Impact of a non-Gaussian density field on Sunyaev-Zeldovich observables

The main statistical properties of the Sunyaev-Zeldovich (S-Z) effect - the power spectrum, cluster number counts, and angular correlation function - are calculated and compared within the framework of two density fields which differ in their predictions of the cluster mass function at high redshifts. We do so for the usual Press and Schechter mass function, which is derived on the basis of a Gaussian density fluctuation field, and for a mass function based on a chi^2 distributed density field. These three S-Z observables are found to be very significantly dependent on the choice of the mass function. The different predictions of the Gaussian and non-Gaussian density fields are probed in detail by investigating the behaviour of the three S-Z observables in terms of cluster mass and redshift. The formation time distribution of clusters is also demonstrated to be sensitive to the underlying mass function. A semi-quantitative assessment is given of its impact on the concentration parameter and the temperature of intracluster gas.Comment: 17 pages, 11 figures, accepted for publication in MNRA

Narrow-line Seyfert 1 Galaxies and the M_BH - sigma Relation

We have studied the location of narrow-line Seyfert 1 (NLS1) galaxies and broad-line Seyfert 1 (BLS1) galaxies on the M_BH - sigma relation of non-active galaxies. We find that NLS1 galaxies as a class - as well as the BLS1 galaxies of our comparison sample - do follow the M_BH-sigma relation of non-active galaxies if we use the width of the [SII]6716,6731 emission lines as surrogate for stellar velocity dispersion, sigma_*. We also find that the width of [OIII]5007 is a good surrogate for sigma_*, but only after (a) removal of asymmetric blue wings, and, more important, after (b) excluding core [OIII] lines with strong blueshifts (i.e., excluding galaxies which have their [OIII] velocity fields dominated by radial motions, presumably outflows). The same galaxies which are extreme outliers in [OIII] still follow the M_BH - sigma relation in [SII]. We confirm previous findings that NLS1 galaxies are systematically off-set from the M_BH - sigma relation if the full [OIII] profile is used to measure sigma. We systematically investigate the influence of several parameters on the NSL1 galaxies' location on the M_BH - sigma plane: [OIII]_core blueshift, L/L_Edd, intensity ratio FeII/H_beta, NLR density, and absolute magnitude. Implications for NLS1 models and for their evolution along the M_BH - sigma relation are discussed.Comment: ApJ Letters, in press (3 figures, one in colour

Galaxy Mergers at z>1 in the HUDF: Evidence for a Peak in the Major Merger Rate of Massive Galaxies

We present a measurement of the galaxy merger fraction and number density from observations in the Hubble Ultra Deep Field for 0.5<z<2.5. We fit the combination of broadband data and slitless spectroscopy of 1308 galaxies with stellar population synthesis models to select merging systems based on a stellar mass of >10^10 M_sol. When correcting for mass incompleteness, the major merger fraction is not simply proportional to (1+z)^m, but appears to peak at z_frac~=1.3+-0.4. From this merger fraction, we infer that ~42% of massive galaxies have undergone a major merger since z~1. We show that the major merger number density peaks at z_dens~1.2, which marks the epoch where major merging of massive galaxies is most prevalent. This critical redshift is comparable to the peak of the cosmic star formation rate density, and occurs roughly 2.6 Gyr earlier in cosmic time than the peak in the number density of X-ray selected active galactic nuclei. These observations support an indirect evolutionary link between merging, starburst, and active galaxies.Comment: Accepted to ApJ. 7 pages, 6 figures, 1 table. Uses and includes emulateapj.cls. In the initial submission, Figures 1 and 2 where switche

Galaxy Zoo: Multimergers and the Millennium Simulation

We present a catalogue of 39 multiple mergers, found using the mergers catalogue of the Galaxy Zoo project for z <0.1, and compare them to corresponding semi-analytical galaxies from the Millennium Simulation. We estimate the (volume-limited) multimerger fraction of the local Universe using our sample and find it to be at least 2 orders of magnitude less than binary mergers - in good agreement with the simulations (especially the Munich group). We then investigate the properties of galaxies in binary mergers and multimergers (morphologies, colours, stellar masses and environment) and compare these results with those predicted by the semi-analytical galaxies. We find that multimergers favour galaxies with properties typical of elliptical morphologies and that this is in qualitative agreement with the models. Studies of multimergers thus provide an independent (and largely corroborating) test of the Millennium semi-analytical models.Peer reviewe

How young are early-type cluster galaxies ? Quantifying the young stellar component in a rich cluster at z=0.41

We present a new method of quantifying the mass fraction of young stars in galaxies by analyzing near-ultraviolet (NUV)-optical colors. We focus our attention on early-type cluster galaxies, whose star formation history is at present undetermined. Rest-frame NUV (F300W) and optical (F702W) images of cluster Abell 851 (z=0.41) using HST/WFPC2 allow us to determine a NUV-optical color-magnitude relation, whose slope is incompatible with a monolithic scenario for star formation at high redshift. A degeneracy between a young stellar component and its fractional mass contribution to the galaxy is found, and a photometric analysis comparing the data with the predictions for a simple two-stage star formation history is presented. The analysis shows that some of the early-type galaxies may have fractions higher than 10% of the total mass content in stars formed at z~0.5. An increased scatter is found in the color-magnitude relation at the faint end, resulting in a significant fraction of faint blue early-type systems. This would imply that less massive galaxies undergo more recent episodes of star formation, and this can be explained in terms of a positive correlation between star formation efficiency and luminosity.Comment: Accepted for publication in ApJ Letters. Uses emulateapj.sty. 5 pages with 3 embedded EPS figure

3D simulations of the early stages of AGN jets: geometry, thermodynamics and backflow

We investigate the interplay between jets from Active Galactic Nuclei (AGNs) and the surrounding InterStellar Medium (ISM) through full 3D, high resolution, Adaptive Mesh Refinement simulations performed with the FLASH code. We follow the jet- ISM system for several Myr in its transition from an early, compact source to an extended one including a large cocoon. During the jet evolution, we identify three major evolutionary stages and we find that, contrary to the prediction of popular theoretical models, none of the simulations shows a self-similar behavior. We also follow the evolution of the energy budget, and find that the fraction of input power deposited into the ISM (the AGN coupling constant) is of order of a few percent during the first few Myr. This is in broad agreement with galaxy formation models employing AGN feedback. However, we find that in these early stages, this energy is deposited only in a small fraction (< 1%) of the total ISM volume. Finally we demonstrate the relevance of backflows arising within the extended cocoon generated by a relativistic AGN jet within the ISM of its host galaxy, previously proposed as a mechanism for self-regulating the gas accretion onto the central object. These backflows tend later to be destabilized by the 3D dynamics, rather than by hydrodynamic (Kelvin- Helmholtz) instabilities. Yet, in the first few hundred thousand years, backflows may create a central accretion region of significant extent, and convey there as much as a few millions of solar masses.Comment: Accepted in MNRAS - 16 pages, 12 figures - Multimedia available on the author's webpage: http://www.mpia.de/~ciel