Thorsten Lisker

THE BULK OF THE BLACK HOLE GROWTH SINCE z ∼ 1 OCCURS IN A SECULAR UNIVERSE: NO MAJOR MERGER-AGN CONNECTION*

What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGNs) activity? To answer this long-standing question, we analyze 140 XMM-Newton-selected AGN host galaxies and a matched control sample of 1264 inactive galaxies over z ~ 0.3–1.0 and M_∗ < 10^(11.7) M_⊙ with high-resolution Hubble Space Telescope/Advanced Camera for Surveys imaging from the COSMOS field. The visual analysis of their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies in the AGN and control samples, i.e., quantifying the signature of recent mergers which might potentially be responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies do not show strong distortions and (2) there is no significant difference in the distortion fractions between active and inactive galaxies. Our findings provide the best direct evidence that, since z ~ 1, the bulk of black hole (BH) accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e., internal secular processes and minor interactions, are the leading triggers for the episodes of major BH growth.We also exclude an alternative interpretation of our results: a substantial time lag between merging and the observability of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to (1) recent major mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGNs, (2) the lack of a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low levels of soft X-ray obscuration for AGNs hosted by interacting galaxies, in contrast to model predictions.

Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. III. Subpopulations: Distributions, Shapes, Origins

From a quantitative analysis of 413 Virgo Cluster early-type dwarf galaxies (dEs) with SDSS imaging data, we find that the dE class can be divided into multiple subpopulations that differ significantly in their morphology and clustering properties. Three dE subclasses are shaped like thick disks and show no central clustering: (1) dEs with disk features like spiral arms or bars, (2) dEs with central star formation, and (3) ordinary, bright dEs that have no or only a weak nucleus. These populations probably formed from infalling progenitor galaxies. In contrast, ordinary nucleated dEs follow the picture of classical dwarf elliptical galaxies in that they are spheroidal objects and are centrally clustered like E and S0 galaxies, indicating that they have resided in the cluster for a long time or were formed along with it. These results define a morphology-density relation within the dE class. We find that the difference in the clustering properties of nucleated dEs and dEs with no or only a weak nucleus is not caused by selection biases, as opposed to previously reported suggestions. The correlation between surface brightness and observed axial ratio favors oblate shapes for all subclasses, but our derivation of intrinsic axial ratios indicates the presence of at least some triaxiality. We discuss possible interrelations and formation mechanisms (ram pressure stripping, tidally induced star formation, harassment) of these dE subpopulations.

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey III. DA white dwarfs

Context. The ESO Supernova Ia Progenitor Survey (SPY) took high-resolution spectra of more than 1000 white dwarfs and pre-white dwarfs. About two thirds of the stars observed are hydrogen-dominated DA white dwarfs. Here we present a catalog and detailed spectroscopic analysis of the DA stars in the SPY. Aims. Atmospheric parameters effective temperature and surface gravity are determined for normal DAs. Double-degenerate binaries, DAs with magnetic fields or dM companions, are classified and discussed. Methods. The spectra are compared with theoretical model atmospheres using a χ 2 fitting technique. Results. Our final sample contains 615 DAs, which show only hydrogen features in their spectra, although some are double-degenerate binaries. 187 are new detections or classifications. We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).

Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. II. Early-Type Dwarfs with Central Star Formation

Despite the common picture of an early-type dwarf galaxy (dE) as a quiescent galaxy with no star formation and little gas, we identify 23 dEs that have blue central colors caused by recent or ongoing star formation in our sample of 476 Virgo Cluster dEs. In addition, 14 objects that were mostly classified as (candidate) blue compact dwarfs have similar properties. Among the certain cluster members, the dEs with blue centers reach a fraction of more than 15% of the dE population at brighter (mB ≤ 16) magnitudes. A spectral analysis of the centers of 16 galaxies reveals in all cases an underlying old population that dominates the mass, with Mold ≥ 90% for all but one object. Therefore, the majority of these galaxies will appear like ordinary dEs within approximately one gigayear or less after the last episode of star formation. Their overall gas content is less than that of dwarf irregular galaxies but higher than that of ordinary dEs. Their flattening distribution suggests the shape of a thick disk, similar to what has been found for dEs with disk features in Paper I of this series. Their projected spatial distribution shows no central clustering, and their distribution with projected local density follows that of irregular galaxies, indicative of an unrelaxed population. This is corroborated by their velocity distribution, which displays two side peaks characteristic of recent infall. We discuss possible formation mechanisms (ram-pressure stripping, tidally induced star formation, and harassment) that might be able to explain both the disk shape and the central star formation of the dEs with blue centers.

Dwarf galaxy populations in present-day galaxy clusters – I. Abundances and red fractions

We compare the galaxy population in the Virgo, Fornax, Coma and Perseus cluster to a state-of-the-art semi-analytic model, focusing on the regime of dwarf galaxies with luminosities from approximately 108 to 109 L⊙. We find that the number density profiles of dwarfs in observed clusters are reproduced reasonably well, and that the red fractions of model clusters provide a good match to Coma and Perseus. On the other hand, the red fraction among dwarf galaxies in Virgo is clearly lower than in model clusters. We argue that this is mainly caused by the treatment of environmental effects in the model. This explanation is supported by our finding that the colours of central (‘field’) dwarf galaxies are reproduced well, in contrast to previous claims. Finally, we find that the dwarf-to-giant ratio in model clusters is too high. This may indicate that the current model prescription for tidal disruption of faint galaxies is still not efficient enough.

VIRGO CLUSTER EARLY-TYPE DWARF GALAXIES WITH THE SLOAN DIGITAL SKY SURVEY. IV. THE COLOR-MAGNITUDE RELATION

We present an analysis of the optical colors of 413 Virgo cluster early-type dwarf galaxies (dEs), based on Sloan Digital Sky Survey imaging data. Our study comprises (1) a comparison of the color-magnitude relation (CMR) of the different dE subclasses that we identified in paper III of this series, (2) a comparison of the shape of the CMR in low- and high-density regions, (3) an analysis of the scatter of the CMR, and (4) an interpretation of the observed colors with ages and metallicities from population synthesis models. We find that the CMRs of nucleated (dE(N)) and non-nucleated (dE(nN)) dEs are significantly different from each other, with similar colors at fainter magnitudes (m r 17 mag), but increasingly redder colors of the dE(N)s at brighter magnitudes. We interpret this with older ages and/or higher metallicities of the brighter dE(N)s. The dEs with disk features have similar colors as the dE(N)s and seem to be only slightly younger and/or less metal-rich on average. Furthermore, we find a small but significant dependence of the CMR on local projected galaxy number density, consistently seen in all of u − r, g − r, and g − i, and weakly i − z. We deduce that a significant intrinsic color scatter of the CMR is present, even when allowing for a distance spread of our galaxies. No increase of the CMR scatter at fainter magnitudes is observed down to m r ≈ 17 mag (M r ≈ −14 mag). The color residuals, i.e. the offsets of the data points from the linear fit to the CMR, are clearly correlated with each other in all colors for the dE(N)s and for the full dE sample, implying that, at a given magnitude, a galaxy with an older stellar population than average typically also exhibits a higher metallicity than average. Given the observational data for Virgo dEs presented here and in the previous papers of this series, we conclude that there must be at least two different formation channels for early-type dwarfs in order to explain the heterogeneity of this class of galaxy.

The Sizes of Early-Type Galaxies

In this letter we present a study of the size luminosity relation of 475 early-type galaxies in the Virgo Cluster with Sloan Digital Sky Survey imaging data. The analysis of our homogeneous, model-independent data set reveals that giant and dwarf early-type galaxies do not form one common sequence in this relation. The dwarfs seem to show weak or no dependence on luminosity, and do not fall on the extension of the rather steep relation of the giants. Under the assumption that the light profile shape varies continuously with magnitude, a curved relation of size and magnitude would be expected. While the galaxies do roughly follow this trend overall, we find that the dwarf galaxies are significantly larger and the low-luminosity giants are significantly smaller than what is predicted. We come to the conclusion that in this scaling relation there is not one common sequence from dwarfs to giants, but a dichotomy that cannot be resolved by varying profile shapes. The comparison of our data to a semianalytic model supports the idea of a physical origin for this dichotomy.

DISSECTING EARLY-TYPE DWARF GALAXIES INTO THEIR MULTIPLE COMPONENTS

Early-type dwarf galaxies, once believed to be simple systems, have recently been shown to exhibit an intriguing diversity in structure and stellar content. To analyze this further, we started the SMAKCED project (Stellar content, MAss and Kinematics of Cluster Early-type Dwarfs, http://www.smakced.net) and obtained deep H-band images for 101 early-type dwarf galaxies in the Virgo Cluster in a brightness range of -19 mag

ON THE COLOR-MAGNITUDE RELATION OF EARLY-TYPE GALAXIES

In this Letter, we present a study of the color-magnitude relation (CMR) of 468 early-type galaxies in the Virgo Cluster with Sloan Digital Sky Survey imaging data. The analysis of our homogeneous, model-independent data set reveals that, in all colors (u – g, g – r, g – i, i – z) similarly, giant and dwarf early-type galaxies follow a continuous CMR that is best described by an S shape. The magnitude range and quality of our data allows us to clearly confirm that the CMR in Virgo is not linear. Additionally, we analyze the scatter about the CMR and find that it increases in the intermediate-luminosity regime. Nevertheless, despite this observational distinction, we conclude from the similarly shaped CMR of semianalytic model predictions that dwarfs and giants could be of the same origin.

The sensitivity of harassment to orbit: mass loss from early-type dwarfs in galaxy clusters

We conduct a comprehensive numerical study of the orbital dependence of harassment on early-type dwarfs consisting of 168 different orbits within a realistic, Virgo-like cluster, varying in eccentricity and pericentre distance. We find harassment is only effective at stripping stars or truncating their stellar discs for orbits that enter deep into the cluster core. Comparing to the orbital distribution in cosmological simulations, we find that the majority of the orbits (more than three quarters) result in no stellar mass loss. We also study the effects on the radial profiles of the globular cluster systems of early-type dwarfs. We find these are significantly altered only if harassment is very strong. This suggests that perhaps most early-type dwarfs in clusters such as Virgo have not suffered any tidal stripping of stars or globular clusters due to harassment, as these components are safely embedded deep within their dark matter halo. We demonstrate that this result is actually consistent with an earlier study of harassment of dwarf galaxies, despite the apparent contradiction. Those few dwarf models that do suffer stellar stripping are found out to the virial radius of the cluster at redshift = 0, which mixes them in with less strongly harassed galaxies. However when placed on phase-space diagrams, strongly harassed galaxies are found offset to lower velocities compared to weakly harassed galaxies. This remains true in a cosmological simulation, even when haloes have a wide range of masses and concentrations. Thus phase-space diagrams may be a useful tool for determining the relative likelihood that galaxies have been strongly or weakly harassed.