arXiv subjects

Anja von der Linden

Publications and source records attributed to Anja von der Linden.

66 records · Page 4Linked to original sources

arXiv2009

Evolution of the Early-Type Galaxy Fraction in Clusters since z = 0.8

We study the morphological content of a large sample of high-redshift clusters to determine its dependence on cluster mass and redshift. Quantitative morphologies are based on bulge+disk decompositions of cluster and field galaxies on deep VLT/FORS2 images of 18 optically-selected clusters at 0.45 < z < 0.80 from the ESO Distant Cluster Survey (EDisCS). Morphological content is given by the early-type galaxy fraction f_et, and early-type galaxies are selected based on their bulge fraction and image smoothness. A set of 158 SDSS clusters is analyzed exactly as the EDisCS sample to provide a robust local comparison. Our main results are: (1) f_et values for the SDSS and EDisCS clusters exhibit no clear trend as a function of sigma. (2) Mid-z EDisCS clusters around sigma = 500 km/s have f_et ~= 0.5 whereas high-z EDisCS clusters have f_et ~= 0.4 (~25% increase over 2 Gyrs). (3) There is a marked difference in the morphological content of EDisCS and SDSS clusters. None of the EDisCS clusters have f_et greater than 0.6 whereas half of the SDSS clusters lie above this value. This difference is seen in clusters of all velocity dispersions. (4) There is a strong correlation between morphology and star formation in SDSS and EDisCS clusters. This correlation holds independent of sigma and z even though the fraction of [OII] emitters decreases from z~0.8 to z~0.06 in all environments. Our results pose an interesting challenge to structural transformation and star formation quenching processes that strongly depend on the global cluster environment and suggest that cluster membership may be of lesser importance than other variables in determining galaxy properties. (ABRIDGED)

astro-ph.CO↗

arXiv2009

Star formation and AGN activity in SDSS cluster galaxies

We investigate the recent and current star formation activity of galaxies as function of distance from the cluster center in a sample of 521 SDSS clusters at z<0.1. We show that when the BCGs are excluded from the galaxy sample, there is no evidence for mass segregation in the clusters, so that differences in cluster and field populations cannot simply be attributed to different mass functions. We find a marked star formation-radius relation in that almost all galaxies in the cluster core are quiescent, i.e. have terminated star formation a few Gyr ago. This star formation-radius relation is most pronounced for low-mass galaxies and is very weak or absent beyond the virial radius. The typical star formation rate of non-quiescent galaxies declines by approximately a factor of two towards the cluster center. However, the fraction of galaxies with young stellar populations indicating a recently completed starburst or a truncation of star formation does not vary significantly with radius. The fraction of star-forming galaxies which host a powerful optical AGN is also independent of clustercentric radius, indicating that the link between star formation and AGN in these galaxies operates independent of environment. The fraction of red galaxies which host a weak optical AGN decreases, however, towards the cluster center, with a similar timescale as the decline of star forming galaxies. Our results favor a scenario in which star formation is quenched slowly, on timescales similar to the cluster crossing time, and rule out significant contributions from more violent processes at least beyond cluster radii >0.1 R_200.

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arXiv2009

The UV-optical colours of brightest cluster galaxies in optically and X-ray selected clusters

Many brightest cluster galaxies (BCGs) at the centers of X-ray selected clusters exhibit clear evidence for recent star formation. However, studies of BCGs in optically-selected clusters show that star formation is not enhanced when compared to control samples of non-BCGs of similar stellar mass. Here we analyze a sample of 113 BCGs in low redshift (z<0.1), optically-selected clusters, a matched control sample of non-BCGs, and a smaller sample of BCGs in X-ray selected clusters. We convolve the SDSS images of the BCGs to match the resolution of the GALEX data and we measure UV-optical colours in their inner and outer regions. We find that optically-selected BCGs exhibit smaller scatter in optical colours and redder inner NUV-r colours than the control galaxies, indicating that they are a homogenous population with very little ongoing star formation. The BCGs in the X-ray selected cluster sample span a similar range in optical colours, but have bluer NUV-r colours. Among X-ray selected BCGs, those located in clusters with central cooling times of less than 1 Gyr are significantly bluer than those located in clusters where the central gas cooling times are long. Our main conclusion is that the location of a galaxy at the centre of its halo is not sufficient to determine whether or not it is currently forming stars. One must also have information about the thermodynamic state of the gas in the core of the halo.

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arXiv2009

The Rest-Frame Optical Luminosity Function of Cluster Galaxies at z<0.8 and the Assembly of the Cluster Red Sequence

We present the rest-frame optical luminosity function (LF) of red sequence galaxies in 16 clusters at 0.4<z<0.8 drawn from the ESO Distant Cluster Survey (EDisCS). We compare our clusters to an analogous sample from the Sloan Digital Sky Survey (SDSS) and match the EDisCS clusters to their most likely descendants. We measure all LFs down to M M* + (2.5 - 3.5). At z<0.8, the bright end of the LF is consistent with passive evolution but there is a significant build-up of the faint end of the red sequence towards lower redshift. There is a weak dependence of the LF on cluster velocity dispersion for EDisCS but no such dependence for the SDSS clusters. We find tentative evidence that red sequence galaxies brighter than a threshold magnitude are already in place, and that this threshold evolves to fainter magnitudes toward lower redshifts. We compare the EDisCS LFs with the LF of co-eval red sequence galaxies in the field and find that the bright end of the LFs agree. However, relative to the number of bright red galaxies, the field has more faint red galaxies than clusters at 0.6<z<0.8 but fewer at 0.4<z<0.6, implying differential evolution. We compare the total light in the EDisCS cluster red sequences to the total red sequence light in our SDSS cluster sample. Clusters at 0.4<z<0.8 must increase their luminosity on the red sequence (and therefore stellar mass in red galaxies) by a factor of 1-3 by z=0. The necessary processes that add mass to the red sequence in clusters predict local clusters that are over-luminous as compared to those observed in the SDSS. The predicted cluster luminosities can be reconciled with observed local cluster luminosities by combining multiple previously known effects.

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arXiv2009

The ESO Distant Cluster Sample: galaxy evolution and environment out to z=1

The ESO Distant Cluster Survey (EDisCS, P.I. Simon D.M. White, LP 166.A-0162) is an ESO large programme aimed at studying clusters and cluster galaxies at z=0.4-1. How different is the evolution of the star formation activity in clusters, in groups and in the field? Does it depend on cluster mass and/or the local galaxy density? How relevant are starburst and post-starburst galaxies in the different environments? Is there an evolution in the galaxies' structures, and if so, is this related to the changes in their star formation activity? These are some of the main questions that have been investigated using the EDisCS dataset.

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arXiv2008

The environments of starburst and post-starburst galaxies at z=0.4-0.8

Post-starburst (E+A or k+a) spectra, characterized by their exceptionally strong Balmer lines in absorption and the lack of emission lines, belong to galaxies in which the star formation activity ended abruptly sometime during the past Gyr. We perform a spectral analysis of galaxies in clusters, groups, poor groups and the field at z=0.4-0.8 based on the ESO Distant Cluster Survey. The incidence of k+a's at these redshifts depends strongly on environment. K+a's reside preferentially in clusters and, unexpectedly, in a subset of the sigma = 200-400 km/s groups, those that have a low fraction of [OII] emitters. In these environments, 20-30% of the recently star-forming galaxies have had their star formation activity recently truncated. In contrast, there are proportionally fewer k+a's in the field, the poor groups and groups with a high [OII] fraction. The incidence of k+a galaxies correlates with the cluster velocity dispersion: more massive clusters have higher proportions of k+a's. Spectra of dusty starburst candidates, with strong Balmer absorption and emission lines, present a very different environmental dependence from k+a's. They are numerous in all environments at z=0.4-0.8, but they are especially numerous in all types of groups, favoring the hypothesis of triggering by a merger. Our observations are consistent with previous suggestions that cluster k+a galaxies are observed in a transition phase as massive S0 and Sa galaxies, evolving from star-forming later types to passively evolving early-type galaxies. The correlation between k+a fraction and cluster sigma supports the hypothesis that k+a galaxies in clusters originate from processes related to the intracluster medium, while several possibilities are discussed for the origin of the k+a frequency in low-[OII] groups.(abr.)

astro-ph↗

arXiv2008

Revealing the properties of dark matter in the merging cluster MACSJ0025.4-1222

We constrain the physical nature of dark matter using the newly identified massive merging galaxy cluster MACSJ0025.4-1222. As was previously shown by the example of the Bullet Cluster (1E0657-56), such systems are ideal laboratories for detecting isolated dark matter, and distinguishing between cold dark matter (CDM) and other scenarios (e.g. self-interacting dark matter, alternative gravity theories). MACSJ0025.4-1222 consists of two merging subclusters of similar richness at z=0.586. We measure the distribution of X-ray emitting gas from Chandra X-ray data and find it to be clearly displaced from the distribution of galaxies. A strong (information from highly distorted arcs) and weak (using weakly distorted background galaxies) gravitational lensing analysis based on Hubble Space Telescope observations and Keck arc spectroscopy confirms that the subclusters have near-equal mass. The total mass distribution in each of the subclusters is clearly offset (at >4sigma significance) from the peak of the hot X-ray emitting gas (the main baryonic component), but aligned with the distribution of galaxies. We measure the fractions of mass in hot gas (0.09^{+0.07}_{-0.03}) and stars (0.010^{+0.007}_{-0.004}), consistent with those of typical clusters, finding that dark matter is the dominant contributor to the gravitational field. Under the assumption that the subclusters experienced a head-on collision in the plane of the sky, we obtain an order-of-magnitude estimate of the dark matter self-interaction cross-section of sigma/m < 4cm^2/g, re-affirming the results from the Bullet Cluster on the collisionless nature of dark matter.

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arXiv2008

Radio loud AGN and the L_X - σrelation of galaxy groups and clusters

We use the ROSAT All-Sky Survey to study the X-ray properties of a sample of 625 groups and clusters of galaxies selected from the Sloan Digital Sky Survey. We stack clusters with similar velocity dispersions and investigate whether their average X-ray luminosities and surface brightness profiles vary with the radio activity level of their central galaxies. We find that at a given value of $σ$, clusters with a central radio AGN have more concentrated X-ray surface brightness profiles, larger central galaxy masses, and higher X-ray luminosities than clusters with radio-quiet central galaxies. The enhancement in X-ray luminosity is more than a factor of two, is detected with better than 6$σ$ significance, and cannot be explained by X-ray emission from the radio AGN itself. This difference is largely due to a subpopulation of radio-quiet, high velocity dispersion clusters with low mass central galaxies. These clusters are underluminous at X-ray wavelengths when compared to otherwise similar clusters where the central galaxy is radio-loud, more massive, or both.

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arXiv2008

The relation between star formation, morphology and local density in high redshift clusters and groups

We investigate how the [OII] properties and the morphologies of galaxies in clusters and groups at z=0.4-0.8 depend on projected local galaxy density, and compare with the field at similar redshifts and clusters at low-z. In both nearby and distant clusters, higher-density regions contain proportionally fewer star-forming galaxies, and the average [OII] equivalent width of star-forming galaxies is independent of local density. However, in distant clusters the average current star formation rate (SFR) in star-forming galaxies seems to peak at densities ~15-40 galaxies Mpc^{-2}. At odds with low-z results, at high-z the relation between star-forming fraction and local density varies from high- to low-mass clusters. Overall, our results suggest that at high-z the current star formation (SF) activity in star-forming galaxies does not depend strongly on global or local environment, though the possible SFR peak seems at odds with this conclusion. We find that the cluster SFR normalized by cluster mass anticorrelates with mass and correlates with the star-forming fraction. These trends can be understood given a) that the average star-forming galaxy forms about 1 Msun/yr in all clusters; b) that the total number of galaxies scales with cluster mass and c) the dependence of star-forming fraction on cluster mass. We present the morphology-density (MD) relation for our z=0.4-0.8 clusters, and uncover that the decline of the spiral fraction with density is entirely driven by galaxies of types Sc or later. For galaxies of a given Hubble type, we see no evidence that SF properties depend on local environment. In contrast with recent findings at low-z, in our distant clusters the SF-density relation and the MD-relation are equivalent, suggesting that neither of the two is more fundamental than the other.(abr.)

astro-ph↗

arXiv2008

Spectroscopy of clusters in the ESO distant cluster survey (EDisCS).II. Redshifts, velocity dispersions, and substructure for clusters in the last 15 fields

AIMS. We present spectroscopic observations of galaxies in 15 survey fields as part of the ESO Distant Cluster Survey (EDisCS). We determine the redshifts and velocity dispersions of the galaxy clusters located in these fields, and we test for possible substructure in the clusters. METHODS. We obtained multi-object mask spectroscopy using the FORS2 instrument at the VLT. We reduced the data with particular attention to the sky subtraction. We implemented the method of Kelson for performing sky subtraction prior to any rebinning/interpolation of the data. From the measured galaxy redshifts, we determine cluster velocity dispersions using the biweight estimator and test for possible substructure in the clusters using the Dressler-Shectman test. RESULTS. The method of subtracting the sky prior to any rebinning/interpolation of the data delivers photon-noise-limited results, whereas the traditional method of subtracting the sky after the data have been rebinned/interpolated results in substantially larger noise for spectra from tilted slits. Redshifts for individual galaxies are presented and redshifts and velocity dispersions are presented for 21 galaxy clusters. For the 9 clusters with at least 20 spectroscopically confirmed members, we present the statistical significance of the presence of substructure obtained from the Dressler-Shectman test, and substructure is detected in two of the clusters. CONCLUSIONS. Together with data from our previous paper, spectroscopy and spectroscopic velocity dispersions are now available for 26 EDisCS clusters with redshifts in the range 0.40-0.96 and velocity dispersions in the range 166-1080 km/s.

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arXiv2007

How special are Brightest Group and Cluster Galaxies?

We use the Sloan Digital Sky Survey to construct a sample of 625 brightest group and cluster galaxies (BCGs) together with control samples of non-BCGs matched in stellar mass, redshift, and color. We investigate how the systematic properties of BCGs depend on stellar mass and on their privileged location near the cluster center. The groups and clusters that we study are drawn from the C4 catalogue of Miller et al. (2005) but we have developed improved algorithms for identifying the BCG and for measuring the cluster velocity dispersion. Since the SDSS photometric pipeline tends to underestimate the luminosities of large galaxies in dense environments, we have developed a correction for this effect which can be readily applied to the published catalog data. We find that BCGs are larger and have higher velocity dispersions than non-BCGs of the same stellar mass, which implies that BCGs contain a larger fraction of dark matter. In contrast to non-BCGs, the dynamical mass-to-light ratio of BCGs does not vary as a function of galaxy luminosity. Hence BCGs lie on a different fundamental plane than ordinary elliptical galaxies. BCGs also follow a steeper Faber-Jackson relation than non-BCGs, as suggested by models in which BCGs assemble via dissipationless mergers along preferentially radial orbits. We find tentative evidence that this steepening is stronger in more massive clusters. BCGs have similar mean stellar ages and metallicities to non-BCGs of the same mass, but they have somewhat higher alpha/Fe ratios, indicating that star formation may have occurred over a shorter timescale in the BCGs. Finally, we find that BCGs are more likely to host radio-loud active galactic nuclei than other galaxies of the same mass, but are less likely to host an optical AGN. The differences we find are more pronounced for the less massive BCGs.

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arXiv2006

The build-up of the colour-magnitude relation in galaxy clusters since z~0.8

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.

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