The K-band Hubble diagram for the brightest cluster galaxies: a test of hierarchical galaxy formation models

We analyse the K -band Hubble diagram for a sample of brightest cluster galaxies (BCGs) in the redshift range 0< z <1. In good agreement with earlier studies, we confirm that the scatter in the absolute magnitudes of the galaxies is small (0.3 mag). The BCGs exhibit very little luminosity evolution in this redshift range: if q ₀=0.0, we detect no luminosity evolution; for q ₀=0.5, we measure a small negative evolution (i.e., BCGs were about 0.5 mag fainter at z =1 than today). If the mass in stars of these galaxies had remained constant over this period of time, substantial positive luminosity evolution would be expected: BCGs should have been brighter in the past, since their stars were younger. A likely explanation for the observed zero or negative evolution is that the stellar mass of the BCGs has been assembled over time through merging and accretion, as expected in hierarchical models of galaxy formation. The colour evolution of the BCGs is consistent with that of an old stellar population ( z _for>2) that is evolving passively. We can thus use evolutionary population synthesis models to estimate the rate of growth in stellar mass for these systems. We find that the stellar mass in a typical BCG has grown by a factor ≃2 since z ≃1 if q ₀=0.0, or by factor ≃4 if q ₀=0.5. These results are in good agreement with the predictions of semi-analytic models of galaxy formation and evolution set in the context of a hierarchical scenario for structure formation. The models predict a scatter in the luminosities of the BCGs that is somewhat larger than the observed one, but that depends on the criterion used to select the model clusters.     

The evolution of [O ii] emission from cluster galaxies

We investigate the evolution of the star formation rate in cluster galaxies. We complement data from the Canadian Network for Observational Cosmology 1 (CNOC1) cluster survey  (0.15 < z < 0.6)  with measurements from galaxy clusters in the Two-degree Field (2dF) galaxy redshift survey  (0.05 < z < 0.1)  and measurements from recently published work on higher-redshift clusters, up to almost   z = 1  . We focus our attention on galaxies in the cluster core, i.e. galaxies with   r < 0.7  h ⁻¹₇₀ Mpc  . Averaging over clusters in redshift bins, we find that the fraction of galaxies with strong [O  ii ] emission is ≲20 per cent in cluster cores, and the fraction evolves little with redshift. In contrast, field galaxies from the survey show a very strong increase over the same redshift range. It thus appears that the environment in the cores of rich clusters is hostile to star formation at all the redshifts studied. We compare this result with the evolution of the colours of galaxies in cluster cores, first reported by Butcher and Oemler. Using the same galaxies for our analysis of the [O  ii ] emission, we confirm that the fraction of blue galaxies, which are defined as galaxies 0.2 mag bluer in the rest-frame B – V than the red sequence of each cluster, increases strongly with redshift. Because the colours of galaxies retain a memory of their recent star formation history, while emission from the [O  ii ] line does not, we suggest that these two results can best be reconciled if the rate at which the clusters are being assembled is higher in the past, and the galaxies from which it is being assembled are typically bluer.     

The Epochs of Early-Type Galaxy Formation

By means of population synthesis models with variable α/Fe ratios we derive average ages, metallicities, and [α/Fe] element enhancements for a sample of 126 field and cluster early-type galaxies. We find a clear positive relation between [α/Fe] and velocity dispersion. Zero-point, slope, and scatter of this correlation are the same for cluster and field galaxies. In particular, the [α/Fe] ratios and mean ages of cluster ellipticals are positively correlated. This strongly reinforces the view that the [α/Fe] element enhancement in ellipticals is produced by star formation timescales rather than by variations of the initial mass function. These results indicate that the more massive the galaxy, the shorter is its star formation timescale, and the higher is the redshift of the bulk of star formation. This finding is not compatible with the predictions from models of hierarchical galaxy formation. The lenticular and field galaxies of the investigated sample do not follow the correlation between age and [α/Fe], but contain a non-negligible fraction of galaxies with young average ages and high [α/Fe] ratios. This revised version was published online in September 2006 with corrections to the Cover Date.     

The Colour–Magnitude Relation of Early-Type Galaxies in the Hubble Deep Field North

We present the rest-frame colour–magnitude diagram for 35 early-type galaxies in the Hubble Deep Field with median redshift 0.9.Although with considerable scatter, a red sequence well described by the passive evolution of an intrinsically old stellar population is observed. Comparison with the passively evolved colour–magnitude relation of the rich Coma Cluster (z = 0.023) indicates that at least ∼1/2 of the early-type galaxies in the field at this redshift are as old as those in rich clusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Intrinsic Redshifts and the Tully–Fisher Distance Scale

The Tully–Fisher relationship (TFR) has been shown to have a relatively small observed scatter of ∼±0.35 mag implying an intrinsic scatter < ±0.30 mag. However, when the TFR is calibrated from distances derived from the Hubble relation for field galaxies scatter is consistently found to be ±0.64 to ±0.84 mag. This significantly larger scatter requires that intrinsic TFR scatter is actually much larger than ±0.30 mag, that field galaxies have an intrinsic TFR scatter much larger than cluster spirals, or that field galaxies have a velocity dispersion relative to the Hubble flow in excess of 1000 km s⁻¹. Each of these potential explanations faces difficulties and contradicted by available data and the results of previous studies. An alternative explanation is that the measured redshifts of galaxies are composed of a cosmological redshift component predicted from the value of the Hubble constant and a superimposed intrinsic redshift component previously identified in other studies. This intrinsic redshift component may exceed 5000 km s⁻¹ in individual galaxies. In this alternative scenario a possible value for the Hubble constant is 55–60 km s⁻¹ Mpc⁻¹.     

Evolution of Quasar Host Galaxies

Near-IR images, obtained at the ESO-VLT during excellent seeing conditions, of a sample of 17 radio-loud and radio-quiet quasars in the redshift range 1 < z < 2 are presented. The host galaxies of both types of quasars appear to follow the expected trend in luminosity of massive ellipticals undergoing simple passive evolution. We find a systematic difference by a factor ∼2 in the host luminosity between RLQs and RQQs that does not change significantly from z = 2 to the present epoch. Quasar hosts appear thus to be already well formed at z ∼ 2 and similar to massive inactive spheroids. These findings are in disagreement with the predictions of models for the joint formation and evolution of galaxies and active nuclei based on the hierarchical structure formation scenario.     

The ages and metallicities of ellipticals from continuum colors

We present the results of a narrow band imaging project of dwarf and giant ellipticals in the Fornax (z = 0.01), Coma (z = 0.02), A2218 (z= 0.17) and A2125 (z = 0.24) cluster. Differing from spectral line projects, we determine the mean age and metallicity of the underlying stellar populations in galaxies by measurement of the position of the RGB and MS turnoff through continuum colors (3500 Å, 4100 Å, 4675 Å and5500 Å in rest system, i.e. modified Strømgren colors). Our sample includes 120 galaxies between M = –16 and –23 in Fornax and Coma plus over 300 galaxies in distant clusters. We find the color-magnitude relation to be linear for only the brightest galaxies with an increasing amount of scatter for low luminosity ellipticals. Bright ellipticals are found to have a metallicity between –0.5 and +0.5, but low luminosity ellipticals have values that range from –2 to solar. Our age index finds a weak correlation between luminosity and mean stellar age in ellipticals such that bright ellipticals are 2 to 3 Gyrs younger than low luminosity ellipticals.     

The stellar content of brightest cluster galaxies

We present near-infrared K -band spectroscopy of 21 elliptical or cD brightest cluster galaxies (BCGs), for which we have measured the strength of the 2.293-μm CO stellar absorption feature. We find that the strength of this feature is remarkably uniform among these galaxies, with a smaller scatter in equivalent width than for the normal elliptical population in the field or clusters. The scatter for BCGs is 0.156 nm, compared with 0.240 nm for Coma cluster ellipticals, 0.337 nm for ellipticals from a variety of other clusters, and 0.422 nm for field ellipticals. We interpret this homogeneity as being the result of a greater age, or more uniform history of star formation in BCGs than in other ellipticals; only a small fraction of the scatter can be caused by metallicity variations, even in the BCGs. Notwithstanding the small scatter, correlations are found between CO strength and various galaxy properties, including R -band absolute magnitude, which could improve the precision of these galaxies as distance indicators in measurements of cosmological parameters and velocity flows.     

The evolution of the galaxy red sequence in simulated clusters and groups

N -body/hydrodynamical simulations of the formation and evolution of galaxy groups and clusters in a Λ cold dark matter (ΛCDM) cosmology are used in order to follow the building-up of the colour–magnitude relation in two clusters and in 12 groups. We have found that galaxies, starting from the more massive, move to the red sequence (RS) as they get aged over times and eventually set upon a 'dead sequence' (DS) once they have stopped their bulk star formation activity. Fainter galaxies keep having significant star formation out to very recent epochs and lie broader around the RS. Environment plays a role as galaxies in groups and cluster outskirts hold star formation activity longer than the central cluster regions. However, galaxies experiencing infall from the outskirts to the central parts keep star formation on until they settle on to the DS of the core galaxies. Merging contributes to mass assembly until z ∼ 1, after which major events only involve the brightest cluster galaxies.
The emerging scenario is that the evolution of the colour–magnitude properties of galaxies within the hierarchical framework is mainly driven by star formation activity during dark matter haloes assembly. Galaxies progressively quenching their star formation settle to a very sharp 'red and dead' sequence, which turns out to be universal, its slope and scatter being almost independent of the redshift (since at least z ∼ 1.5) and environment.
Differently from the DS, the operatively defined RS evolves more evidently with z , the epoch when it changes its slope being closely corresponding to that at which the passive galaxies population takes over the star-forming one: this goes from z ≃ 1 in clusters down to 0.4 in normal groups.     

The history of mass assembly of faint red galaxies in 28 galaxy clusters since z= 1.3

We measure the relative evolution of the number of bright and faint (as faint as  0.05 L *)  red galaxies in a sample of 28 clusters, out of which 16 are at  0.50 ≤ z ≤ 1.27  , all observed through a pair of filters bracketing the 4000-Å break rest frame. The abundance of red galaxies, relative to bright ones, is constant over all the studied redshift range,  0 < z < 1.3  , and rules out a differential evolution between bright and faint red galaxies as large as claimed in some past works. Faint red galaxies are largely assembled and in place at   z = 1.3  and their abundance does not depend on cluster mass, parametrized by velocity dispersion or X-ray luminosity. Our analysis, with respect to the previous one, samples a wider redshift range, minimizes systematics and put a more attention to statistical issues, keeping at the same time a large number of clusters.     

Optical properties of low z radio galaxies

The optical morphological and photometric properties of 79 low redshift radio galaxies are discussed. It is found that most radio galaxies are luminous bulge dominated systems similar to normal non-radio giant ellipticals. The average absolute magnitude of the sample is 〈M_HOST(tot)〉=−23.98, with a clear trend for FR I sources to be ∼0.5 mag brighter than FR II galaxies. In about 40% of the objects we find an excess of light in the nucleus attributable to the presence of a nuclear point source. This contributes on average for ∼1–2% of the total flux from the host galaxy. Radio galaxies follow the same μ_e−R_e relationship of normal (non-active) elliptical galaxies. The distribution of ellipticity, the amount of twisting and shape of isophotes do not differ significantly from other ellipticals. These results support a scenario where radio emission is little related with the overall properties of the host galaxy.     

The cluster environments of the z ∼ 1 3CR radio galaxies

An analysis of the environments around a sample of 28 3CR radio galaxies with redshifts 0.6< z <1.8 is presented, based primarily upon K -band images down to K ∼20 taken using the UK Infrared Telescope (UKIRT). A net overdensity of K -band galaxies is found in the fields of the radio galaxies, with the mean excess counts being comparable to that expected for clusters of Abell Class 0 richness. A sharp peak is found in the angular cross-correlation amplitude centred on the radio galaxies that, for reasonable assumptions about the luminosity function of the galaxies, corresponds to a spatial cross-correlation amplitude between those determined for low-redshift Abell Class 0 and 1 clusters.
These data are complemented by J -band images also from UKIRT, and by optical images from the Hubble Space Telescope . The fields of the lower redshift ( z ≲0.9) radio galaxies in the sample generally show well-defined near-infrared colour–magnitude relations with little scatter, indicating a significant number of galaxies at the redshift of the radio galaxy; the relations involving colours at shorter wavelengths than the 4000 Å break show considerably greater scatter, suggesting that many of the cluster galaxies have low levels of recent or on-going star formation. At higher redshifts the colour–magnitude sequences are less prominent owing to the increased field galaxy contribution at faint magnitudes, but there is a statistical excess of galaxies with the very red infrared colours ( J − K ≳1.75) expected of old cluster galaxies at these redshifts.
Although these results are appropriate for the mean of all of the radio galaxy fields, there exist large field-to-field variations in the richness of the environments. Many, but certainly not all, powerful z ∼1 radio galaxies lie in (proto)cluster environments.     

The galaxy environment of a quasar at z= 1.226: a possible cluster merger

We have conducted ultra-deep optical and deep near-infrared observations of a field around the z =1.226 radio-quiet quasar 104420.8+055739 from the Clowes–Campusano LQG of 18 quasars at z ∼1.3, in search of associated galaxy clustering. Galaxies at these redshifts are distinguished by their extremely red colours, with I − K >3.75, and we find a factor ∼11 overdensity of such galaxies in a 2.25×2.25 arcmin² field centred on the quasar. In particular, we find 15–18 galaxies that have colours consistent with being a population of passively evolving massive ellipticals at the quasar redshift. They form 'fingers' in the V − K K , I − K K colour–magnitude plots at V − K ≃6.9, I − K ≃4.3 comparable to the red sequences observed in other z ≃1.2 clusters. We find suggestive evidence for substructure among the red sequence galaxies in the K image, in the form of two compact groups, 40 arcsec to the north, and 60 arcsec to the south-east of the quasar. An examination of the wider optical images indicates that this substructure is significant, and that the clustering extends to form a large-scale structure 2–3  h ⁻¹ Mpc across. We find evidence for a high (≳50 per cent) fraction of blue galaxies in this system, in the form of 15–20 'red outlier' galaxies with I − K >3.75 and V − I <2.00, which we suggest are dusty, star-forming galaxies at the quasar redshift. Within 30 arcsec of the quasar we find a concentration of blue ( V − I <1) galaxies in a band that bisects the two groups of red sequence galaxies. This band of blue galaxies is presumed to correspond to a region of enhanced star formation. We explain this distribution of galaxies as the early stages of a cluster merger which has triggered both the star formation and the quasar.     

Spectroscopic constraints on the stellar population of elliptical galaxies in the Coma cluster

Near-infrared spectra for a sample of 31 elliptical galaxies in the Coma cluster are obtained. The galaxies are selected to be ellipticals (no lenticulars), with a large spatial distribution, covering both the core and outskirts of the cluster (i.e. corresponding to regions with large density contrasts). CO_sp (2.3-μm) absorption indices, measuring the contribution from intermediate-age red giant and supergiant stars to the near-infrared light of the ellipticals, are then estimated.
It is found that the strength of CO_sp features in elliptical galaxies increases from the core ( r <02) to the outskirts ( r >02) of the Coma cluster. Using the Mg₂ strengths, it is shown that the observed effect is not caused by metallicity and is mostly caused by the presence of a younger population (giant and supergiant stars) in ellipticals in the outskirts (low-density region) of the cluster.
Using the CO_sp features, the origin of the scatter on the near-infrared Fundamental Plane (the relation between the effective diameter, effective surface brightness and velocity dispersion) of elliptical galaxies is studied. Correcting this relation for contributions from the red giant and supergiant stars, the rms scatter reduces from 0.077 to 0.073 dex. Although measurable, the contribution from these intermediate-age stars to the scatter on the near-infrared Fundamental Plane of ellipticals is only marginal.
A relation is found between the CO_sp and V − K colours of ellipticals, corresponding to a slope of 0.036±0.016, significantly shallower than that from the Mg₂–( V − K ) relation. This is studied using stellar synthesis models.     

The elliptical galaxy colour–magnitude relation as a discriminant between the monolithic and merger paradigms

The colour–magnitude relation (CMR) of cluster elliptical galaxies has been widely used to constrain their star formation histories (SFHs) and to discriminate between the monolithic collapse and merger paradigms of elliptical galaxy formation. We use a Λ cold dark matter hierarchical merger model of galaxy formation to investigate the existence and redshift evolution of the elliptical galaxy CMR in the merger paradigm. We show that the SFH of cluster ellipticals predicted by the model is quasi-monolithic , with only ∼10 per cent of the total stellar mass forming after   z ∼ 1  . The quasi-monolithic SFH results in a predicted CMR that agrees well with its observed counterpart in the redshift range  0 < z < 1.27  . We use our analysis to argue that the elliptical-only CMR can be used to constrain the SFHs of present-day cluster ellipticals only if we believe a priori in the monolithic collapse model. It is not a meaningful tool for constraining the SFH in the merger paradigm, since a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star-forming systems at higher redshift, which cannot be ignored when deriving the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between the two competing theories of elliptical galaxy evolution.     

The evolution of 3CR radio galaxies from z1

We present the results of a comprehensive re-analysis of the images of a virtually complete sample of 28 powerful 3CR radio galaxies with redshifts 0.6< z <1.8 from the Hubble Space Telescope ( HST ) archive. Using a two-dimensional modelling technique we have derived scalelengths and absolute magnitudes for a total of 16 3CR galaxies with a median redshift of z =0.8. Our results confirm the basic conclusions of Best, Longair & Röttgering in that we also find z =1 3CR galaxies to be massive, well-evolved ellipticals, the infrared emission of which is dominated by starlight. However, we in fact find that the scalelength distribution of 3CR galaxies at z ≃1 is completely indistinguishable from that derived for their low-redshift counterparts from our own recently completed HST study of active galactic nuclei hosts at z ≃0.2. There is thus no evidence that 3CR radio galaxies at z ≃1 are dynamically different from 3CR galaxies at low redshift. Moreover, for a 10-object subsample we have determined the galaxy parameters with sufficient accuracy to demonstrate, for the first time, that the z ≃1 3CR galaxies follow a Kormendy relation that is indistinguishable from that displayed by low-redshift ellipticals if one allows for purely passive evolution. The implied rather modest level of passive evolution since z ≃1 is consistent with that predicted from spectrophotometric models provided one assumes a high formation redshift ( z ≥4) within a low-density universe. We conclude that there is no convincing evidence for significant dynamical evolution among 3CR galaxies in the redshift interval 0< z <1, and that simple passive evolution remains an acceptable interpretation of the K – z relation for powerful radio galaxies.     

The evolution of active galaxies in clusters

The results of deep radio, sub-mm and X-ray observations of samples of high redshift (z∼1) clusters are presented. These reveal significant excesses of active galaxies associated with the clusters at all three wavelengths. The cluster radio source population shows evolution consistent with the (1+z)³ evolution typical of many AGN classes. A large fraction of the AGN are hosted by apparently passive early-type galaxies, often with a close companion. These results essentially constitute the detection of a counterpart of the Butcher-Oemler effect for both strongly star bursting galaxies and AGN. This revised version was published online in July 2006 with corrections to the Cover Date.     

Galactic bulges from Hubble Space Telescope NICMOS observations: ages and dust

We present optical and near-infrared colour maps of the central regions of bulges of S0 and spiral galaxies obtained with WFPC2 and NICMOS on the Hubble Space Telescope ( HST ). By combined use of HST and ground-based data, the colour information spans a region from a few tens of pc to a few kpc. In almost all galaxies, the colour profiles in the central 100–200 pc become more rapidly redder. We attribute the high central colour indices to a central concentration of dust. We infer an average extinction at the centre of A _V =0.6–1.0 mag. Several objects show central dust rings or discs at subkpc scales similar to those found by others in giant ellipticals. For galactic bulges of types S0 to Sb, the tightness of the B − I versus I − H relation suggests that the age spread among bulges of early-type galaxies is small, at most 2 Gyr. Colours at 1 R _eff, where we expect extinction to be negligible, are similar to those of elliptical galaxies in the Coma cluster, suggesting that these bulges formed at the same time as the bright galaxies in Coma. Furthermore, the galaxy ages are found to be independent of their environment. As it is likely that Coma was formed at redshift z >3, our bulges, which are in groups and in the field, must also have been formed at this epoch. Bulges of early-type spirals cannot be formed by secular evolution of bars at recent epochs, because such bulges would be much younger. There are three galaxies of type Sbc and later; their bulges are younger and could perhaps arise from secular evolution of transient bars. Our results are in good agreement with semi-analytic predictions, which also predict that bulges, in clusters and in the field, are as old as giant ellipticals in clusters.     

Compact groups in theory and practice – II. Comparing the observed and predicted nature of galaxies in compact groups

We examine the properties of galaxies in compact groups (CGs) identified in a mock galaxy catalogue based upon the Millennium Run simulation. The overall properties of groups identified in projection are in general agreement with the best available observational constraints. However, only ∼30 per cent of these simulated groups are found to be truly compact in three dimensions, suggesting that interlopers strongly affect our observed understanding of the properties of galaxies in CGs. These simulations predict that genuine CG galaxies are an extremely homogeneous population, confined nearly exclusively to the red sequence: they are best described as 'red and dead' ellipticals. When interlopers are included, the population becomes much more heterogeneous, due to bluer, star-forming, gas-rich, late-type galaxies incorrectly identified as CG members. These models suggest that selection of members by redshift, such that the line-of-sight velocity dispersion of the group is less than 1000 km s⁻¹, significantly reduces contamination to the 30 per cent level. Selection of members by galaxy colour, a technique used frequently for galaxy clusters, is also predicted to dramatically reduce contamination rates for CG studies.     

Starburst Galaxies in Clusters

The nature of the starburst phenomenon in galaxies is investigated using a narrow-band colour system designed to study colour evolution in distant clusters. Work on zero redshift, luminous far-IR galaxies, calibrated by starburst models, demonstrates the usefulness of this colour system in isolating starburst from normal star-forming colours, and also demonstrates a strong correlation with far-IR colours despite reddening effects. The same colour system applied to distant clusters finds that a majority of the faint blue cluster population are starburst dwarf galaxies, probably the progenitors of the current population of dwarf ellipticals in nearby clusters. This revised version was published online in July 2006 with corrections to the Cover Date.