge IR Survey Instrument (CIRSI), reaches H \~ 20-21 over ~1 deg^2. We present results for 744 arcmin^2 in which public UBVRI data exist. We compare optical-IR colours with predictions of a semi-analytic hierarchical model (SAM) and find reasonable agreement. We also determine photometric redshifts, finding a median z of z_m ~ 0.55. We compare N(z) of different spectral types with models, showing that the observations are inconsistent with simple PLE models while the SAM provides a reasonable fit to the total N(z) but underestimates the number of z ~ 1 red spectral types. We also present N(z) for samples of red objects (EROs). We find that EROs with R - H > 4 and H < 20.5 have z_m ~ 1; redder EROs have higher z_m. For 19 < H < 20, EROs with R - H > 4 comprise ~18% of the observed galaxy population while in the SAM they contribute only ~4%. We also determine the angular correlation function w(theta) for magnitude, colour, spectral type and photo-z selected samples and use the estimated N(z) to derive the spatial clustering xi(r). Parametrizing xi(r) by xi(r_c,z)=(r_c/r_*(z))^(-1.8) (r_c comoving), we find that r_*(z) increases by ~1.5-2 times from z = 0 to z ~ 1.2. We interpret this as a selection effect - the galaxies selected at z ~ 1.2 are intrinsically very luminous. When galaxies are selected by absolute magnitude we find no evidence for evolution in r_* over this z range. Extrapolated to z = 0, we find r_*(0) ~ 6.5 h^-1Mpc for red galaxies and r_*(0) ~ 2-4 h^-1Mpc for blue galaxies. We also find that while the angular clustering amplitude of EROs with R - H > 4 or I - H > 3 is up to four times that of the whole galaxy population, the spatial clustering length r_*(z=1) is ~7.5-10.5 h^-1Mpc which is only a factor of ~1.7 times r_* for non-EROs lying in a similar z and luminosity range., Comment: Accepted by MNRAS, 32 pages, 39 figures, 9 tables; accepted version - typos corrected, 2 new figures

(Abridged) The Las Campanas Infrared Survey, based on broad-band optical and near-infrared photometry, is designed to robustly identify a statistically significant and representative sample of evolved galaxies at redshifts z>1. We have completed an H-band imaging survey over 1.1 square degrees of sky in six separate fields. The average 5 sigma detection limit in a four arcsecond diameter aperture is H ~ 20. Here we describe the design of the survey, the observation strategies, data reduction techniques, and object identification procedures. We present sample near-infrared and optical photometric catalogs for objects identified in two survey fields. We perform object detection in all bandpasses and identify ~ 54,000 galaxies over 1,408 square arcminutes of sky in the two fields. Of these galaxies, ~ 14,000 are detected in the H-band and ~ 2,000 have the colors of evolved galaxies, I - H >3, at z > 1. We find that (1) the differential number counts N(m) for the H-band detected objects has a slope of 0.44 at H < 19 and 0.27 for H > 19. In addition, we find that (2) the differential number counts for the H detected red objects has a slope of 0.85 at H < 20 and 0.32 at H > 20, with a mean surface density ~ 3,000 degree^{-2} mag^{-1} at H=20. Finally, we find that (3) galaxies with red optical to near-IR colors (I-H > 3) constitute ~ 20% of the H detected galaxies at H ~ 21, but only 2% at H = 19. We show that red galaxies are strongly clustered, which results in a strong field to field variation in their surface density. Comparisons of observations and predictions based on various formation scenarios indicate that these red galaxies are consistent with mildly evolving early-type galaxies at z ~ 1, although with a significant amount of on-going star formation as indicated by the large scatter in their V-I colors., Comment: 48 pages, 13 figures, accepted for publication in the Astrophysical Journal

(Abridged) We have identified a population of faint red galaxies from a 0.62 square degree region of the Las Campanas Infrared Survey whose properties are consistent with their being the progenitors of early-type galaxies. The optical and IR colors, number-magnitude relation and angular clustering together indicate modest evolution and increased star formation rates among the early-type field population at redshifts between one and two. The counts of red galaxies with $H$ magnitudes between 17 and 20 rise with a slope that is much steeper than that of the total H sample. The surface density of red galaxies drops from roughly 3000 per square degree at H = 20.5, I-H > 3 to ~ 20 per square degree at H = 20, I-H > 5. The V-I colors are approximately 1.5 magnitudes bluer on average than a pure old population and span a range of more than three magnitudes. The colors, and photometric redshifts derived from them, indicate that the red galaxies have redshift distributions adequately described by Gaussians with sigma_z ~ 0.2$ centered near redshift one, with the exception that galaxies having $V-I<1.6$ and $I-H>3$ are primarily in the 1.5 < z < 2 range. We find co-moving correlation lengths of 9-10 Mpc at z ~ 1, comparable to, or larger than, those found for early-type galaxies at lower redshifts. A simple photometric evolution model reproduces the counts of the red galaxies, with only a ~ 30% decline in the underlying space density of early-type galaxies at z ~ 1.2. We suggest on the basis of the colors, counts, and clustering that these red galaxies are the bulk of the progenitors of present day early-type galaxies., Comment: 5 pages, 3 figures, accepted for publication in the ApJ Letters

Establishing the morphological history of ordinary galaxies was one of the original goals for the Hubble Space Telescope, and remarkable progress toward achieving this this goal has been made. How much of this progress has been at the expense of the Hubble sequence? As we probe further out in redshift space, it seems time to re-examine the underlying significance of Hubble's tuning fork in light of the the spectacular and often bizarre morphological characteristics of high redshift galaxies. The aim of this review is to build a morphological bridge between high-redshift and low-redshift galaxy populations, by using quantitative morphological measures to determine the maximum redshift for which the Hubble sequence provides a meaningful description of the galaxy population. I will outline the various techniques used to quantify high-redshift galaxy morphology, highlight the aspects of the Hubble sequence being probed by these techniques, and indicate what is getting left behind. I will argue that at higher redshifts new techniques (and new ideas) that place less emphasis on classical morphology and more emphasis on the link between morphology and resolved stellar populations are needed in order to probe the evolutionary history of high-redshift galaxies., Comment: To appear in "Toward a New Millennium in Galaxy Morphology", eds. D. L. Block, I. Puerari, A. Stockton and D. Ferreira (Kluwer, Dordrecht)

(Abridged) We analyse the morphology, kinematics and ionization structure of the extended emission-line regions in four ULIRGs at 0.1181000 km/s within 1" of the nucleus; an isolated blob ~8kpc from the nucleus with a much lower FWHM may lie in a photoionization cone. Line ratios suggest AGN-photoionization for the systemic gas, and shock+precursor ionization for a second, spatially-unresolved, nuclear component blueshifted by 990 km/s. The gas in IRAS F23060+0505 is in dipolar rotation but with an asymmetric rotation curve and an increased FWHM on one side; an asymmetry to the narrow-line profiles is attributed to a spatially-resolved outflow, and shock+precursor ionization is dominant. At lower luminosity, IRAS F01217+0122 has a featureless velocity field with a high FWHM, an AGN-like emission-line spectrum and a ~1 Gyr old stellar population. IRAS F01003-2238 has a dipolar velocity field and an HII-region emission-line spectrum with a strong blue continuum which can be reproduced with ~10^7 O5 stars, sufficient to account for the bolometric luminosity of the entire galaxy. This diversity is accommodated within the merger-driven evolutionary scenario for ULIRGs: the merger status is assessed from the kinematics in a way which is consistent with the colour and morphology of the galaxy, or with the inferred age of the stellar population and the dominance of the AGN., Comment: 23 pages, 25 figures (7 in gif format); to appear in MNRAS

The frequency of barred spiral galaxies as a function of redshift contains important information on the gravitational influence of stellar disks in their dark matter halos and also may distinguish between contemporary theories for the origin of galactic bulges. In this paper we present a new quantitative method for determining the strength of barred spiral structure, and verify its robustness to redshift-dependent effects. By combining galaxy samples from the Hubble Deep Field North with newly available data from the Hubble Deep Field South, we are able to define a statistical sample of 18 objectively-defined low-inclination barred spiral systems with $I_{814W}<23.2$ mag. Analysing the proportion of barred spiral galaxies seen as a function of redshift, we find a significant decline in the barred fraction beyond redshifts $z\simeq 0.5$. The physical significance of this effect remains unclear, but several possibilities include dynamically hotter (or increasingly dark-matter dominated) high-redshift discs, or an enhanced efficiency in bar destruction at high redshifts. By investigating the formation of the ``orthogonal'' axis of Hubble's classification tuning fork, our result complements studies of evolution in the early--late sequence, and pushes to later epochs the redshift at which the Hubble classification sequence is observed to be in place., Comment: Submitted to MNRAS. 8 pages. LaTeX MNRAS format. The colour montage embedded in the paper is also available as a jpeg image

In hierarchical models, where spheroidals are primarily produced via the merger of disk galaxies, the number of intrinsically red systems at faint limits will be substantially lower than that expected in "traditional" models where the bulk of star formation was completed at high redshifts. We analyse the optical--near-infrared colour distribution of a large flux-limited sample of field spheroidal galaxies identified morphologically from archival Hubble Space Telescope data. The $I_{814}-HK'$ colour distribution for a sample jointly limited at $I_{814}<$23 mag and $HK'<$19.5 mag is used to constrain the star formation history of elliptical and S0 galaxies. We detect a significant deficit of intrinsically red spheroids. From limited spectroscopic and deeper photometric redshift data, we demonstrate that this deficit results from a strong evolutionary effect. The space density of spheroidals of all colours at faint limits is lower than expected for a population of constant comoving density. We conclude that field ellipticals cannot have formed all of their stars at high redshift, as has been suggested for the bulk of their clustered counterparts. We compare alternative evolutionary histories with our data, including an analytical representation of recent models based on the hierarchical assembly of ellipticals, which provides a better, though not ideal fit. The accumulation of spectroscopic redshifts for our sample will allow a quantitative discrimination between models with extended star formation., Comment: 16 pages, 12 figures, accepted for publication in MNRAS. Substantially modified version to take into account referee's comments

In the first section of these lectures I outline the classical framework of the Hubble classification system. Because of space limitations I will focus on points of controversy concerning the physical interpretation of the Hubble sequence, showing how morphological ideas shape our understanding of galaxy evolution. I will then present an overview of the remarkable progress made in recent years in understanding how the local morphological composition has transformed into that seen the distant Universe, highlighting work from the Hubble Deep Field (HDF). Recent studies show quite clearly that the Hubble system does not provide a useful framework for describing the appearance of galaxies at redshifts z >1.5. I argue that as a result of this work the Hubble system needs to be replaced by a system that is more objective, more physically meaningful, and which is applicable across a wider range of redshifts., Comment: Invited lectures on galaxy morphology given at the Les Houches late-summer school ("Galaxy formation and evolution"). LaTeX source uses l-school.sty file. 19 pages, including 6 figures

We analyse the spatially resolved colours of distant galaxies of known redshift in the Hubble Deep Field, using a new technique based on matching resolved four-band internal colour data to the predictions of evolutionary synthesis models. We quantify the relative age, dispersion in age, ongoing star-formation rate, star-formation history, and dust content of these galaxies. To demonstrate the potential of the method, we study the near-complete sample of 32 I<21.9 mag galaxies with ~ 0.5 studied by Bouwens et al (1997). The dispersion of the internal colours of a sample of 0.4

We discuss the galaxy population of the rich cluster MS1621.5+2640 at z=0.4274, based on spectra and imaging in a field of size 9 arcmin by 23 arcmin (~2 by 5 h**-1 Mpc). The sample comprises 277 galaxies, of which 112 are cluster members, 7 are `near-members', and 47 are field galaxies in the redshift range 0.37<=z<=0.50. The results are analyzed and compared with the z=0.2279 rich cluster Abell 2390. MS1621.5+2640 has a higher blue fraction, a younger stellar population, and is a less evolved cluster. We do not find strong evidence of significant excess star formation compared with the field, although there is a small population of outlying near-members that is unusually blue and that may be affected by the cluster. There is a substantial population of red galaxies with significant Hdelta absorption, which are not easily explained by any simple form of modeled star formation history. We detect two distinct populations of cluster galaxies: those where star formation ceased some time ago, and those with a gradual decrease over many Gyr. Our data suggests that the cluster formed by accretion from the field, with truncation of the star formation beginning at very large radii (>2 times the virial radius). The truncation process does not seem to be a sharp one though, in that lower-luminosity early-type galaxies in the inner core of the cluster are seen with significant Hdelta absorption, indicating some star formation within the last 1-2 Gyr. Some combination of stripping of gas from the outer parts of the galaxy, together with gradual exhaustion of the gas in the inner parts would be consistent with our data., Comment: 46 pages, 20 figures. Accepted for publication in ApJ. Full text and figures available at http://www.hia.nrc.ca/STAFF/slm/ms1621/

The current evidence for morphologically peculiar galaxy populations at high-redshifts is outlined. After describing various techniques which can be used to quantify the importance of ``morphological K-corrections'', and to objectively classify galaxy morphology in the presence of these biases, it is concluded that observational biases are not sufficient to explain the increase in the fraction of peculiar galaxies on deep HST images. A new technique is then described which models the spatially resolved internal colors of high redshift galaxies, as a probe of the processes driving galaxy evolution. This ``morphophotometric'' approach investigates directly the evolutionary history of stellar populations, and is a sensitive test of the mechanisms through which galaxies build up and evolve in the field. As a case study, we analyse several ``chain galaxies'' in the Hubble Deep Field. These chain galaxies are shown to be protogalaxies undergoing their first significant episodes of star-formation, and not simply distant edge-on spirals., Comment: Invited review given at "The Ultraviolet Universe at Low and High Redshifts", College Park, Maryland, 1997

Evolution in the merger rate as a function of redshift is in principle the key observable testing hierarchical models for the formation and evolution of galaxies. However, in practice, direct measurement of this quantity has proven difficult. In this opening review I outline the current best estimates for the merger rate as a function of cosmic epoch, focussing mostly upon recent advances made possible by deep ground-based redshift surveys and morphological studies undertaken with HST. I argue that a marriage of these techniques, in an attempt to determine the space density of mergers amongst the abundant morphologically peculiar population at high redshifts, is probably the most promising currently-available avenue for determining the prevalence of mergers at high redshifts. However, resolved kinematical studies, which seem set to become available in the next few years, are probably the best hope for a definitive determination of the space density of mergers at high redshifts., Comment: Invited review given at IAU symposium 186 "Galaxy Interactions at Low and High Redshifts" (Kyoto). LaTeX source, style file, and 6 figures. Four figures are embedded in the processed LaTeX file, while two big images are supplied as jpeg files

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was made with the ROSAT PSPC and reaches a flux limit of 1.6x10^-15 erg cm^-2 s^-1 (0.5--2.0 keV). Above a flux limit of 2x10^-15 erg cm^-2 s^-1 we define a complete sample of 70 sources of which 59 are identified. Some (5) other sources have tentative identifications and in a further 4 the X-ray error-boxes are blank to R=23 mag. At the brighter flux levels (>= 10^-14 erg cm^-2 s^-1) we confirm the results of previous less deep X-ray surveys with 84% of the sources begin QSOs. At fainter fluxes, however, the survey is dominated by a population of galaxies with narrow optical emission lines (NELGs). In addition, a number of groups and clusters of galaxies are found at intermediate fluxes. Most of these are poor systems of low X-ray luminosity and are generally found at redshifts of > 0.3. Their numbers are consistent with a zero evolutionary scenario, in contrast to the situation for high luminosity clusters at the same redshift. We discuss the significance of these results to the determination of the cosmic soft X-ray background (XRB) and show that at 2x10^-15 erg cm^-2 s^-1, we have resolved more than 50% of the background. We also briefly consider the probable importance of NELG objects to the residual background and look at some of the properties of these unusual objects., Comment: 28 pages, LaTeX, 17 figures from 32 postscript files. Uses mn.sty and epsf.sty. Accepted by MNRAS. Revised version now contains data table. For more information see http://www.astro.soton.ac.uk/~amn/UKdeep

In the deepest optically identified X-ray survey yet performed, we have identified 32 X-ray selected QSOs to a flux limit of 2x10^{-15} erg cm^{-2} s^{-1} (0.5-2 keV). The survey, performed with the ROSAT PSPC, has 89% spectroscopic completeness. The QSO log(N)-log(S) relation is found to have a break to a flat slope at faint fluxes. The surface density of QSOs at the survey limit is 230+/-40 per square degree, the largest so far of any QSO survey. We have used this survey to measure the QSO X-ray luminosity function at low luminosities (Lx<10^{44.5} erg s^{-1}) and high redshifts (1

The galaxy population in the intermediate-redshift ($z=0.228$) rich cluster Abell 2390 is investigated. We present velocities, colors, and morphological information for an exceptionally large sample of 323 galaxies (216 cluster members) in a 46$^\prime \times 7^\prime$ (6 $h^{-1}$ Mpc $\times$ 1 $h^{-1}$ Mpc) strip centered on the cD galaxy. This sample of confirmed cluster members is second only to that for the Coma cluster in terms of sample size and spatial coverage in the cluster rest frame, and is the first to trace the transition between a rich cluster and the field at intermediate redshift. The galaxy population in the cluster changes gradually from a very evolved, early-type population in the inner 0.4 \hmpc\ of the cluster to a progressively later-type population in the extensive outer envelope of the cluster from 1 to 3 \hmpc\/ in radius. Radial gradients in galaxy $g-r$ color, 4000 \AA\/ break, H$\delta$ and [O II] line strengths and morphology are seen in the cluster, and are investigated by comparing the data to models computed with the GISSEL spectral synthesis package. The results suggest that the cluster has been gradually built up by the infall of field galaxies over $\sim 8$ Gyr and that star formation has been truncated in infalling galaxies during the accretion process. The morphological composition of the cluster is shown to be consistent with such a scenario. If true for other clusters, infall-truncated star formation as seen in Abell 2390 may explain both the Butcher-Oemler effect and the large fraction of S0 galaxies in clusters. Only $\simlt$5\% of the galaxies observed in Abell 2390 exhibit evidence for star formation at levels stronger than those seen in typical late-type systems. This suggests that starbursts do not play a major role in driving cluster galaxy evolution at the redshift of, Comment: AAS LaTeX source (a2390.tex), 19 Encapsulated PostScript figures (Figure_*.tex), and 3 plano tables (Table_*.tex). Two figures (Figures 2 and 10) are large and have been reduced to low-resolution bitmaps to conserve bandwidth. A copy of the paper (1.5 MB gzipped tar file) with full-size figures is available at ftp://ftp.ast.cam.ac.uk/pub/abraham/a2390_accepted.tar.gz

The morphological properties of galaxies between $21 {\rm~mag} < I < 25 {\rm~mag}$ in the {\em Hubble Deep Field} are investigated using a quantitative classification system based on measurements of the central concentration and asymmetry of galaxian light. The class distribution of objects in the {\em Hubble Deep Field} is strongly skewed towards highly asymmetric objects, relative to distributions from both the {\em HST Medium Deep Survey} at $I < 22 {\rm~mag}$ and an artificially redshifted sample of local galaxies. The steeply rising number count-magnitude relation for irregular/peculiar/merging systems at $I < 22 {~\rm mag}$ reported in Glazebrook \etal\ (1995a) continues to at least $I=25~{\rm mag}$. Although these peculiar systems are predominantly blue at optical wavelengths, a significant fraction also exhibit red $U-B$ colours, which may indicate they are at high redshift. Beyond Glazebrook \etal 's magnitude limit the spiral counts appear to rise more steeply than high-normalization no-evolution predictions, whereas those of elliptical/S0 galaxies only slightly exceed such predictions and may turn-over beyond $I \sim 24~{\rm mag}$. These results are compared with those from previous investigations of faint galaxy morphology with HST and the possible implications are briefly discussed. The large fraction of peculiar/irregular/merging systems in the {\em Hubble Deep Field} suggests that by $I\sim 25 {\rm~mag}$ the conventional Hubble system no longer provides an adequate description of the morphological characteristics of a high fraction of field galaxies., Comment: 6 pages LaTeX. 4 EPS Figures. 2 Colour PS Plates. Accepted for publication in MNRAS

We have begun a program to discover high-redshift supernovae ($z \approx$ 0.25--0.5), and study them with follow-up photometry and spectroscopy. We report here our first discovery, a supernova at $z = 0.458$. The photometry for this supernova closely matches the lightcurve calculated for this redshift from the template of well-observed nearby Type Ia supernovae. We discuss the measurement of the deceleration parameter $q_0$ using such high-redshift supernovae, and give the best fit value assuming this one supernova is a normal, unextincted Type Ia. We describe the main sources of error in such a measurement of $q_0$, and ways to reduce these errors., Comment: Latex using aasms.sty and three postcript files uuencoded compressed. Ap.J.Lett., 440:L41, 1995 (posted for easy access). Complete postscript version also available at http://www-supernova.lbl.gov/public/index.html

Some 32 quasi-stellar objects (QSOs) were identified to a flux limit of 2 x 10(exp -15) erg/sq cm in the deepest optically identified X-ray survey performed to date. This Rosat position sensitive proportional counter survey (PSPC) is spectroscopically 86 percent complete. The surface density of quasars at this flux limit is 210 +/- 37 per sq deg. The QSO X-ray luminosity function was measured at low luminosities and high redshifts, the highest of which was 3.4.

Observations of the X-ray-selected BL Lac H0414 + 009 with the VLA indicate a steep-spectrum radio tail extending about 40 arcsec from the core. Deep V, R, and I CCD images show this object to be located in a galaxy whose colors and surface brightness are fitted by an elliptical model. The BL Lac and the cluster of galaxies are physically associated, indicating that BL Lacs do not avoid the cluster environment but are found in groups and clusters as often as are normal radio galaxies; this supports the contention that BL Lacs are normal radio galaxies seen 'end on'.