Showing papers on "Brightest cluster galaxy published in 2006"
TL;DR: The Millennium Simulation of the concordancecold dark matter (CDM) cosmogony was used to study how the star formation histories, ages and metallicities of elliptical galaxies depend on environment and on stellar mass.
Abstract: We take advantage of the largest high-resolution simulation of cosmic structure growth ever carried out - the Millennium Simulation of the concordancecold dark matter (CDM) cosmogony - to study how the star formation histories, ages and metallicities of elliptical galaxies depend on environment and on stellar mass. We concentrate on a galaxy formation model which is tuned to fit the joint luminosity/colour/morphology distribution of low-redshift galaxies. Massive ellipticals in this model have higher metal abundances, older luminosity- weighted ages and shorter star formation time-scales, but lower assembly redshifts, than less massive systems. Within clusters the typical masses, ages and metal abundances of ellipticals are predicted to decrease, on average, with increasing distance from the cluster centre. We also quantify the effective number of progenitors of ellipticals as a function of present stellar mass, finding typical numbers below two for M∗ < 10 11 M� , rising to approximately five for the most massive systems. These findings are consistent with recent observational results that suggest 'down-sizing' or 'antihierarchical' behaviour for the star formation history of the elliptical galaxy population, despite the fact that our model includes all the standard elements of hierarchical galaxy formation and is implemented on the standard, � CDM cosmogony.
1,037 citations
TL;DR: In this article, it was shown that metal-poor GCs formed in low-mass dark matter halos in the early universe and that their properties reflect biased galaxy assembly, while metal-rich GCs were born in the subsequent dissipational buildup of their parent galaxies.
Abstract: Globular cluster (GC) systems have now been studied in galaxies ranging from dwarfs to giants and spanning the full Hubble sequence of morphological types. Imaging and spectroscopy with the Hubble Space Telescope and large ground-based telescopes have together established that most galaxies have bimodal color distributions that reflect two subpopulations of old GCs: metal-poor and metal-rich. The characteristics of both subpopulations are correlated with those of their parent galaxies. We argue that metal-poor GCs formed in low-mass dark matter halos in the early universe and that their properties reflect biased galaxy assembly. The metal-rich GCs were born in the subsequent dissipational buildup of their parent galaxies and their ages and abundances indicate that most massive early-type galaxies formed the bulk of their stars at early times. Detailed studies of both subpopulations offer some of the strongest constraints on hierarchical galaxy formation that can be obtained in the near-field.
844 citations
National Research Council1, European Southern Observatory2, University of Oxford3, Washington State University4, Johns Hopkins University5, New Jersey Institute of Technology6, Rochester Institute of Technology7, California Institute of Technology8, Fairchild Semiconductor International, Inc.9, University of Hawaii10, University of Hawaii at Hilo11
TL;DR: In this paper, a detailed analysis of the morphology, isophotal parameters, and surface brightness profiles for 100 early-type members of the Virgo Cluster, from dwarfs (MB = -15.1 mag) to giants (GB = -21.8 mag), was presented.
Abstract: We present a detailed analysis of the morphology, isophotal parameters, and surface brightness profiles for 100 early-type members of the Virgo Cluster, from dwarfs (MB = -15.1 mag) to giants (MB = -21.8 mag), imaged in the g and z passbands using the Advanced Camera for Surveys on board the Hubble Space Telescope. Dust and complex morphological structures are common. Dust is detected in 42% of galaxies brighter than BT = 12.15 mag, while kiloparsec-scale stellar disk, bars, and nuclear stellar disks are seen in 60% of galaxies with intermediate luminosity. Isophotal parameters are derived typically within 8 kpc from the center for the brightest galaxies, and 1.5 kpc for the faintest systems, with a resolution of 7 pc. For most galaxies, the surface brightness profiles are well described by a Sersic model with index n that increases steadily with the galaxy luminosity; only for 8 of the 10 brightest galaxies are the inner profiles (typically within 100 pc of the center) lower than expected based on an extrapolation of the outer Sersic model, and are better described by a single power-law function. Contrary to previous claims, we find no evidence in support of a strong bimodal behavior of the logarithmic slope of the inner surface brightness profile, γ; in particular the γ distribution for galaxies that do not show evidence of multiple morphological components is unimodal across the entire magnitude range spanned by the ACSVCS galaxies. Although the brightest galaxies have shallow inner profiles, the shallowest profiles are found in faint dwarf systems. The widely adopted separation of early-type galaxies between core and power-law types is questioned based on the present study.
606 citations
TL;DR: The evolution of the B-band galaxy luminosity function is measured using a sample of more than 11,000 galaxies with spectroscopic redshifts from the DEEP2 Redshift Survey as mentioned in this paper.
Abstract: The evolution of the B-band galaxy luminosity function is measured using a sample of more than 11,000 galaxies with spectroscopic redshifts from the DEEP2 Redshift Survey. The rest-frame MB versus U - B color-magnitude diagram of DEEP2 galaxies shows that the color-magnitude bimodality seen in galaxies locally is still present at redshifts z > 1. Dividing the sample at the trough of this color bimodality into predominantly red and blue galaxies, we find that the luminosity function of each galaxy color type evolves differently. Blue counts tend to shift to brighter magnitudes at constant number density, while the red counts remain largely constant at a fixed absolute magnitude. Using Schechter functions with fixed faint-end slopes we find that M for blue galaxies brightens by ~1.3 ? 0.14 mag per unit redshift, with no significant evolution in number density. For red galaxies M brightens somewhat less with redshift, while the formal value of * declines. When the population of blue galaxies is subdivided into two halves using the rest-frame color as the criterion, the measured evolution of both blue subpopulations is very similar.
434 citations
Herzberg Institute of Astrophysics1, University of Padua2, Rochester Institute of Technology3, European Southern Observatory4, University of Oxford5, Washington State University6, Rutgers University7, Johns Hopkins University8, New Jersey Institute of Technology9, University of Hawaii10, University of Hawaii at Hilo11
TL;DR: In this paper, the authors combine HST imaging for early-type galaxies from the ACS Virgo Cluster Survey with ground-based long-slit spectra from KPNO to show that the masses of compact stellar nuclei in Virgo cluster galaxies obey a tight correlation with the mass of the host galaxies.
Abstract: Imaging surveys with the Hubble Space Telescope (HST) have shown that ?50%-80% of low- and intermediate-luminosity galaxies contain a compact stellar nucleus at their center, regardless of host galaxy morphological type. We combine HST imaging for early-type galaxies from the ACS Virgo Cluster Survey with ground-based long-slit spectra from KPNO to show that the masses of compact stellar nuclei in Virgo Cluster galaxies obey a tight correlation with the masses of the host galaxies. The same correlation is obeyed by the supermassive black holes (SBHs) found in predominantly massive galaxies. The compact stellar nuclei in the Local Group galaxies M33 and NGC 205 are also found to fall along this same scaling relation. These results indicate that a generic by-product of galaxy formation is the creation of a central massive object (CMO)?either an SBH or a compact stellar nucleus?that contains a mean fraction, ?0.2%, of the total galactic mass. In galaxies with masses greater than gal ~ a few × 1010 ?, SBHs appear to be the dominant mode of CMO formation.
398 citations
TL;DR: A spectroscopic redshift of z = 6.96 is reported for a galaxy whose spectrum clearly shows Lyman-α emission at 9,682 Å, indicating active star formation at a rate of ∼10M[circdot] yr-1, which demonstrates that galaxy formation was under way when the Universe was only ∼6 per cent of its present age.
Abstract: The galaxy described on page 186 may be, for the moment, the most distant and hence oldest galaxy known. Large samples of galaxies have been found at redshifts of z∼6, but detections at earlier times tend to be uncertain and unreliable. But this 'new' old galaxy has a spectroscopic redshift of z=6.96, corresponding to just 750 million years after the Big Bang; and a Lyman-α emission line in its spectrum suggests that active star formation was under way when the Universe was only about 6% of its present age. This galaxy was detected during a survey using the Subaru Suprime-Cam on the summit of Mauna Kea. Looking at the galaxy population as a whole, the same survey produced a number density of galaxies at z≈7 that is only 18-36% that at z=6.6. A separate search for galaxies at at z∼7–8 using data from the Hubble Space Telescope yielded (conservatively) only one candidate galaxy, where 10 would be expected if there were no evolution in the galaxy population between z∼7 and z∼6. The simplest explanation for this is that the Universe is just too young to have built up many luminous galaxies at z∼7–8 by hierarchical merging of small galaxies. This paper reports a spectroscopic redshift of z=6.96, corresponding to just 750 million years after the Big Bang, for a galaxy whose spectrum clearly shows Lyman-alpha emission at 9682 A. When galaxy formation started in the history of the Universe remains unclear. Studies of the cosmic microwave background indicate that the Universe, after initial cooling (following the Big Bang), was reheated and reionized by hot stars in newborn galaxies at a redshift in the range 6 7 have been identified photometrically2,3, galaxies with spectroscopically confirmed redshifts have been confined to z < 6.6 (refs 4–8). Here we report a spectroscopic redshift of z = 6.96 (corresponding to just 750 Myr after the Big Bang) for a galaxy whose spectrum clearly shows Lyman-α emission at 9,682 A, indicating active star formation at a rate of ∼10M⊙ yr-1, where M⊙ is the mass of the Sun. This demonstrates that galaxy formation was under way when the Universe was only ∼6 per cent of its present age. The number density of galaxies at z ≈ 7 seems to be only 18–36 per cent of the density at z = 6.6.
369 citations
TL;DR: In this paper, the authors studied how the proportion of star-forming galaxies evolves between z ¼ 0:8 and 0 as a function of galaxy environment, using the OIilineinemissionasasignatureofongoing starformation.
Abstract: We study how the proportion of star-forming galaxies evolves between z ¼ 0:8 and 0 as a function of galaxy environment,usingtheOiilineinemissionasasignatureofongoingstarformation.Our high-zdatasetcomprises16 clusters, 10 groups, and another 250 galaxies in poorer groups and the field at z ¼ 0:4 0:8 from the ESO Distant Cluster Survey, plus another 9 massive clusters at similar redshifts. As a local comparison, we use galaxy systems selected from the Sloan Digital Sky Survey (SDSS) at 0:04 < z < 0:08. At high z most systems follow a broad anticorrelation between the fraction of star-forming galaxies and the system velocity dispersion. At face value, this suggests that at z ¼ 0:4 0:8 the mass of the system largely determines the proportion of galaxies with ongoing star formation. At these redshifts the strength of star formation (as measured by the O ii equivalent width) in star-forming galaxies is also found to vary systematically with environment. SDSS clusters have much lower fractions of starforming galaxies than clusters at z ¼ 0:4 0:8 and, in contrast with the distant clusters, show a plateau for velocity dispersions � 550kms � 1 ,where thefraction ofgalaxieswithOiiemission doesnotvarysystematicallywithvelocity dispersion. We quantify the evolution of the proportion of star-forming galaxies as a function of the system velocity dispersion and find that it is strongest in intermediate-mass systems (� � 500 600 km s � 1 at z ¼ 0). To understandtheoriginoftheobservedtrends,weusethePress-Schechter formalismandtheMillenniumSimulationandshow thatgalaxystarformationhistoriesmaybecloselyrelatedtothegrowthhistoryofclustersandgroups.Ifthescenariowe propose is roughly correct, the link between galaxy properties and environment is extremely simple to predict purely from a knowledge of the growth of dark matter structures. Subject headings: cosmology: observations — galaxies: clusters: general — galaxies: evolution — galaxies: fundamental parameters — galaxies: stellar content
337 citations
TL;DR: In this paper, the modified acceleration law obtained from Einstein gravity coupled to a massive skew-symmetric field Fμνλ was applied to the problem of explaining galaxy rotation curves without exotic dark matter.
Abstract: We apply the modified acceleration law obtained from Einstein gravity coupled to a massive skew-symmetric field Fμνλ to the problem of explaining galaxy rotation curves without exotic dark matter. Our sample of galaxies includes low surface brightness (LSB) and high surface brightness (HSB) galaxies and an elliptical galaxy. In those cases for which photometric data are available, a best fit via the single parameter (M/L)stars to the luminosity of the gaseous (H I plus He) and luminous stellar disks is obtained. In addition, a best fit to the rotation curves of galaxies is obtained in terms of a parametric mass distribution (independent of luminosity observations)—a two-parameter fit to the total galactic mass (or mass-to-light ratio M/L) and a core radius associated with a model of the galaxy cores—using a nonlinear least-squares fitting routine including estimated errors. The fits are compared to those obtained using Milgrom's phenomenological MOND model and to the predictions of the Newtonian/Kepler acceleration law.
322 citations
TL;DR: In this paper, the authors used a simple model in which relative sizes of elliptical galaxies of a given mass correlate with the fraction of stars formed in a starburst during a major merger event, and were able to reproduce the observed redshift-size evolution.
Abstract: We use semianalytical modeling of galaxy formation to predict the redshift-size evolution of elliptical galaxies. Using a simple model in which relative sizes of elliptical galaxies of a given mass correlate with the fraction of stars formed in a starburst during a major merger event, we are able to reproduce the observed redshift-size evolution. The size evolution is a result of the amount of cold gas available during the major merger. Mergers at high redshifts are gas-rich and produce ellipticals with smaller sizes. In particular, we find a power-law relation between the sizes at different redshifts, with the power-law index giving a measure of the relative amount of dissipation during the mergers that lead to the formation of an elliptical. The size evolution is found to be stronger for more massive galaxies as they involve more gas at high redshifts when they form, compared to less massive ellipticals. Local ellipticals more massive than 5 × 1011 M☉ will be approximately 4 times larger than their counterparts at z = 2. Our results indicate that the scatter in the size of similar massive present-day elliptical galaxies is a result of their formation epoch, with smaller ellipticals being formed earlier.
307 citations
Abstract: A long-standing problem for models of galaxy formation has been the mismatch between the predicted shape of the mass function of dark matter halos and the observed shape of the luminosity function of galaxies. The number of massive haloes is predicted to decrease as a power law (N ∝ M −2 ) out to very large masses, while the galaxy luminosity function cuts off exponentially at luminosities above L∗ . This implies that the efficiency with which gas cools onto massive systems is lower than expected. This letter investigates the role of radioloud active galactic nuclei (AGN) in continually re-heating the cooling gas. By combining two observational results, the time-averaged energy output associated with recurrent radio source activity is determined, as a function of the black hole mass of the host galaxy: ¯ H = 10 21.4 (MBH/M� ) 1.6 W. It is shown that for massive elliptical galaxies this radio-source heating balances the radiative energy losses from the hot gas surrounding the galaxy. The recurrent radio-loud AGN activity may therefore provide a self-regulating feedback mechanism capable of controlling the rate of growth of galaxies.
306 citations
TL;DR: In this article, the authors derived B-band luminosity distributions and selected bivariate brightness distributions for the galaxy population subdivided by eyeball morphology; Sersic index (n); two-degree Field Galaxy Redshift Survey (2dFGRS) η parameter; rest-(u - r) colour (global and core); MGC continuum shape; half-light radius; (extrapolated) central surface brightness; and inferred stellar mass-to-light ratio.
Abstract: Using 10095 galaxies (B < 20 mag) from the Millennium Galaxy Catalogue, we derive B-band luminosity distributions and selected bivariate brightness distributions for the galaxy population subdivided by eyeball morphology; Sersic index (n); two-degree Field Galaxy Redshift Survey (2dFGRS) η parameter; rest-(u - r) colour (global and core); MGC continuum shape; half-light radius; (extrapolated) central surface brightness; and inferred stellar mass-to-light ratio. All subdivisions extract highly correlated subsets of the galaxy population which consistently point towards two overlapping distributions: an old, red, inert, predominantly luminous, high central-surface brightness subset; and a young, blue, star forming, intermediate surface brightness subset. A clear bimodality in the observed distribution is seen in both the rest-(u - r) colour and log (n) distributions. Whilst the former bimodality was well established from Sloan Digital Sky Survey data, we show here that the rest-(u - r) colour bimodality becomes more pronounced when using the core colour as opposed to global colour. The two populations are extremely well separated in the colour-log(n) plane. Using our sample of 3314 (B < 19 mag) eyeball classified galaxies, we show that the bulge-dominated, early-type galaxies populate one peak and the bulge-less, late-type galaxies occupy the second. The early- and mid-type spirals sprawl across and between the peaks. This constitutes extremely strong evidence that the fundamental way to divide the luminous galaxy population (M BMGC -5 log h < -16 mag, i.e. dwarfs not included) is into bulges (old red, inert, high concentration) and discs (young, blue, star forming, low concentration) and that the galaxy bimodality reflects the two-component nature of galaxies and not two distinct galaxy classes. We argue that these two components require two independent formation mechanisms/processes and advocate early bulge formation through initial collapse and ongoing disc formation through splashback, infall and merging/accretion. We calculate the B-band luminosity densities and stellar mass densities within each subdivision and estimate that the z ≈ 0 stellar mass content in spheroids, bulges and discs is 35 ± 2, 18 ± 7 and 47 ± 7 per cent, respectively.
TL;DR: In this paper, the projected two-point correlation function for samples of luminous and massive galaxies in the COMBO-17 photometric redshift survey was calculated, focusing particularly on the amplitude of the correlation function at small projected radii.
Abstract: We calculate the projected two-point correlation function for samples of luminous and massive galaxies in the COMBO-17 photometric redshift survey, focusing particularly on the amplitude of the correlation function at small projected radii and exploring the constraints such measurements can place on the galaxy merger rate. For nearly volume-limited samples with 0.4 2.5 × 1010 M☉ is 5% ± 1%. Incorporating close pair fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid evolution of the merger fraction of massive galaxies between z = 0.8 and the present day. Assuming that the major merger timescale is of order the dynamical timescale for close massive galaxy pairs, we tentatively infer that ~50% (70%) of all galaxies with present-day masses M* > 5 × 1010 M☉ (remnants of mergers between galaxies with M* > 2.5 × 1010 M☉) have undergone a major merger since z = 0.8(1): major mergers between massive galaxies are a significant driver of galaxy evolution over the last eight billion years.
TL;DR: In this article, the authors studied the mean environment of galaxies in the DEEP2 Galaxy Redshift Survey as a function of rest-frame color, luminosity, and [OII] 3727u equivalent width.
Abstract: We study the mean environment of galaxies in the DEEP2 Galaxy Redshift Survey as a function of rest-frame color, luminosity, and [OII] 3727u equivalent width. The local galaxy overdensity for > 14,000 galaxies at 0.75 < z < 1.35 is estimated using the projected 3 rd -nearest-neighbor surface density. Of the galaxy properties studied, mean environment is found to depend most strongly on galaxy color; all major features of the correlation between mean overdensity and rest-frame color observed in the local universe were already in place at z ∼ 1. In contrast to local results, we find a substantial slope in the mean dependence of environment on luminosity for blue, star-forming galaxies at z ∼ 1, with brighter blue galaxies being found on average in regions of greater overdensity. We discuss the roles of galaxy clusters and groups in establishing the observed correlations between environment and galaxy properties at high redshift, and we also explore the evidence for a “downsizing of quenching” from z ∼ 1 to z ∼ 0. Our results add weight to existing evidence that the mechanism(s) that result in star-formation quenching are efficient in group environments as well as clusters. This work is the first of its kind at high redshift and represents the first in a series of papers addressing the role of environment in galaxy formation at 0 < z < 1. Subject headings: galaxies:high-redshift, galaxies:evolution, galaxies:statistics, galaxies:fundamental parameters, large-scale structure of universe
TL;DR: In this paper, the authors present new (B, I) photometry for the globular cluster systems in eight brightest cluster galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope, and find that all have strongly bimodal color distributions that are clearly resolved by the metallicity-sensitive (B - I) index.
Abstract: We present new (B, I) photometry for the globular cluster systems in eight brightest cluster galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions that are clearly resolved by the metallicity-sensitive (B - I) index. Furthermore, the mean colors and internal color range of the blue subpopulation are remarkably similar from one galaxy to the next, to well within the ±0.02-0.03 mag uncertainties in the foreground reddenings and photometric zero points. By contrast, the mean color and internal color range for the red subpopulation differ from one galaxy to the next by twice as much as the blue population. All the BCGs show population gradients, with much higher relative numbers of red clusters within 5 kpc of their centers, consistent with their having formed at later times than the blue, metal-poor population. A striking new feature of the color distributions emerging from our data is that for the brightest clusters (MI < -10.5) the color distribution becomes broad and less obviously bimodal. This effect was first noticed by Ostrov et al. and Dirsch et al. for the Fornax giant NGC 1399; our data suggest that it may be a characteristic of many BCGs and perhaps other large galaxies. Our data indicate that the blue (metal-poor) clusters brighter than MI -10 become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z ~ M0.55. A basically similar relation has been found for M87 by Strader et al. (2005). We argue that these GCS characteristics are consistent with a hierarchical-merging galaxy formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 107-1010 M☉, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more preenrichment could occur.
TL;DR: In this paper, the light distribution in 10095 galaxies from the Millennium Galaxy Catalogue (MGC) is modelled, providing publically available structural catalogues for a large, rep- resentative sample of galaxies in the local Universe.
Abstract: We have modelled the light distribution in 10095 galaxies from the Millennium Galaxy Catalogue (MGC), providing publically available structural catalogues for a large, rep- resentative sample of galaxies in the local Universe. Three different models were used: (1) a single Sersic function for the whole galaxy, (2) a bulge-disc decomposition model using a de Vaucouleurs (R 1/4 ) bulge plus exponential disc, (3) a bulge-disc decomposi- tion model using a Sersic (R 1/n ) bulge plus exponential disc. Repeat observations for � 700 galaxies demonstrate that stable measurements can be obtained for object com- ponents with a half-light radius comparable to, or larger than, the seeing half-width at half maximum. We show that with careful quality control, robust measurements can be obtained for large samples such as the MGC. We use the catalogues to show that the galaxy colour bimodality is due to the two-component nature of galaxies (i.e. bulges and discs) and not to two distinct galaxy populations. We conclude that under- standing galaxy evolution demands the routine bulge-disc decomposition of the giant galaxy population at all redshifts.
TL;DR: In this paper, the bimodal distribution of the galaxy population commonly seen in the local universe was revisited in terms of spectral synthesis products, such as mean stellar ages and stellar masses.
Abstract: We revisit the bimodal distribution of the galaxy population commonly seen in the local universe. Here, we address the bimodality observed in galaxy properties in terms of spectral synthesis products, such as mean stellar ages and stellar masses, derived from the application of this powerful method to a volume-limited sample, with magnitude limit cut-off M(r ) =− 20.5, containing about 50 000 luminous galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 2 (DR2). In addition, galaxies are classified according to their emission-line properties in three distinct spectral classes: star-forming galaxies, with young stellar populations; passive galaxies, dominated by old stellar populations; and hosts of active nuclei, which comprise a mix of young and old stellar populations. We show that the extremes of the distribution of some galaxy properties, essentially galaxy colours, 4000 A break index and mean stellar ages, are associated to star-forming galaxies at one side, and passive galaxies at another. We find that the mean light-weighted stellar age of galaxies is directly responsible for the bimodality seen in the galaxy population. The stellar mass, in this view, has an additional role since most of the star-forming galaxies present in the local universe are low-mass galaxies. Our results also give support to the existence of a ‘downsizing’ in galaxy formation, where massive galaxies seen nowadays have stellar populations formed at early times.
TL;DR: In this paper, the authors present the results of a series of empirical computations regarding the role of major mergers in forming the stellar masses of modern galaxies based on measured galaxy merger and star formation histories from z ~ 0.5 to 3.
Abstract: We present the results of a series of empirical computations regarding the role of major mergers in forming the stellar masses of modern galaxies based on measured galaxy merger and star formation histories from z ~ 0.5 to 3. We reconstruct the merger history of normal field galaxies from z ~ 3 to z ~ 0 as a function of initial mass using published pair fractions and merger fractions from structural analyses. We calibrate the observed merger timescale and mass ratios for galaxy mergers using self-consistent N-body models of mergers with mass ratios from 1?:?1 to 1?:?5 at various orbital properties and viewing angles. We use these simulations to determine the timescales and mass ratios that produce structures that would be identified as major mergers. Based on these calculations, we argue that a typical massive galaxy at z ~ 3 with M* > 1010 M? undergoes 4.4 major mergers at z > 1. We find that by z ~ 1.5 the stellar mass of an average massive galaxy is relatively established, a scenario qualitatively favored in a ?-dominated universe. We argue that the final masses of these systems increase by as much as a factor of 100, allowing Lyman break galaxies, which tend to have low stellar masses, to become the most massive galaxies in today's universe with M > M*. Induced star formation, however, only accounts for 10%-30% of the stellar mass formed in these galaxies at z < 3. A comparison to semianalytic models of galaxy formation shows that cold dark matter (CDM) models consistently underpredict the merger fraction, and rate of merging, of massive galaxies at high redshift. This suggests that massive galaxy formation occurs through more merging than predicted in CDM models, rather than a rapid early collapse.
TL;DR: In this paper, a new three-dimensional galaxy classification system designed to account for the diversity of galaxy properties in the nearby universe is presented, which is based on the Spearman rank and principal component analyses (PCAs).
Abstract: We present in this paper a new three-dimensional galaxy classification system designed to account for the diversity of galaxy properties in the nearby universe. To construct this system we statistically analyse a sample of >22 000 galaxies at v < 15 000 km s−1 (z < 0.05) with Spearman rank and principal-component analyses (PCAs). Fourteen major galaxy properties are considered, including: Hubble type, size, colour, surface brightness, magnitude, stellar mass, internal velocities, H i gas content and an index that measures dynamical disturbances. We find, to a high degree, that most galaxy properties are correlated, with in particular Hubble type, colour and stellar mass all strongly related. We argue that this tight three-way correlation is a result of evolutionary processes that depend on galaxy mass, as we show that the relation between colour and mass is independent of Hubble type. Various PCAs reveal that most of the variation in nearby galaxy properties can be accounted for by eigenvectors dominated by (i) the scale of a galaxy, such as its stellar mass, (ii) the spectral type and (iii) the degree of dynamical disturbances. We suggest that these three properties: mass, star formation and interactions/mergers are the major features that determine a galaxy's physical state, and should be used to classify galaxies. As shown by Conselice et al., these properties are measurable within the CAS (concentration, asymmetry, clumpiness) structural system, thus providing an efficient mechanism for classifying galaxies in optical light within a physical meaningful framework. We furthermore discuss the fraction and number density of galaxies in the nearby universe as a function of Hubble type, for comparison with higher redshift populations.
TL;DR: In this article, the authors measured line-of-sight velocity dispersions from integrated emission for 1089 galaxies with median redshift 0.637 and spatially resolved kinematics for a subsample of 380 galaxies.
Abstract: We present kinematic measurements of a large sample of galaxies from the Team Keck Redshift Survey in the GOODS-N field. We measure line-of-sight velocity dispersions from integrated emission for 1089 galaxies with median redshift 0.637 and spatially resolved kinematics for a subsample of 380 galaxies. This is the largest sample of galaxies to z ~ 1 with kinematics to date and allows us to measure kinematic properties without morphological preselection. Emission-line widths provide a dynamical measurement for the bulk of blue galaxies. To fit the spatially resolved kinematics, we construct models that fit both line-of-sight rotation amplitude and velocity dispersion. Integrated line width correlates well with a combination of the velocity gradient and dispersion and is a robust measure of galaxy kinematics. The spatial extents of emission and continuum are similar, and there is no evidence that line widths are affected by nuclear or clumpy emission. The measured rotation gradient depends strongly on slit position angle alignment with galaxy major axis, but integrated line width does not. Even for galaxies with well-aligned slits, some have kinematics dominated by dispersion (V/σ < 1) rather than rotation. These are probably objects with disordered velocity fields, not dynamically hot stellar systems. About 35% of the spatially resolved sample are dispersion dominated; galaxies that are both dispersion dominated and bright exist at high redshift but appear rare at low redshift. This kinematic morphology may probe galaxies' evolutionary state. It is linked to photometric morphology in HST ACS images: dispersion-dominated galaxies include a higher fraction of irregulars and chain galaxies, while rotation-dominated galaxies are mostly disks and irregulars. Only one-third of chain/hyphen galaxies are dominated by rotation; high-redshift elongated objects cannot be assumed to be inclined disks.
TL;DR: In this paper, the authors used the Hubble Space Telescope Advanced Camera for Surveys (SCAS) to determine accurate distances for 20 galaxies from measurements of the luminosity of the brightest red giant branch stars.
Abstract: The Hubble Space Telescope Advanced Camera for Surveys has been used to determine accurate distances for 20 galaxies from measurements of the luminosity of the brightest red giant branch stars. Five associations of dwarf galaxies that had originally been identified based on strong correlations on the plane of the sky and in velocity are shown to be equally well correlated in distance. Two more associations with similar properties have been discovered. Another association is identified that is suggested to be unbound through tidal disruption. The associations have the spatial and kinematic properties expected of bound structures with ? 1011 M?. However, these entities have little light, with the consequence that the mass-to-light ratios are in the range 100-1000 M? L. Within a well-surveyed volume extending to a 3 Mpc radius, all but one known galaxy lie within one of the groups or associations that have been identified.
TL;DR: In this paper, the authors compare the results of a semi-analytical model for galaxy formation with the observations of the Sloan Digital Sky Survey (SDSS) and show that there are significant discrepancies when the blue fraction of galaxies as a function of mass and luminosity is compared between the observations and the model.
Abstract: Successfully reproducing the galaxy luminosity function (LF) and the bimodality in the galaxy distribution requires a mechanism that can truncate star formation in massive haloes. Current models of galaxy formation consider two such truncation mechanisms: strangulation, which acts on satellite galaxies, and active galactic nucleus (AGN) feedback, which predominantly affects central galaxies. The efficiencies of these processes set the blue fraction of galaxies, f(blue)(L, M), as a function of galaxy luminosity, L, and halo mass, M. In this paper, we use a galaxy group catalogue extracted from the Sloan Digital Sky Survey (SDSS) to determine f(blue)(L, M). To demonstrate the potential power of these data as a benchmark for galaxy formation models, we compare the results to the semi-analytical model for galaxy formation of Croton et al. Although this model accurately fits the global statistics of the galaxy population, as well as the shape of the conditional LF, there are significant discrepancies when the blue fraction of galaxies as a function of mass and luminosity is compared between the observations and the model. In particular, the model predicts (i) too many faint satellites in massive haloes, (ii) a blue fraction of satellites that is much too low, and (iii) a blue fraction of centrals that is too high and with an inverted luminosity dependence. In the same order, we argue that these discrepancies owe to (i) the neglect of tidal stripping in the semi-analytical model, (ii) the oversimplified treatment of strangulation, and (iii) improper modelling of dust extinction and/or AGN feedback. The data presented here will prove useful to test and calibrate future models of galaxy formation and, in particular, to discriminate between various models for AGN feedback and other star formation truncation mechanisms.
TL;DR: In this paper, the authors reported the discovery of an extraordinarily massive young cluster of stars in the Galaxy, having an inferred total initial cluster mass comparable to the most massive young clusters in the galaxy.
Abstract: We report the discovery of an extraordinarily massive young cluster of stars in the Galaxy, having an inferred total initial cluster mass comparable to the most massive young clusters in the Galaxy. Using IRMOS, 2MASS, and Spitzer observations, we conclude that there are 14 red supergiants in the cluster, compared with five, in what was previously thought to be the richest Galactic cluster of such stars. We infer spectral types from near-infrared spectra that reveal deep CO bandhead absorption that can only be fit by red supergiants. We identify a gap of ΔKs ~ 4 mag between the stars and the bulk of the other stars in the region that can only be fit by models if the brightest stars in the cluster are red supergiants. We estimate a distance of 5.8 kpc to the cluster by associating an OH maser with the envelope of one of the stars. We also identify a "yellow" supergiant of G6 I type in the cluster. Assuming a Salpeter IMF, we infer an initial cluster mass of 20,000-40,000 M☉ for cluster ages of 7-12 Myr. Continuing with these assumptions, we find that 80% of the initial mass and 99% of the number of stars remain at the present time. We associate the cluster with an X-ray source (detected by ASCA and Einstein), a recently discovered very high energy γ-ray source (detected by INTEGRAL and HESS), and several nonthermal radio sources, finding that these objects are likely related to recent supernovae in the cluster. In particular, we claim that the cluster has produced at least one recent supernova remnant with properties similar to the Crab Nebula.
TL;DR: In this paper, a multi-wavelength analysis of a compact group of galaxies infalling at high speed into the dynamically young cluster Abell 1367 is presented, and the properties of this group suggest that environmental effects within infalling groups may have represented a preprocessing step of the galaxy evolution during the high redshift cluster assembly phase.
Abstract: We present a multiwavelength analysis of a compact group of galaxies infalling at high speed into the dynamically young cluster Abell 1367. Peculiar morphologies and unusually high Halpha emission are associated with two giant galaxies and at least ten dwarfs/extragalactic HII regions, making this group the region with the highest density of star formation activity ever observed in the local clusters. Moreover Halpha imaging observations reveal extraordinary complex trails of ionized gas behind the galaxies, with projected lengths exceeding 150 kpc. These unique cometary trails mark the gaseous trajectory of galaxies, witnessing their dive into the hot cluster intergalactic medium. Under the combined action of tidal forces among group members and the ram-pressure by the cluster ambient medium, the group galaxies were fragmented and the ionized gas was blown out. The properties of this group suggest that environmental effects within infalling groups may have represented a preprocessing step of the galaxy evolution during the high redshift cluster assembly phase.
TL;DR: It is found that a coeval group of old clusters with a unimodal metallicity spread can exhibit color bimodality, and this scenario gives simple and cohesive explanations for all the key observations and could simplify theories of elliptical galaxy formation.
Abstract: The colors of globular clusters in most large elliptical galaxies are bimodal. This is generally taken as evidence for the presence of two cluster subpopulations that have different geneses. However, here we find that, because of the nonlinear nature of the metallicity-to-color transformation, a coeval group of old clusters with a unimodal metallicity spread can exhibit color bimodality. The models of cluster colors indicate that horizontal-branch stars are the main drivers behind the empirical nonlinearity. We show that the scenario gives simple and cohesive explanations for all the key observations and could simplify theories of elliptical galaxy formation.
TL;DR: In this paper, the alignment between the orientation of the central galaxy (defined as the brightest group member) and the distribution of satellite galaxies was examined. But the alignment on scales smaller than the halo virial radius was not considered.
Abstract: We use galaxy groups selected from the Sloan Digital Sky Survey to examine the alignment between the orientation of the central galaxy (defined as the brightest group member) and the distribution of satellite galaxies. By construction, we therefore only address the alignment on scales smaller than the halo virial radius. We find a highly significant alignment of satellites with the major axis of their central galaxy. This is in qualitative agreement with the recent study of Brainerd, but inconsistent with several previous studies who detected a preferential minor-axis alignment. The alignment strength in our sample is strongest between red central galaxies and red satellites. On the contrary, the satellite distribution in systems with a blue central galaxy is consistent with isotropic. We also find that the alignment strength is stronger in more massive haloes and at smaller projected radii from the central galaxy. In addition, there is a weak indication that fainter (relative to the central galaxy) satellites are more strongly aligned. We present a detailed comparison with previous studies, and discuss the implications of our findings for galaxy formation.
TL;DR: In this paper, the authors report the discovery of two peculiar galaxies infalling into the lensing clusters of galaxies Abell 1689 (z∼0.18) and 2667 (z ∼0.23).
Abstract: We report the discovery of two peculiar galaxies infalling into the lensing clusters of galaxies Abell 1689 (z∼0.18) and 2667 (z∼0.23). Hubble Space Telescope images show
extraordinary trails composed by blue bright knots and stellar streams associated with both these systems, an ∼L ∗ and ∼0.1L ∗ galaxy. Under the combined action of tidal interaction with the cluster potential and of ram pressure by the intra-cluster medium the morphologies and star
formation histories of these two galaxies are strongly perturbed. While in the massive system tidal interactions are the dominant effect and are able to produce a sinking of gas towards the galaxy center triggering a strong burst of star formation and changing galaxy’s morphology, in the smaller galaxy the effects of gravitation are reduced by ram pressure stripping which blows away the neutral hydrogen from the galactic disk, quenching the star formation activity and transforming a gas rich late type spiral into quiescent disk dominated early type system.This result is a new additional evidence that galaxy mass represents the main driver of galaxy evolution, even during their dive into the harsh cluster environment and can give additional insights on the origin of S0s and dwarf cluster galaxies.
TL;DR: In this article, the authors construct merger trees for galaxies identified in a cosmological hydrodynamic simulation and use them to characterize predicted merger rates as a function of redshift, galaxy mass, and merger mass ratio.
Abstract: We construct merger trees for galaxies identified in a cosmological hydrodynamic simulation and use them to characterize predicted merger rates as a function of redshift, galaxy mass, and merger mass ratio. At z = 0.3, we find a mean rate of 0.054 mergers per galaxy per Gyr above a 1 : 2 mass ratio threshold for massive galaxies (baryonic mass above 6.4 × 1010 M☉), but only 0.018 Gyr-1 for lower mass galaxies. The mass ratio distribution is ∝R for the massive galaxy sample, so high-mass mergers dominate the total merger growth rate. The predicted rates increase rapidly with increasing redshift, and they agree reasonably well with observational estimates. A substantial fraction of galaxies do not experience any resolved mergers during the course of the simulation, and even for the high-mass sample, only 50% of galaxies experience a greater than 1 : 4 merger since z = 1. Typical galaxies thus have fairly quiescent merger histories. We assign bulge-to-disk ratios to simulated galaxies by assuming that mergers above a mass ratio threshold Rmajor convert stellar disks into spheroids. With Rmajor values of 1 : 4, we obtain a fairly good match to the observed dependence of the early-type fraction on galaxy mass. However, the predicted fraction of truly bulge-dominated systems (fbulge > 0.8) is small, and producing a substantial population of bulge-dominated galaxies may require a mechanism that shuts off gas accretion at late times and/or additional processes (besides major mergers) for producing bulges.
TL;DR: In this article, the photometric and structural properties of spectroscopically confirmed members in the two massive X-ray-selected z ≈ 0.83 galaxy clusters MS 1054-03 and RX J0152.7-1357 were studied.
Abstract: We study the photometric and structural properties of spectroscopically confirmed members in the two massive X-ray‐selected z ≈ 0.83 galaxy clusters MS 1054‐03 and RX J0152.7‐1357 using three-band mosaic imaging with the Hubble Space TelescopeAdvanced Camera for Surveys. The samples include 105 and 140 members of RX J0152.7‐1357 and MS 1054‐03, respectively, with ACS F775W magnitude i775 < 24.0. We fit the 2-D galaxy light profiles to determine effective radii and Se rsic indices; deviations from the smooth profiles are quantified by the ratio of the rms residuals to the mean of t he galaxy model. Galaxies are then classified according to a combination of this rms/mean ratio and the Sersic index; the resulting classes correlate well with visually classified morphological types, but are less affec ted by orientation. We find the size‐surface brightness relations in the two clusters to be very similar, supporting recent results on the evolution of this relationship with redshift. We examine in detail the color‐magnitude relations in these clusters and systematic effects on the residuals with respect to these relations. The color-ma gnitude residuals correlate with the local density, as measured from both galaxy numbers and weak lensing. These correlations are strongest for the full galaxy samples (commensurate with the morphology‐density relation), but are also present at lower significance levels for the early- and late-type samples individually. Weaker correlations are found with cluster radius, resulting from the more fundamental dependence on local density. We identify a threshold surface mass density of � ≈ 0.1, in units of the critical density, above which there are rel atively few blue (star-forming) galaxies. In RX J0152.7‐1357, there is an offset of 0.006 ± 0.002 in the mean redshifts of the early- and late-type galaxies, which produces a trend in the color residuals with velocity and may result from an infalling foreground association of late-type galaxies. Comparison of the color‐color diagrams for these clusters to stellar population models implies that a range of star formation time-scales are needed to reproduce the locus of galaxy colors. We also identify two galaxies, one in each cluster, whose colors can only be explained by large amounts, AV ∼ 1 mag, of internal dust extinction. Converting to rest-frame bandpasses, we find elliptical galaxy color scatters of 0.03 ± 0.01 mag in (U-B) and 0.07 ± 0.01 mag in (U-V), indicating mean ages of ∼ 3.5 Gyr, similar to the estimates from the mean colors and corresponding to formation at z ≈ 2.2. Thus, when the universe was half its present age, cluster ellipticals were half the age of the universe at that epoch; the same is coincidentally true of the median ages of ellipticals today. However, the most massive local cluster ellipticals have ages & 10 Gyr, consistent with our results for their likely progenitors at z & 0.8.
TL;DR: In this paper, the luminosity functions (LFs) of a subsample of 69 clusters from the RASS-SDSS galaxy cluster catalog were analyzed for early-and late-type galaxies.
Abstract: We analyze the Luminosity Functions (LFs) of a subsample of 69 clusters from the RASS-SDSS galaxy cluster catalog When calculated within the cluster physical sizes, given by r 200 or r 500 , all the cluster LFs appear to have the same shape, well fitted by a composite of two Schechter functions with a marked upturn and a steepening at the faint-end Previously reported cluster-to-cluster variations of the LF faint-end slope are due to the use of a metric cluster aperture for computing the LF of clusters of different masses We determine the composite LF for early- and late-type galaxies, where the typing is based on the galaxy u - r colors The late-type LF is well fitted by a single Schechter function with a steep slope (α = -20 in the r band, within r 200 ) The early-type LF instead cannot be fitted by a single Schechter function, and a composite of two Schechter functions is needed The faint-end upturn of the global cluster LF is due to the early-type cluster galaxies The shape of the bright-end tail of the early-type LF does not seem to depend upon the local galaxy density or the distance from ihe cluster center The late-type LF shows a significant variation only very near the cluster center On the other hand, the faint-end tail of the early-type LF shows a significant and continuous variation with the environment We provide evidence that the process responsible for creating the excess population of dwarf early type galaxies in clusters is a threshold process that occurs when the density exceeds ∼500 times the critical density of the Universe We interpret our results in the context of the harassment scenario where faint early-type cluster galaxies are predicted to be the descendants of tidally-stripped late-type galaxies
TL;DR: In this article, the authors measured the redshift-space luminosity-weighted or "marked" correlation function in the Sloan Digital Sky Survey (SDSS) and compared it with a model in which the luminosity function and luminosity dependence of clustering are the same as that observed, and the form of luminosity weighted correlation function is entirely a consequence of the fact that massive halos populate dense regions.
Abstract: We present measurements of the redshift-space luminosity-weighted or ‘marked’ correlation function in the Sloan Digital Sky Survey (SDSS). These are compared with a model in which the luminosity function and luminosity dependence of clustering are the same as that observed, and in which the form of the luminosity-weighted correlation function is entirely a consequence of the fact that massive haloes populate dense regions. We do this by using mock catalogues which are constrained to reproduce the observed luminosity function and the luminosity dependence of clustering, as well as by using the language of the redshift-space halo model. These analyses show that marked correlations may show a signal on large scales even if there are no large-scale physical effects – the statistical correlation between haloes and their environment will produce a measurable signal. Our model is in good agreement with the measurements, indicating that the halo mass function in dense regions is top heavy; the correlation between halo mass and large-scale environment is the primary driver for correlations between galaxy properties and environment; and the luminosity of the central galaxy in a halo is different from (in general, brighter than) that of the other objects in the halo. Thus our measurement provides strong evidence for the accuracy of these three standard assumptions of galaxy formation models. These assumptions also form the basis of current halo-model-based interpretations of galaxy clustering.
When the same galaxies are weighted by their u-, g- or r-band luminosities, then the marked correlation function is stronger in the redder bands. When the weight is galaxy colour rather than luminosity, then the data suggest that close pairs of galaxies tend to have redder colours. This wavelength dependence of marked correlations is in qualitative agreement with galaxy formation models, and reflects the fact that the mean luminosity of galaxies in a halo depends more strongly on halo mass in the r-band than in u. The luminosity and colour dependence we find are consistent with models in which the galaxy population in clusters is more massive than the population in the field. If the u-band luminosity is a reliable tracer of star formation, then our results suggest that cluster galaxies have lower star formation rates. The virtue of this measurement of environmental trends is that it does not require classification of galaxies into field, group and cluster environments.