We study the formation and evolution of voids in the dark matter distribution using various simulations of the popular $\Lambda$ Cold Dark Matter Universe. We identify voids by requiring them to be spherical or elliptical regions of space with a mean overdensity of $-0.8$ or less. The distribution of void sizes in the different simulations shows good overlap. The size of a void is related to the depth of the smoothed density field at that position in the initial conditions. The rescaled mass profiles of voids in the different simulations agree remarkably well. We find a universal void mass profile of the form $\rho(\ltr)/\rho(r_{\rm eff}) \propto \exp[(r/r_{\rm eff})^\alpha]$ where $r_{\rm eff}$ is the effective void radius and $\alpha\sim 2$. The mass function of haloes in voids is steeper than that of haloes which populate denser regions.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We have established under which conditions core collapse of a spherical cluster occurs before massive stars have time to evolve off the main sequence (MS). We consider cluster central velocity dispersions of 100 km s−1 and higher, appropriate for galactic nuclei. At such high velocities, binary stars play little dynamical role and are therefore neglected. On the other hand whether collisions allow the growth of very massive stars (VMS, with $\ensuremath{M_{\ast}} \gg 100\,\ensuremath{M_{\odot}}$) or, on the contrary, grind them down is a central unknown addressed in this work. We find that, in spite of the high relative velocities, run-away growth of a VMS, a likely progenitor for an intermediate-mass BH (IMBH), occurs in all clusters with short enough a core collapse time.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

In this paper we present some preliminary results of a 3-mm (86 GHz) radio continuum survey of selected Seyfert galaxies made with the Berkley-Maryland Illinois Millimeter Array (BIMA) in D configuration. The images shown here have spatial resolutions of about two arcminutes after gaussian tapering. At this observing wavelength, we expect to have a negligible non-thermal emission as the spectra is certainly dominated by free-free radio emission. We have measured flux densities and 3 mm luminosities which agree with a thermal blackbody spectra expected from data obtained with the IRAS satellite at wavelengths shorter than 80 μm. Data is presented for NGC3982, NGC5597, NGC5253, NGC1667, and NGC2997 and the main astrophysical consequences of such studies are discussed in this poster. We also argue that radio continuum at millimeter wavelengths is a better observational tool to study the star-formation in active galaxies than conventional infrared photometry.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

More than half of all low-redshift AGN exhibit UV and X-ray absorption by highly ionized gas. The observed UV and X-ray absorption lines are almost aiways blue-shifted at velocities of hundreds of km s−1, indicating that the absorbing gas is outflowing from the active nucleus. In some cases the inferred mass flux rivals the Eddington limit of the central black hole, an indication that these outflows are intimately related to the mass accretion and energy generation mechanism in AGN. The ejected material can also have an affect on the interstellar medium of the host galaxy and the surrounding intergalactic medium. Over the past several years, coordinated UV and X-ray observations of several bright AGN at high spectral resolution using HST, FUSE, Chandra, and XMM-Newton have contributed greatly to our understanding of these outflows. I will give an overview of these recent observations, summarize our FUSE survey of low-redshift AGN, and interpret the results in the context of models of winds from accretion disks and thermally driven winds from the obscuring torus.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present completed results of a high resolution radio imaging survey of the Palomar Spectroscopic Sample of all (∼470) nearby bright northern galaxies. Almost half the Palomar sample's galaxies have nuclei with emission-lines characteristic of AGN but with LHα ≤ 1040 erg s−1. These are referred to as low-luminosity AGNs or LLAGNs. The power source of such LLAGNs has been long debated. High resolution radio surveys of the sample – with the VLA at 15 GHz (150 mas resolution), and the VLBA at 5 GHz (2 mas resolution) – have now revealed a high incidence of pc-scale radio cores with implied brightness temperatures [gsim]108K, and sub-parsec scale jets. The results support the presence of accreting black holes in ≥50% of all LLAGNs; there is no evidence against all LLAGNs being mini-AGNs. The detected parsec-scale radio nuclei are preferentially found in massive ellipticals and in type 1 nuclei (i.e. nuclei with broad Hα emission). These nuclei follow the usual correlations between radio and emission-line gas properties found in more powerful AGNs. The radio luminosity function (RLF) of Palomar Sample LLAGNs extends three orders of magnitude below, and is continuous with, that of ‘classical’ AGNs. We find marginal evidence for a low-power turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of accretion in the local universe. Low accretion rates (≤10−2−10−3 of the Eddington rate) are implied in both advection- and jet-type models. Within the context of jet models, the accretion energy output is dominated by the energy in the observed jets rather than the radiated bolometric luminosity. These jets would be able to dump sufficient energy into the innermost parsecs to significantly slow the accretion inflow. Detailed results can be found in Nagar et al. (2002, 2004) and references therein.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present some preliminary results from an on-going study of the evolution of stellar populations in rich clusters of galaxies. Our baseline sample contains core line-strength measurements from 183 galaxies with $b_{\rm J} \le 19.5$ from four clusters with $\bar{z}\sim0.04$, against which observations of higher-redshift clusters can be compared. Using predictions from stellar population models to compare with our measured line strengths we can derive relative luminosity-weighted mean ages and metallicities for the stellar populations in each of our clusters. It must be stressed that these ages and metallicities are only accurate when used in a relative sense as the stellar population models, due to differing abundance ratios used in the models compared to those observed in elliptical galaxies, provide inaccurate absolute ages and metallicities. We also investigate the ${\rm Mg}b^\prime$-$\sigma_0$ and H$\beta_G^\prime$-$\sigma_0$ scaling relations. We find that ${\rm Mg}b^\prime$ is correlated with $\sigma_0$, the likely explanation being that larger galaxies are better at retaining their heavier elements due to their larger potentials. ${\rm H}\beta_G^\prime$, on the other hand, we find to be anti-correlated with $\sigma_0$. This result implies that the stellar populations in larger galaxies are older than in smaller galaxies.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The intrinsic emission of obscured active galactic nuclei (AGN) cannot be measured directly, but the obscured examples uniquely allow determination of the physical conditions near the central engine on 10–100 pc scales. The reprocessed radiation that emerges at X-ray and infrared energies, in particular, reveals the distribution of the obscuring medium. In general, X-ray spectroscopy shows the total column density along the line of sight, and in extremely obscured (Compton thick) cases, the fluorescent iron Kα emission line is sensitive to viewing angle and covering fraction. Observed near- and mid-infrared spectra require an inhomogeneous distribution of material around the AGN, and they specifically depend on parameters such as the optical depth per obscuring cloud and the number of clouds along the line of sight. The total spatial extent of the obscuring region determines the shape of the far-infrared spectrum.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We study the effects of radiative cooling, star formation and stellar feedback on the properties and evolution of galaxy clusters using high-resolution Adaptive Mesh Refinement N-body+gasdynamics simulations of clusters forming in the $\Lambda$CDM universe. Cooling leads to the condensation of gas in the inner regions of clusters, which in turn leads to steepening of the dark matter profile. The cooling gas is replaced by the higher-entropy gas from the outer regions, which raises the entropy and temperature of gas in the cluster core. The magnitude of these effects is likely overestimated in the current simulations because they suffer from the overcooling problem: a much larger fraction of baryons is in the form of cold gas and stars than is observed. We find that the thermal stellar feedback alone does not remedy this problem. Additional ad-hoc preheating can lower the amount of cold gas but a simple uniform preheating results in incorrect star formation history, as it delays the bulk of star formation until $z<1$. Our analysis shows that the overcooling in a cluster as a whole is really the overcooling in the central galaxy and its progenitors at high redshifts. This indicates that an additional heating mechanism that can continuously heat the gas in the cluster core is required to reproduce the observed cluster properties. Energy injection by the Active Galactic Nuclei, which may provide such heating, may thus be an important missing ingredient in the current theoretical models of cluster formation.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Here we present a self-consistent stationary solution for spherically symmetric winds driven by massive star clusters under the impact of radiative cooling. We demonstrate that cooling may modify drastically the distribution of temperature and the observational appearance of star cluster winds if the rate of injected energy approaches a critical value, and that the stationary wind solution does not exist if the energy deposition rate exceeds this limit.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The discovery of diffuse synchrotron radio emission and, more recently, of the hard X-ray (HXR) tails have triggered a growing interest about non-thermal phenomena in galaxy clusters. After a brief review of the most important evidences for non-thermal emission, I will focus on the origin of the emitting particles and of the hadronic component. In particular I will describe the particle-injection and -acceleration mechanisms at work in the intra-cluster medium (ICM) and, at the same time, discuss the possibility to test current modellings of these phenomena with future radio, HXR, and gamma ray observatories.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present a method for deriving light-weighted stellar metallicities and ages from high-resolution galaxy spectra, based on the new population synthesis code of Bruzual & Charlot (2003). The method relies on the simultaneous fit of several optical spectral absorption features that are sensitive to either age or metallicity, but not to the $\alpha$-elements abundance ratio. We have constructed a library of stochastic star formation histories, which we have used to derive median-likelihood estimates of ages and metallicities for $\sim10^5$ galaxies extracted from the Sloan Digital Sky Survey Data Release One (SDSS DR1), spanning the full range in star formation activities, from dormant early type to actively star forming. Here we discuss the results for early-type galaxies. We show that the $g-r$, $M_r$ color-magnitude relation for these galaxies is driven primarily by changes in metallicity and in heavy-element abundance ratios. Changes in light-weighted age contribute mainly to the scatter about the relation. This is consistent with previous interpretations of this relation based on lower resolution models.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Spectroscopic optical data (∼4200-6300 Å in the rest-frame) acquired during the monitoring of some QSOs seem to confirm that AGN spectra normally harden at higher luminosities. Focusing only on the global trend (i.e. comparing different objects), the implications in deriving luminosities using optical photometric data (through the K-correction) is briefly outlined here, as well as the result of its parameterization for calculating BH masses through the M$_{BH} \propto$ L$_V^\gamma$ (FWHM Hβ)2 virial relation.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

In this contribution we present the detection of two WHIM filaments in the Chandra-LETG spectrum of Mkn 421. This spectrum has been obtained following 2 of our pre-approved Target of Opportunity requests to observe Blazars in outburst to efficiently ‘X-ray’ the IGM at high spectral resolution. These observations caught the source at the unprecedented levels of 60 and 40 mCrab in the soft X-ray (0.5-2 keV) band. We detect, for the first time, two WHIM filaments at redshifts of z=0.011 and z=0.027, respectively. Based on these two detections and on the upper limits on associated HI and OVI absorption as inferred from the HST and FUSE spectra of Mkn 421, we estimate a number of WHIM filaments per unit redshift, with He-like ion columns $N_{X} \gs 8 \times 10^{14}$ cm$^{-2}$ of $dN/dz = 67^{+88}_{-43}$, and a baryon mass density of $\Omega_b = 0.021^{+0.028}_{-0.014}$, virtually all of the ‘missing baryons’ at $z\lt1$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We report the status of our study of the central region of M 83. Recently a binary mass concentration at the nucleus was suggested by Thatte et al. (2000), and we determined the possible position of the optically hidden mass concentration with 2D Hα kinematics. We continue our study of this region trough IR 3D spectroscopy performed with CIRPASS, attached to the Gemini South telescope. Almost 1500 spectra were obtained, centered in the wavelength 1.3 μm. The velocity field in a region of 13″ × 9″ around the optical nucleus is resolved, with 0.36″ (6.4 pc) sampling. We confirm that the optical nucleus of M 83 is not located at the most important center of symmetry of the velocity field. The largest black hole that could fit to the circular motion in this kinematic center should have a mass less than $2\times10^7M_{\odot}$.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss the discovery of six bright Lyman break galaxy candidates in the Sloan Digital Sky Survey and the follow-up observations necessary to determine whether they are powered by star formation or by active galactic nuclei.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

In this work we investigate the stellar population, metallicity distribution and ionized gas in the elliptical galaxy NGC 5044, using long-slit spectroscopy and a stellar population synthesis method. We found differences in the slope of FeI and Mg2 lines gradients, which suggests an enhancement of α elements, particularly towards the central region. The stellar population synthesis shows that the contribution, at λ5870, of the most metallic and old stellar population ($Z/Z_\odot \sim 0.0$ and 1010 year) is dominant in NGC 5044. The presence of a non-thermal ionization source, such as a low-luminosity AGN and/or shock ionization, is implied by the large values of the ratio [NII]/Hα observed in all sampled regions. However, the emission lines observed in the external regions indicate the presence of an additional ionization source, probably hot, post-AGB stars.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present spatially resolved X-ray spectra taken with the EPIC cameras of XMM-Newton of a sample of 17 cooling clusters and three non-cooling clusters for comparison. The deprojected spectra are analyzed with a single-temperature model. All cooling clusters show a central decrement of the average temperature, most of them of a factor of $\sim$2. Three clusters show a weak temperature decrement, while two others have a very strong temperature decrement. We investigate the role of heat conduction by electrons and find that the theoretically predicted conductivity rates are not high enough to balance radiation losses.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We discuss evidence in local, present-day clusters of galaxies (from the ENACS survey) about the way in which those clusters have evolved and about the evolutionary relationships between the galaxies of different morphological types in them. This evidence is complementary to that obtained from the study of clusters at intermediate and high redshifts. We argue that the spatial distribution and the kinematics of the various types of galaxies in and outside substructures support the following picture.

The elliptical and S0 galaxies have been around for a long time and have obtained an isotropic velocity distribution. The spatial distribution and kinematics of the early spirals are consistent with the idea that many of their kind have transformed into an S0, but that they have survived, most likely because of their velocities. The distribution and kinematics of the late spirals are consistent with a picture in which they have been accreted fairly recently. They have mildly radial orbits and hardly populate the central regions, most likely because they suffer tidal disruption. Finally, the distribution and kinematics of the galaxies in substructures, when taken at face value, imply tangential velocity anisotropy for these galaxies, but this result may be (partly) due to the procedure by which these galaxies are selected. A first attempt to take the effects of selection into account shows that isotropic (or even mildly radial) orbits of subcluster galaxies cannot be excluded.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

To understand cluster formation and evolution it is important to understand the evolution of cluster substructure. The work described here focuses upon a set of fully self-consistent numerical simulations within a $\Lambda$CDM cosmology. Within the simulations we follow in detail the temporal and spatial properties of individually tracked satellite galaxies. The host galaxy clusters were chosen to sample a variety of formation histories, ages, and triaxialities. Despite their obvious differences, we find striking similarities within the associated substructure populations. Namely, the satellite galaxy orbital distributions are indistinguishable between the galaxy clusters.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We present our project on galaxy evolution in the environment of distant rich clusters aiming at disentangling the importance of specific interaction and galaxy transformation processes from the hierarchical evolution of field galaxies. Spatially resolved MOS spectra were gained with VLT/FORS to analyze the internal kinematics of disk galaxies. First results are shown for the clusters MS 1008.1–1224 ($z=0.30$), Cl 0303+1706 ($z=0.42$), and Cl 0413–6559 ($z=0.51$). Out of 35 late-type cluster members, 13 galaxies exhibit a rotation curve of the universal form rising in the inner region and passing over into a flat part. The other members have peculiar kinematics. The 13 cluster galaxies for which a maximum rotation velocity could be derived are distributed in the Tully-Fisher diagram very similarly to field galaxies from the FORS Deep Field with corresponding redshifts. The same is true for seven non-cluster galaxies observed in the cluster fields. The TF-cluster spirals do not show any significant luminosity evolution as might be expected from certain phenomena specific to clusters. Contrary to that, the disturbed kinematics of the non–TF cluster spirals indicate ongoing or recent interaction processes.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html