Dark and luminous matter connections 

  Context The past two decades have witnessed the emergence of a successful class of ab-initio theoretical models to explain the formation of the structures in the Universe. The initial conditions consist of the cosmological parameters, Ω0, Λ, H0, and of an initial fluctuation spectrum such as the CDM power spectrum. The remaining parameter, i.e. the amplitude of these initial fluctuations, is calibrated by the measured anisotropies of the microwave background. Numerical simulations have become an important tool for exploring the detailed predictions of these ab-initio models. However, extending these studies to understand the formation of luminous components of galaxies is difficult since our knowledge of the star formation process and its interaction (feedback) with the surrounding interstellar medium is still rather limited. Moreover, computational resources strongly constrain their baryonic resolution, thus most of the simulations that focused on the formation of individual galaxies were not performed in a cosmological scenario.

 Smoothed particle hydrodynamic (SPH) simulations implemented with chemo-photometric evolutionary population synthesis models, able  to predict the spectral energy distribution from UV to 1 mm, as described in previous papers (Mazzei et al. 1992, Mazzei et al. 1994,, Mobasher & Mazzei 2000), are a useful tool to shed light on several open questions (Mazzei & Curir 2001, Mazzei & Curir 2003).

We performed high resolution cosmological simulations to explore, for the first time in a self-consistent way, the growth and the evolution of the bar instability in baryonic disks embedded at high redshift in a suitable dark matter halo and evolving in a fully consistent cosmological framework.
We aim at pointing out the impact of the cosmological framework, of different disk-to-halo mass ratios, different gas fractions as so as the role of the onset of the star formation, on the evolution of the disks. We used the 2.6 version of the tree-SPH parallel code GADGET (courtesy of V. Springel). The simulations run on the SP4 and CLX computers located at the CINECA computing centre and, partially, also on local workstations.

Results
A long living bar, lasting about 10 Gyr, appears in all our simulations of pure stellar disks evolving in a fully cosmological context (Curir et al. 2006).
Disks expected to be stable according to classical criteria, i.e. dark matter (DM) dominated disks, form indeed weak bars living up to z=0 (see figure). This is due to the dynamical properties of our cosmological halo which is far from stability and isotropy, at variance with the classical halos used in the literature. Stronger bars arise in the more massive disks whose morphology at z=0 is driven by the initial value of the Toomre parameter of the disk. Moreover, inside DM dominated disks, a stellar bar, lasting 10 Gyr, is still living at z=0 even if the gaseous fraction exceeds half of the disk mass (Curir et al. 2007). However, in the most massive disks we find a threshold value of the gas fraction (0.2) able to destroy the bar.
Long living bars appear in all the simulations when the star formation rate is switched on, even in the more massive disks with gas fraction larger than the previous threshold value (Curir et al. 2008).

Future work
We aim at investigating:
i) the role of the star formation rate on the disk evolution by performing cosmological simulations of the same disk-to-halo mass systems as in a previous work, where the star formation was switched off.
ii) the impact of the disk evolution on the halo with particular attention to its geometry as given by its triaxiality ratio.

People:P. Mazzei

Collaboration: A. Curir, G. Murante (INAF OA Torino)

Publications: Curir et al. (2008), A&A, 481,651; Curir et al. (2007), A&A, 467,509