Julien Devriendt

Abstract:

The Active Galactic Nuclei In Cosmological Simulations (AGNICS) project includes AGN into the GalICS hybrid (N-body + semi-analytic) model to explore the quasar-galaxy link in a cosmological perspective. The key problems are the quasar fuelling mechanism, the origin of the BH to bulge mass relation, the causal and chronological link between BH growth and galaxy formation, the properties of quasar hosts and the role of AGN feedback in galaxy formation. This first paper has two goals: to describe the general structure and the assumptions of the method, and to apply AGNICS to studying the joint formation of BHs and spheroids in galaxy mergers. We investigate under what conditions this scenario can reproduce the local BH distribution and the evolution of the quasar population. AGNICS contains two star formation modes: a quiescent one in discs and a starburst one in protospheroids, the latter triggered by mergers and disc instabilities. We assume that BH growth is linked to the starburst mode. The simplest version of this scenario, in which the BH accretion rate and the star formation rate in the starburst component are simply related by a constant of proportionality, does not to reproduce the cosmic evolution of the quasar population. A model in which this relation is modulated by a scaling with the square root of the starburst gas density can explain the evolution of the quasar luminosity function in B-band and X-rays (accounting for the obscuration inferred from X-ray studies). The scatter and the tilt that this model introduces in the BH-to-bulge mass relation are within the observational constraints. The quasar contribution grows with bolometric luminosity and for a given bulge mass the most massive BHs are in the bulges with the oldest stars.

Abstract:

Molecular clouds (MCs) are highly structured and ``turbulent''. Colliding gas streams of atomic hydrogen have been suggested as a possible source of MCs, imprinting the filamentary structure as a consequence of dynamical and thermal instabilities. We present a 2D numerical analysis of MC formation via converging HI flows. Even with modest flow speeds and completely uniform inflows, non-linear density perturbations as possible precursors of MCs arise. Thus, we suggest that MCs are inevitably formed with substantial structure, e.g., strong density and velocity fluctuations, which provide the initial conditions for subsequent gravitational collapse and star formation in a variety of galactic and extragalactic environments.

Abstract:

We address the degree and rapidity of generation of small-scale power over the course of structure formation in cosmologies where the primordial power spectrum is strongly suppressed beyond a given wavenumber. We first summarize the situations where one expects such suppressed power spectra and point out their diversity. We then employ an exponential cut-off, which characterizes warm dark matter (WDM) models, as a template for the shape of the cut-off and focus on damping scales ranging from 106 to 109 h -1 M⊙. Using high-resolution simulations, we show that the suppressed part of the power spectrum is quickly (re)generated and catches up with both the linear and the non-linear evolution of the unsuppressed power spectrum. From z = 2 onwards, a power spectrum with a primordial cut-off at 109 h-1 MŁódź, becomes virtually indistinguishable from an evolved cold dark matter (CDM) power spectrum. An attractor such as that described in Zaldarriaga, Scoccimarro & Hui for power spectra with different spectral indices also emerges in the case of truncated power spectra. Measurements of z ∼ 0 non-linear power spectra at ∼100 h-1 kpc cannot rule out the possibility of linear power spectra damped below ∼109 h-1 M ⊙. Therefore, WDM or scenarios with similar features should be difficult to exclude in this way. © 2005 RAS.