The evolution of the brightest cluster galaxies sincez∼ 1 from the ESO Distant Cluster Survey (EDisCS) (original) (raw)
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The bright end of the colour-magnitude relation of cluster galaxies
Monthly Notices of The Royal Astronomical Society, 2011
We investigate the physical processes involved in the development of the red sequence (RS) of cluster galaxies by using a combination of cosmological N-body simulations of clusters of galaxies and a semi-analytic model of galaxy formation. Results show good agreement between the general trend of the simulated RS and the observed colour-magnitude relation (CMR) of early-type galaxies in different magnitude planes. However, in many clusters, the most luminous galaxies (?) depart from the linear fit to observed data, as traced by less luminous ones, displaying almost constant colours. With the aim of understanding this particular behaviour of galaxies in the bright end of the RS, we analyse the dependence with redshift of the fraction of stellar mass contributed to each galaxy by different processes, i.e. quiescent star formation, and starbursts triggered by disc instability and merger events. We find that the evolution of galaxies in the bright end since z≈ 2 is mainly driven by minor and major dry mergers, while minor and major wet mergers are relevant in determining the properties of less luminous galaxies. Since the most luminous galaxies have a narrow spread in ages (1.0 × 1010 yr < t < 1.2 × 1010 yr), their metallicities are the main factor that affects their colours. Their mean iron abundances are close to the solar value and have already been reached at z≈ 1. This fact is consistent with several observational evidence that favour a scenario in which both the slope and scatter of the CMR are in place since z≈ 1.2. Galaxies in the bright end reach an upper limit in metallicity as a result of the competition of the mass of stars and metals provided by the star formation occurring in the galaxies themselves and by the accretion of merging satellites. Star formation activity in massive galaxies (stellar mass M★≳ 1010 M⊙) that takes place at low redshifts contributes with stellar components of high metallicity, but the fraction of stellar mass contributed since z≈ 1 is negligible with respect to the total mass of the galaxy at z= 0. On the other hand, mergers contribute with a larger fraction of stellar mass (≈10-20 per cent), but the metallicity of the accreted satellites is lower by ≈0.2 dex than the mean metallicity of galaxies they merge with. The effect of dry mergers is to increase the mass of galaxies in the bright end, without significantly altering their metallicities. Hence, very luminous galaxies present similar colours that are bluer than those expected if recent star formation activity were higher, thus giving rise to a break in the RS. These results are found for simulated clusters with different virial masses (1014- 1015 h-1 M⊙), supporting the idea of the universality of the CMR in agreement with observational results.
The build-up of the colour-magnitude relation in galaxy clusters since z 0.8
Monthly Notices of the Royal Astronomical Society, 2007
Using galaxy clusters from the ESO Distant Cluster Survey, we study how the distribution of galaxies along the colour-magnitude relation has evolved since z ∼ 0.8. While red-sequence galaxies in all these clusters are well described by an old, passively evolving population, we confirm our previous finding of a significant evolution in their luminosity distribution as a function of redshift. When compared to galaxy clusters in the local Universe, the high redshift EDisCS clusters exhibit a significant deficit of faint red galaxies. Combining clusters in three different redshift bins, and defining as 'faint' all galaxies in the range 0.4 L/L * 0.1, we find a clear decrease in the luminous-to-faint ratio of red galaxies from z ∼ 0.8 to z ∼ 0.4. The amount of such a decrease appears to be in qualitative agreement with predictions of a model where the blue bright galaxies that populate the colour-magnitude diagram of high redshift clusters, have their star formation suppressed by the hostile cluster environment. Although model results need to be interpreted with caution, our findings clearly indicate that the red-sequence population of high-redshift clusters does not contain all progenitors of nearby red-sequence cluster galaxies. A significant fraction of these must have moved onto the red-sequence below z ∼ 0.8.
The K-band Hubble diagram for brightest cluster galaxies in X-ray clusters
Monthly Notices of the Royal Astronomical Society, 1998
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 magnitudes). 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 magnitudes 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 f orm > 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.
Revisiting Brightest Cluster Galaxy Evolution with the Las Campanas Distant Cluster Survey
The Astrophysical Journal, 2002
We investigate the influence of environment on brightest cluster galaxy (BCG) evolution using a sample of 63 clusters at 0.3 ≤ z ≤ 0.9 drawn primarily from the Las Campanas Distant Cluster Survey and follow-up V , I, and K ′ photometry. The luminosity evolution of the entire BCG sample is not adequately described by a single evolutionary model. Using the integrated light from the cluster detection as a proxy for cluster L x and the suggestion by Burke, Collins, & Mann, we set L x = 2 × 10 44 ergs s −1 to be the division between high and low luminosity clusters. At high redshift (z > 0.6) BCGs from low-L x clusters are fainter, on average, than those from high-L x clusters and are best modeled as having constant luminosity with redshift. The BCGs from high-L x cluster are best modeled as having a stellar population that formed at large redshift (z f orm > 5) and is passively evolving. However, for the entire BCG population, the observed V − I and I−K ′ colors are well described by a single evolutionary model in which the stellar populations have z f orm > 5 and subsequently passively evolve. We conclude that accretion is proportionally more significant for BCGs in lower mass clusters at these redshifts (factor of 2 to 4 increase in mass since z ∼ 1 for the low L x systems; Aragon-Salamanca et al. ) and that the accreted matter is in the form of systems with evolved stellar populations.
The Astrophysical Journal, 2015
The mass and structural evolution of massive galaxies is one of the hottest topics in galaxy formation. This is because it may reveal invaluable insights into the still debated evolutionary processes governing the growth and assembly of spheroids. However, direct comparison between models and observations is usually prevented by the so-called progenitor bias, i.e., new galaxies entering the observational selection at later epochs, thus eluding a precise study of how pre-existing galaxies actually evolve in size. To limit this effect, we here gather data on high-redshift brightest group and cluster galaxies, evolve their (mean) host halo masses down to z = 0 along their main progenitors, and assign as their "descendants" local Sloan Digital Sky Survey central galaxies matched in host halo mass. At face value, the comparison between high redshift and local data suggests a noticeable increase in stellar mass of a factor of 2 since z ∼ 1, and of 2.5 in mean effective radius. We then compare the inferred stellar mass and size growth with those predicted by hierarchical models for central galaxies, selected at high redshifts to closely match the halo and stellar mass bins as in the data. Only hierarchical models characterized by very limited satellite stellar stripping and parabolic orbits are capable of broadly reproducing the stellar mass and size increase of a factor of ∼2-4 observed in cluster galaxies since z ∼ 1. The predicted, average (major) merger rate since z ∼ 1 is in good agreement with the latest observational estimates.
Stellar population gradients in brightest cluster galaxies
Monthly Notices of the Royal Astronomical Society, 2012
We present the stellar population and velocity dispersion gradients for a sample of 24 brightest cluster galaxies (BCGs) in the nearby Universe for which we have obtained high quality long-slit spectra at the Gemini telescopes. With the aim of studying the possible connection between the formation of the BCGs and their host clusters, we explore the relations between the stellar population gradients and properties of the host clusters as well as the possible connections between the stellar population gradients and other properties of the galaxies. We find mean stellar population gradients (negative ∆[Z/H]/log r gradient of-0.285±0.064; small positive ∆log (age)/log r gradient of 0.069±0.049; and null ∆[E/Fe]/log r gradient of-0.008±0.032) that are consistent with those of normal massive elliptical galaxies. However, we find a trend between metallicity gradients and velocity dispersion (with a negative slope of-1.616±0.539) that is not found for the most massive ellipticals. Furthermore, we find trends between the metallicity gradients and K-band luminosities (with a slope of 0.173±0.081) as well as the distance from the BCG to the X-ray peak of the host cluster (with a slope of-7.546±2.752). The latter indicates a possible relation between the formation of the cluster and that of the central galaxy.
Evolution of the Hubble sequence in hierarchical models for galaxy formation
Monthly Notices of the Royal Astronomical Society, 1996
We present a model for the broad morphological distinction between the disk and spheroidal components of galaxies. Elaborating on the hierarchical clustering scheme of galaxy formation proposed by Cole et al, we assume that galaxies form stars quiescently in a disk until they are disrupted into a spheroidal configuration by mergers. Bulges and spheroids may continue to accrete gas from their hot coronae, and so they may grow disks again. Thus, an individual galaxy may pass through various phases of disk or spheroid dominance during its lifetime. To distinguish between disks and spheroids we add one additional free parameter to the semianalytic model of Cole et al. which we fix by requiring that the predicted morphological mix should match that observed locally. Assuming an Ω = 1, standard cold dark matter cosmology, we calculate formation and merging histories, and the evolution in colour, luminosity and morphology of the galaxy populations in different environments. Our model predicts that the bulges of spirals were assembled before the spheroids of ellipticals and the spheroids of cluster ellipticals were assembled before those of field ellipticals. About 50% of ellipticals, but only about 15% of spirals, have undergone a major merger during the redshift interval 0.0 ≤ z ≤ 0.5. In spite of their violent formation history, elliptical galaxies turn out to have colourmagnitude diagrams with remarkably small scatter. Apart from a general blueing of the galaxy population with redshift, the colour-magnitude diagrams are remarkably similar at redshift z = 0.5 and at the present day. The morphological mix of galaxies that become rich cluster members at high redshift is dominated by spiral galaxies, due to the long timescale for galaxy mergers compared with the timescale for cluster assembly at high redshift. The assembly of low redshift clusters is slower, allowing more galaxy mergers to occur in the progenitor halos. As a result z = 0 rich clusters become E/S0 dominated and we find a "Butcher-Oemler" effect that becomes weaker for poorer groups at high redshift. The field luminosity function of red galaxies shows little evolution out to z ≃ 1 and the reddest galaxies at these redshifts are as bright as their local counterparts. The blue luminosity function, on the other hand, evolves rapidly with redshift, increasing its characteristic luminosity and becoming steeper at the faint end. These trends are similar to those recently observed in the Canada-France Redshift Survey. Our calculations serve to demonstrate that a simple prescription for the distinction between disks and spheroids that is compatible with hierarchical clustering goes a long way towards explaining many of the systematic trends observed in the galaxy population.
Astronomy and Astrophysics, 2006
We constrain the evolution of the galaxy mass and luminosity functions from the analysis of (public) multi-wavelength data in the Chandra Deep Field South (CDFS) area, obtained from the GOODS and other projects, and including very deep high-resolution imaging by HST/ACS. Our reference catalogue of faint high-redshift galaxies, which we have thoroughly tested for completeness and reliability, comes from a deep (S3.6 ≥ 1 µJy) image by IRAC on the Spitzer Observatory. These imaging data in the field are complemented with extensive optical spectroscopy by the ESO VLT/FORS2 and VIMOS spectrographs, while deep K-band VLT/ISAAC imaging is also used to derive further complementary statistical constraints and to assist the source identification and SED analysis. We have selected a highly reliable IRAC 3.6µm sub-sample of 1478 galaxies with S3.6 ≥ 10 µJy, 47% of which have spectroscopic redshift, while for the remaining objects both COMBO-17 and Hyperz are used to estimate the photometric redshift. This very extensive dataset is exploited to assess evolutionary effects in the galaxy luminosity and stellar mass functions, while luminosity/density evolution is further constrained with the number counts and redshift distributions. The deep ACS imaging allows us to differentiate these evolutionary paths by morphological type, which our simulations show to be reliable at least up to z ∼ 1.5 for the two main early-(E/S0) and late-type (Sp/Irr) classes. These data, as well as our direct estimate of the stellar mass function above M * h 2 = 10 10 M⊙ for the spheroidal subclass, consistently evidence a progressive dearth of such objects to occur starting at z ∼ 0.7, paralleled by an increase in luminosity. A similar trend, with a more modest decrease of the mass function, is also shared by spiral galaxies, while the irregulars/mergers show an increased incidence at higher z. Remarkably, this decrease of the comoving density with redshift of the total population appears to depend on galaxy mass, being stronger for moderate-mass, but almost absent until z = 1.4 for high-mass galaxies, thus confirming previous evidence for a "downsizing" effect in galaxy formation. Our favoured interpretation of the evolutionary trends for the two galaxy categories is that of a progressive morphological transformation (due to gas exhaustion and, likely, merging) from the star-forming to the passively evolving phase, starting at z ≥ 2 and keeping on down to z ∼ 0.7. The rate of this process appears to depend on galaxy mass, being already largely settled by z ∼ 1 for the most massive systems.
Brightest Cluster Galaxies at the Present Epoch
The Astrophysical Journal, 2014
We have obtained photometry and spectroscopy of 433 z ≤ 0.08 brightest cluster galaxies (BCGs) in a full-sky survey of Abell clusters to construct a BCG sample suitable for probing deviations from the local Hubble flow. The BCG Hubble diagram over 0 < z < 0.08 is consistent to within 2% of the Hubble relation specified by a Ω m = 0.3, Λ = 0.7 cosmology. This sample allows us to explore the structural and photometric properties of BCGs at the present epoch, their location in their hosting galaxy clusters, and the effects of the cluster environment on their structure and evolution. We revisit the L m − α relation for BCGs, which uses α, the log-slope of the BCG photometric curve of growth, to predict the metric luminosity in an aperture with 14.3 kpc radius, L m , for use as a distance indicator. Residuals in the relation are 0.27 mag rms. We measure central stellar velocity dispersions, σ, of the BCGs, finding the Faber-Jackson relation to flatten as the metric aperture grows to include an increasing fraction of the total BCG luminosity. A 3-parameter "metric plane" relation using α and σ together gives the best prediction of L m , with 0.21 mag residuals. The distribution of projected spatial offsets, r x of BCGs from the X-ray-defined cluster center is a steep γ = −2.33 power-law over 1 < r x < 10 3 kpc. The median offset is ∼ 10 kpc, but ∼ 15% of the BCGs have r x > 100 kpc. The absolute cluster-dispersion normalized BCG peculiar velocity |∆V 1 |/σ c follows an exponential distribution with scale length 0.39 ± 0.03. Both L m and α increase with σ c. The α parameter is further moderated by both the spatial and velocity offset from the cluster center, with larger α correlated with the proximity of the BCG to the cluster mean velocity or potential center. At the same time, position in the cluster has little effect on L m. Likewise, residuals from the metric plane show no correlation with either the spatial or velocity offset from the cluster center. The luminosity difference between the BCG and second-ranked galaxy, M2, increases as the peculiar velocity of the BCG within the cluster decreases. Further, when M2 is a close luminosity "rival" of the BCG, the galaxy that is closest to either the velocity or X-ray center of the cluster is most likely to have the larger α. We conclude that the inner portions of the BCGs are formed outside the cluster, but interactions in the heart of the galaxy cluster grow and extend the envelopes of the BCGs.