Fraction Of Ellipticals Research Articles

Clustercentric Distance or Local Density? It Depends on Galaxy Morphology

Determining which between projected local density and distance from the cluster center plays a major role in regulating morphological fractions in clusters is a longstanding debate. Reaching a definitive answer will shed light on the main physical mechanisms at play in the most extreme environments. Here we make use of the data from the OmegaWINGS survey, currently the largest survey of clusters in the local universe extending beyond 2 virial radii from the cluster cores, to extend the previous analysis outside the virial radius. Local density and clustercentric distance seems to play different roles for galaxies of different morphology: the fraction of elliptical galaxies mainly depends on local density, suggesting that their formation was linked to the primordial densities, which now correspond to the cluster cores. Only the fraction of low-mass ellipticals shows an anticorrelation with clustercentric distance, suggesting a different origin for these objects. Excluding elliptical galaxies, the relative fraction of S0s and spirals instead depends on local density only far from the cluster cores, while within the virial radius their proportion is regulated by distance, suggesting that cluster-specific processes halt the star formation and transform Sp galaxies into S0s. This interpretation is supported by literature results on the kinematical analysis of early- and late-type galaxies, according to which fast and slow rotators have distinct dependencies on halo mass and local density.

A deep near-infrared view of the Ophiuchus galaxy cluster

Context. The Ophiuchus cluster of galaxies, located at low latitudes in the direction of the Galactic bulge, has been relatively poorly studied in comparison with other rich galaxy clusters, such as Coma, Virgo, and Fornax, despite being the second brightest X-ray cluster in the sky. Aims. Our aim is perform a study of the hidden galaxy population of the massive cluster Ophiuchus located in the Zone of Avoidance. Methods. Deep near-infrared images and photometry from the VISTA Variables in the Vía Láctea eXtended (VVVX) survey were used to detect galaxy member candidates of the Ophiuchus cluster up to 2 Mpc from the cD galaxy 2MASX J17122774-2322108 using criteria from a past paper to select the galaxies among the foreground sources. We also perform a morphological visual classification and generate color-magnitude diagrams and density profiles. Results. We identify 537 candidate galaxy members of the Ophiuchus cluster up to 2 Mpc from the cD galaxy, increasing by a factor of seven the number of reported Ophiuchus galaxies. In addition, we performed a morphological classification of these galaxy candidates finding that the fraction of ellipticals reaches more than 60% in the central region of the cluster. On the other hand, the fraction of spirals fraction is lower than 20%, remaining almost constant throughout the cluster. Moreover, we study the red sequence of galaxy member candidates and use mock catalogs to explore the density profile of the cluster, finding that the value derived from the mock catalog toward an overdense region is in agreement with the galaxy excess of the central zone of the Ophiuchus cluster. Conclusions. Our investigation of the hidden population of Ophiuchus galaxies underscores the importance of this cluster as a prime target for future photometric and spectroscopic studies. Moreover the results of this work highlight the potential of the VVVX survey to study extragalactic objects in the Zone of Avoidance.

Predicting fully self-consistent satellite richness, galaxy growth and starformation rates from the STastical sEmi-Empirical modeL steel.

Abstract Observational systematics complicate comparisons with theoretical models limiting understanding of galaxy evolution. In particular, different empirical determinations of the stellar mass function imply distinct mappings between the galaxy and halo masses, leading to diverse galaxy evolutionary tracks. Using our state-of-the-art STatistical sEmi-Empirical modeL, steel, we show fully self-consistent models capable of generating galaxy growth histories that simultaneously and closely agree with the latest data on satellite richness and star-formation rates at multiple redshifts and environments. Central galaxy histories are generated using the central halo mass tracks from state-of-the-art statistical dark matter accretion histories coupled to abundance matching routines. We show that too flat high-mass slopes in the input stellar-mass-halo-mass relations as predicted by previous works, imply non-physical stellar mass growth histories weaker than those implied by satellite accretion alone. Our best-fit models reproduce the satellite distributions at the largest masses and highest redshifts probed, the latest data on star formation rates and its bi-modality in the local Universe, and the correct fraction of ellipticals. Our results are important to predict robust and self-consistent stellar-mass-halo-mass relations and to generate reliable galaxy mock catalogues for the next generations of extra-galactic surveys such as Euclid and LSST.

Galaxy and Mass Assembly (GAMA): formation and growth of elliptical galaxies in the group environment

There are many proposed mechanisms driving the morphological transformation of disk galaxies to elliptical galaxies. In this paper, we determine if the observed transformation in low mass groups can be explained by the merger histories of galaxies. We measured the group mass-morphology relation for groups from the Galaxy and Mass Assembly group catalogue with masses from 10$^{11}$ - 10$^{15}$ M$_{\odot}$. Contrary to previous studies, the fraction of elliptical galaxies in our more complete group sample increases significantly with group mass across the full range of group mass. The elliptical fraction increases at a rate of 0.163$\pm$0.012 per dex of group mass for groups more massive than 10$^{12.5}$ M$_{\odot}$. If we allow for uncertainties in the observed group masses, our results are consistent with a continuous increase in elliptical fraction from group masses as low as 10$^{11}$M$_{\odot}$. We tested if this observed relation is consistent with merger activity using a GADGET-2 dark matter simulation of the galaxy groups. We specified that a simulated galaxy would be transformed to an elliptical morphology either if it experienced a major merger or if its cumulative mass gained from minor mergers exceeded 30 per cent of its final mass. We then calculated a group mass-morphology relation for the simulations. The position and slope of the simulated relation were consistent with the observational relation, with a gradient of 0.184$\pm$0.010 per dex of group mass. These results demonstrate a strong correlation between the frequency of merger events and disk-to-elliptical galaxy transformation in galaxy group environments.

Redshift and luminosity evolution of the intrinsic alignments of galaxies in Horizon-AGN

Intrinsic galaxy shape and angular momentum alignments can arise in cosmological large-scale structure due to tidal interactions or galaxy formation processes. Cosmological hydrodynamical simulations have recently come of age as a tool to study these alignments and their contamination to weak gravitational lensing. We probe the redshift and luminosity evolution of intrinsic alignments in Horizon-AGN between z=0 and z=3 for galaxies with an r-band absolute magnitude of <-20. Alignments transition from being radial at low redshifts and high luminosities, dominated by the contribution of ellipticals, to being tangential at high redshift and low luminosities, where discs dominate the signal. This cannot be explained by the evolution of the fraction of ellipticals and discs alone: intrinsic evolution in the amplitude of alignments is necessary. The alignment amplitude of elliptical galaxies alone is smaller in amplitude by a factor of ~2, but has similar luminosity and redshift evolution as in current observations and in the nonlinear tidal alignment model at projected separations of > 1 Mpc. Alignments of discs are null in projection and consistent with current low redshift observations. The combination of the two populations yields an overall amplitude a factor of ~4 lower than observed alignments of luminous red galaxies with a steeper luminosity dependence. The restriction on accurate galaxy shapes implies that the galaxy population in the simulation is complete only to an r-band absolute magnitude of <-20. Higher resolution simulations will be necessary to avoid extrapolation of the intrinsic alignment predictions to the range of luminosities probed by future surveys.

Revisiting the original morphology–density relation

In light of recent ndings from the kinematic morphology-density relation, we investigate whether the same trends exist in the original morphology density relation, using the same data as Dressler. In addition to Dressler’s canonical relations, we nd that further renements are possible when considering the average local projected density of galaxies in a cluster. Firstly, the distribution of ellipticals in a cluster depends on the relative local density of galaxies in that cluster: equivalent rises in the elliptical fraction occur at higher local densities for clusters with higher average local densities. This is not true for the late-type fraction, where the variation with local density within a cluster is independent of the average local density of galaxies in that cluster, and is as Dressler originally found. Furthermore, the overall ratio of ellipticals to early-types in a cluster does not depend on the average density of galaxies in that cluster (unlike the ratio of lenticulars to disk systems), and is xed at around 30%. In the paradigm of fast and slow rotators, we show that such an elliptical fraction in the early-type population is consistent with a slow rotator fraction of 15% in the early-type population, using the statistics of the ATLAS 3D survey. We also nd the scatter in the overall ratio of ellipticals to early-types is greatest for clusters with lower average densities, such that clusters with the highest elliptical fractions have the lowest average local densities. Finally, we show that average local projected density correlates well with global projected density, but the latter has diculty in accurately characterising the density of irregular cluster morphologies.

The hierarchical origins of observed galaxy morphology

Galaxies grow primarily via accretion-driven star formation in discs and merger-driven growth of bulges. These processes are implicit in semi-analytical models of galaxy formation, with bulge growth in particular relating directly to the hierarchical build-up of halos and their galaxies. In this paper, we consider several implementations of two semi-analytical models. Focusing on implementations in which bulges are formed during mergers only, we examine the fractions of elliptical galaxies and both passive and star-forming disk galaxies as functions of stellar and halo mass, for central and satellite systems. This is compared to an observational cross-matched SDSS+RC3 z ~ 0 sample of galaxies with accurate visual morphological classifications and M_{stellar} > 10^10.5 M_{sol}. The models qualitatively reproduce the observed increase of elliptical fraction with stellar mass, and with halo mass for central galaxies, supporting the idea that observed ellipticals form during major mergers. However, the overall elliptical fraction produced by the models is much too high compared with the z ~ 0 data. Since the "passive" -- i.e. non-star-forming -- fractions are approximately reproduced, and since the fraction which are star-forming disc galaxies is also reproduced, the problem is that the models overproduce ellipticals at the expense of passive S0 and spiral galaxies. Bulge-growth implementations (tuned to reproduce simulations) which allow the survival of residual discs in major mergers still destroy too much of the disc. Increasing the lifetime of satellites, or allowing significant disc regrowth around merger remnants, merely increases the fraction of star-forming disc galaxies. Instead, it seems necessary to reduce the mass ratios of merging galaxies, so that most mergers produce modest bulge growth in disc-galaxy remnants instead of ellipticals. [Abridged]

What determines the fraction of elliptical galaxies in clusters?

We study the correlation between the morphological mix of cluster galaxies and the assembly history of the parent cluster by taking advantage of two independently developed semi-analytic models for galaxy formation and evolution. In our models, both the number of cluster members and that of elliptical members increase as a function of cluster mass, in such a way that the resulting elliptical fractions are approximately independent of cluster mass. The population of cluster ellipticals exhibits a marked bimodal distribution as a function of galaxy stellar mass, with a dip at masses ∼1010 M⊙. In the framework of our models, this bimodality originates from the combination of a strongly decreasing number of galaxies with increasing stellar mass, and a correspondingly increasing probability of experiencing major mergers. We show that the correlation between the measured elliptical fraction and the assembly history of the parent cluster is weak, and that it becomes stronger in models that adopt longer galaxy merger times. We argue that this results from the combined effect of a decreasing bulge production due to a reduced number of mergers, and an increasing survival probability of pre-existing ellipticals, with the latter process being more important than the former.

Are luminous radio-loud active galactic nuclei triggered by galaxy interactions?

We present the results of a comparison between the optical morphologies of a complete sample of 46 southern 2Jy radio galaxies at intermediate redshifts (0.05<z<0.7) and those of two control samples of quiescent early-type galaxies: 55 ellipticals at redshifts z<0.01 from the Observations of Bright Ellipticals at Yale (OBEY) survey, and 107 early-type galaxies at redshifts 0.2<z<0.7 in the Extended Groth Strip (EGS). Based on these comparisons, we discuss the role of galaxy interactions in the triggering of powerful radio galaxies (PRGs). We find that a significant fraction of quiescent ellipticals at low and intermediate redshifts show evidence for disturbed morphologies at relatively high surface brightness levels, which are likely the result of past or on-going galaxy interactions. However, the morphological features detected in the galaxy hosts of the PRGs (e.g. tidal tails, shells, bridges, etc.) are up to 2 magnitudes brighter than those present in their quiescent counterparts. Indeed, if we consider the same surface brightness limits, the fraction of disturbed morphologies is considerably smaller in the quiescent population (53% at z<0.2 and 48% at 0.2<z<0.7) than in the PRGs (93% at z<0.2 and 95% at 0.2<z<0.7 considering strong-line radio galaxies only). This supports a scenario in which PRGs represent a fleeting active phase of a subset of the elliptical galaxies that have recently undergone mergers/interactions. However, we demonstrate that only a small proportion (<20%) of disturbed early-type galaxies are capable of hosting powerful radio sources.

The evolution of early-type galaxies in clusters from z∼ 0.8 to z ∼ 0: the ellipticity distribution and the morphological mix

We present the ellipticity distribution and its evolution for early-type galaxies in clusters from z~0.8 to z~0, based on the WIde-field Nearby Galaxy-cluster Survey (WINGS)(0.04<z<0.07), and the ESO Distant Cluster Survey (EDisCS)(0.4<z<0.8). We first investigate a mass limited sample and we find that, above a fixed mass limit, the ellipticity distribution of early-types noticeably evolves with redshift. In the local Universe there are proportionally more galaxies with higher ellipticity, hence flatter, than in distant clusters. This evolution is due partly to the change of the mass distribution and mainly to the change of the morphological mix with z (among the early types, the fraction of ellipticals goes from ~70% at high to ~40% at low-z). Analyzing separately the ellipticity distribution of the different morphological types, we find no evolution both for ellipticals and S0s. However, for ellipticals a change with redshift in the median value of the distributions is detected. This is due to a larger population of very round (e<0.05) elliptical galaxies at low-z. To compare our finding to previous studies, we also assemble a magnitude-"delimited" sample that consists of early-type galaxies on the red sequence with -19.3>M_B+1.208z>-21. Analyzing this sample, we do not recover exactly the same results of the mass-limited sample. Hence the selection criteria are crucial to characterize the galaxy properties: the choice of the magnitude-delimited sample implies the loss of many less massive galaxies and so it biases the final results. Moreover, although we are adopting the same selection criteria, our results in the magnitude-delimited sample are also not in agreement with those of Holden et al.(2009). This is due to the fact that our and their low-z samples have a different magnitude distribution because the Holden et al.(2009) sample suffers from incompleteness at faint magnitudes.

BVRISURFACE PHOTOMETRY OF ISOLATED GALAXY TRIPLETS

Optical broadband BVRI observations of 54 galaxies selected from the Catalog of Isolated Triplets of Galaxies in the Northern Hemisphere have been carried out at San Pedro Martir National Observatory to evaluate their photometric and morphological properties. We complement our analysis with Two-Micron All Sky Survey (2MASS) and Sloan Digital Sky Survey (SDSS) images and look for signatures likely related to interactions/mergers. We report apparent/absolute BVRI magnitudes and colors for the 54 galaxies. The membership of these galaxies is re-evaluated by imposing a reasonable condition of concordant redshifts upon the original selection criteria, rendering a final sample of 34 galaxies in 13 triplets, 12 galaxies in close pairs, and 8 galaxy outliers. The triplets are spiral-dominated systems in different dynamical stages from loosely interacting to almost merged objects. The incidence fraction of features likely associated with interactions is {approx}56%, similar to those found in northern and southern compact groups. The average fraction of bars is 35% with a mean value of maximum bar ellipticity {epsilon}{sub max} {approx} 0.4. Bars are hosted in the late-type triplet spirals, almost twice more than in early-type spirals. The global fraction of rings is 20%, all in the late-type components. The overdensity of triplets withmore » respect to the background and their current dynamical status, as devised from our estimate of their dynamical parameters, namely the harmonic radius R{sub H} , velocity dispersion {sigma}, dimensionless crossing time H{sub 0{tau}c}, and virial mass M{sub V} , appear to be sufficient to favor galaxy transformations similar to those seen in dense groups and clusters. By contrast, the lower fraction of bonafide ellipticals and the relatively higher fraction of late-type spirals make these triplets essentially different from the Hickson Compact Groups and more representative of the field. A modest 1.6 enhancement factor in the optical luminosity of the late-type triplet components relative to an isolated galaxy control sample is also interpreted as consistent with interactions in physically bounded aggregates. Our results lead us to suggest that non-negligible populations of physical triplets might be found in complete and well-observed samples. We provide individual mosaics for the 54 galaxies containing (1) logarithmic-scaled R-band images, (2) R-band sharp/filtered images, (3) (B - I) color index maps, (4) RGB images from the SDSS database, (5) co-added J + H + K images generated from the 2MASS archives that were also sharp/filtered, and (6) {epsilon}, position angle radial profiles from a surface photometry analysis of (a) the R band and (b) the co-added near-infrared images, all used for the present analysis.« less

THE BUILDUP OF THE HUBBLE SEQUENCE IN THE COSMOS FIELD

We use ~8,600 >5e10 Msol COSMOS galaxies to study how the morphological mix of massive ellipticals, bulge-dominated disks, intermediate-bulge disks, bulge-less disks and irregular galaxies evolves from z=0.2 to z=1. The morphological evolution depends strongly on mass. At M>3e11 Msol, no evolution is detected in the morphological mix: ellipticals dominate since z=1, and the Hubble sequence has quantitatively settled down by this epoch. At the 1e11 Msol mass scale, little evolution is detected, which can be entirely explained with major mergers. Most of the morphological evolution from z=1 to z=0.2 takes place at masses 5e10 - 1e11 Msol, where: (i) The fraction of spirals substantially drops and the contribution of early-types increases. This increase is mostly produced by the growth of bulge-dominated disks, which vary their contribution from ~10% at z=1 to >30% at z=0.2 (cf. the elliptical fraction grows from ~15% to ~20%). Thus, at these masses, transformations from late- to early-types result in disk-less elliptical morphologies with a statistical frequency of only 30% - 40%. Otherwise, the processes which are responsible for the transformations either retain or produce a non-negligible disk component. (ii) The bulge-less disk galaxies, which contribute ~15% to the intermediate-mass galaxy population at z=1, virtually disappear by z=0.2. The merger rate since z=1 is too low to account for the disappearance of these massive bulge-less disks, which most likely grow a bulge via secular evolution.

Cold gas and young stars in tidally disturbed ellipticals at z = 0

Abstract We present an analysis of the neutral hydrogen and stellar populations of elliptical galaxies in the Tal et al. sample. Our aim is to test their conclusion that the continuing assembly of these galaxies at z ∼ 0 is essentially gas free and not accompanied by significant star formation. In order to do so, we make use of H i data and line-strength indices available in the literature. We look for direct and indirect evidence of the presence of cold gas during the recent assembly of these objects and analyse its relation to galaxy morphological fine structure. We find that ≥25 per cent of ellipticals contain H i at the level of M(H i) &amp;gt; 108 M⊙, and that M(H i) is of the order of a few per cent of the total stellar mass. Available data are insufficient to establish whether galaxies with a disturbed stellar morphology are more likely to contain H i. However, H i interferometry reveals very disturbed gas morphology/kinematics in all but one of the detected systems, confirming the continuing assembly of many ellipticals but also showing that this is not necessarily gas free. We also find that all very disturbed ellipticals have a single-stellar-population-equivalent age &amp;lt;4 Gyr. We interpret this as evidence that ∼0.5–5 per cent of their stellar mass is contained in a young population formed during the past ∼1 Gyr. Overall, a large fraction of ellipticals seem to have continued their assembly over the past few Gyr in the presence of a mass of cold gas of the order of 10 per cent of the galaxy stellar mass. This material is now observable as neutral hydrogen and young stars.

Are dry mergers dry, moist or wet?

We present a spectral analysis of a sample of red-sequence galaxies identified by van Dokkum (2005) as dry merger remnants and ongoing dry mergers. Kinematics, stellar population absorption features, and ionisation from emission lines, are derived. We find that approximately half of the sample showing strong tidal features have younger stellar populations than a control sample at a given velocity dispersion. Conversely, galaxies with weak tidal tails and/or ongoing mergers -- with the exception of one galaxy -- do not show this young component. This seems to indicate that the young stellar populations observed in a significant fraction of ellipticals is the consequence of star formation triggered by mergers. This young component is consistent with a light "frosting" of young stars (<2% by mass) superimposed upon a dominant, old (~11 Gyr), stellar population. In terms of stellar populations, these mergers are, in fact, fairly dry. We found, however, that merger remnants with young stellar populations are supported by rotation, contrary to the expectations of a major dry merger. This suggests that the small amount of gas involved has been sufficient to produce a dynamically cold stellar component. Half of the galaxies with strong tidal distortion, however, are slow rotating and have stellar populations compatible with the control sample at a given velocity dispersion. Remarkably, none of the galaxies with velocity dispersions in excess of 250 km/s have a young stellar component, independent of the merger stage.

The properties and evolution of a K-band selected sample of massive galaxies at z∼ 0.4-2 in the Palomar/DEEP2 survey

We present the results of a study on the properties and evolution of massive (M* > 10 11 M⊙) galaxies at z ∼ 0.4-2 utilizing Keck spectroscopy, near-infrared Palomar imaging, and Hubble, Chandra and Spitzer data covering fields targeted by the DEEP2 galaxy spectroscopic survey. Our sample is K-band selected and stellar mass limited, based on wide-area near-infrared imaging from the Palomar Observatory Wide-Field Infrared Survey, which covers 1.53 deg 2 to a 5σ depth of K s,vega ∼ 20.5. Our primary goal is to obtain a broad census of massive galaxies through measuring how their number and mass densities, morphology, as well as their star formation and active galactic nucleus content evolve from z ∼ 0.4-2. Our major findings include: (i) statistically the mass and number densities of M* > 10 11 M⊙ galaxies show little evolution between z = 0 and 1 and from z ∼ 0 to 2 for M* > 10 11.5 M⊙ galaxies. We however find significant evolution within 1 10 11 M⊙ selected galaxies show a nearly constant elliptical fraction of ∼70-90 per cent at all redshifts. The remaining objects tend to be peculiars possibly undergoing mergers at z > 0.8, while spirals dominate the remainder at lower redshifts. A significant fraction (∼25 per cent) of these early-types contain minor structural anomalies. (iii) We find that only a fraction (∼60 per cent) of massive galaxies with M* > 10 11 M⊙ are on the red sequence at z ∼ 1.4, while nearly 100 per cent evolve on to it by z ∼ 0.4. (iv) By utilizing Spitzer MIPS imaging and [O II] line fluxes we argue that M* > 10 11.5 M⊙ galaxies have a steeply declining star formation rate (SFR) density ∼ (1 + z) 6 . By examining the contribution of star formation to the evolution of the mass function, as well as the merger history through the CAS parameters, we determine that M* > 10 11 M⊙ galaxies undergo on average 0.9 +0.7 -0.5 major mergers at 0.4 10 11 M⊙ galaxies are X-ray emitters. Roughly half of these are morphologically distorted ellipticals or peculiars. Finally, we compare our mass growth with semi-analytical models from the Millennium Simulation, finding relative good agreement at z 10 1.5 M⊙ galaxies are underpredicted by a factor of > 100.

Radiative Feedback from Massive Black Holes in Elliptical Galaxies: AGN Flaring and Central Starburst Fueled by Recycled Gas

The importance of the radiative feedback from massive black holes at the centers of elliptical galaxies is not in doubt, given the well established relations among electromagnetic output, black hole mass and galaxy optical luminosity. We show how this AGN radiative output affects the hot ISM of an isolated elliptical galaxy with the aid of a high-resolution hydrodynamical code, where the cooling and heating functions include photoionization plus Compton heating. We find that radiative heating is a key factor in the self-regulated coevolution of massive black holes and their host galaxies and that 1) the mass accumulated by the central black hole is limited by feedback to the range observed today, and 2) relaxation instabilities occur so that duty cycles are small enough (~0.03) to account for the very small fraction of massive ellipticals observed to be in the "on" -QSO- phase, when the accretion luminosity approaches the Eddington luminosity. The duty cycle of the hot bubbles inflated at the galaxy center during major accretion episodes is of the order of 0.1-0.4. Major accretion episodes caused by cooling flows in the recycled gas produced by normal stellar evolution trigger nuclear starbursts coincident with AGN flaring. During such episodes the central sources are often obscured; but overall, in the bursting phase (1<z<3), the duty cycle of the black hole in its "on" phase is of the order of percents and it is unobscured approximately one-third of the time. Mechanical energy output from non-relativistic gas winds integrates to 2.3 10^{59} erg, with most of it caused by broadline AGN outflows. [abridged]

Evidence for the Evolution of Young Early-Type Galaxies in the GOODS/CDF-S Field

We have developed an efficient photometric technique for identifying young early-type galaxy candidates using a combination of photometric redshifts, spectral-type classification, and optical/near-infrared colors. Applying our technique to the GOODS HST/ACS and VLT/ISAAC data we have selected a complete and homogeneous sample of young elliptical candidates among early-type field galaxies. The distribution of structural parameters for these candidates shows that their selection, which is based on early spectral types, is fully consistent with early morphological types. We investigate the evolution of their luminosities and colors as a function of redshift and galaxy mass and find evidence for an increasing starburst mass fraction in these young early-type galaxy candidates at higher redshifts, which we interpret in terms of massive field galaxies experiencing more massive/intense starbursts at higher redshifts. Moreover, we find indications for a systematically larger young elliptical fraction among sub-L*/2 early-type galaxies compared to their brighter counterparts. The total fraction among the field early-type galaxies increases with redshift, irrespective of galaxy luminosity. Our results are most consistent with galaxy formation scenarios in which stars in massive early-type field galaxies are assembled earlier than in their low-mass counterparts.

The Morphological Content of 10 EDisCS Clusters at 0.5 &lt;z&lt; 0.8

We describe Hubble Space Telescope (HST) imaging of 10 of the 20 ESO Distant Cluster Survey (EDisCS) fields. Each ~40 square arcminute field was imaged in the F814W filter with the Advanced Camera for Surveys Wide Field Camera. Based on these data, we present visual morphological classifications for the ~920 sources per field that are brighter than I_auto=23 mag. We use these classifications to quantify the morphological content of 10 intermediate-redshift (0.5 < z < 0.8) galaxy clusters within the HST survey region. The EDisCS results, combined with previously published data from seven higher redshift clusters, show no statistically significant evidence for evolution in the mean fractions of elliptical, S0, and late-type (Sp+Irr) galaxies in clusters over the redshift range 0.5 < z < 1.2. In contrast, existing studies of lower redshift clusters have revealed a factor of ~2 increase in the typical S0 fraction between z=0.4 and z=0, accompanied by a commensurate decrease in the Sp+Irr fraction and no evolution in the elliptical fraction. The EDisCS clusters demonstrate that cluster morphological fractions plateau beyond z ~ 0.4. They also exhibit a mild correlation between morphological content and cluster velocity dispersion, highlighting the importance of careful sample selection in evaluating evolution. We discuss these findings in the context of a recently proposed scenario in which the fractions of passive (E,S0) and star-forming (Sp,Irr) galaxies are determined primarily by the growth history of clusters.

FORS spectroscopy of galaxies in the Hubble Deep Field-South

We present low resolution multi-object spectroscopy of an I -band magnitude limited ($I_{{\rm AB}} \simeq$ 23–23.5) sample of galaxies located in an area centered on the Hubble Deep Field-South (HDFS). The observations were obtained using the Focal Reducer$/$low dispersion Spectrograph (FORS) on the ESO Very Large Telescope. Thirty-two primary spectroscopic targets in the HST-WFPC2 HDFS were supplemented with galaxies detected in the Infrared Space Observatory's survey of the HDFS and the ESO Imaging Deep Survey to comprise a sample of 100 galaxies for spectroscopic observations. Based on detections of several emission lines, such as [OII] λ 3727, H$_{\beta}$ and [OIII] λ 5007, or of other spectroscopic features, we measured accurate redshifts for 50 objects in the central HDFS and flanking fields. The redshift range of the current sample of galaxies is 0.6–1.2, with a median redshift of 1.13 (at I $\simeq$ 23.5 not corrected for completeness). The sample is dominated by starburst galaxies with only a small fraction of ellipticals (~10%). For the emission line objects, the extinction corrected [OII] λ 3727 line strengths yield estimates of star formation rates in the range 0.5–30 $M_{\odot}$ yr -1 . We used the present data to derive the [OII] λ 3727 luminosity function up to redshift of 1.2. When combined with [OII] λ 3727 luminosity densities for the local and high redshift Universe, our results confirm the steep rise in the star formation rate (SFR) to z $\simeq$ 1.3.

On the origin of isophotal shapes in elliptical galaxies

Using semi-analytical models of galaxy formation, the origin of boxy and disky elliptical galaxies is investigated. We find that the simple scenario, motivated by N-body simulations, in which the isophotal shape is only dependent on the mass ratio of the last major merger, is not able to reproduce the observation that the fraction of boxy and disky ellipticals depends on galaxy luminosity. The observations can however be reproduce with the following reasonable assumptions: (i) equal-mass mergers lead to boxy ellipticals and unequal-mass mergers produce disky ellipticals (as motivated by N-body simulations) (ii) major mergers between bulge-dominated galaxies result always in boxy ellipticals, independent of the mass ratio, (iii) merger remnants that subsequently accrete gas leading to a secondary stellar disk with more than $20 %$ the total stellar fraction are always disky. Boxy and disky ellipticals can be divided into two subclasses, depending on their formation history. Boxy ellipticals either formed by equal mass mergers of disk galaxies or by major mergers of early-type galaxies. We find that disky ellipticals are indicators of unequal-mass mergers or late gas infall. Disky ellipticals with high luminosities preferentially are 1-component systems that result from unequal mass mergers whereas low-luminosity disky ellipticals are more likely to harbor secondary disk components. In addition, the fraction of disky ellipticals with secular disk components should increase in regions with higher galaxy densities. Taking into account the conversion of cuspy cores into flat low-density cores by black hole merging we find that disky ellipticals should contain central density cusps whereas boxy ellipticals should in general be characterized by flat cores.