Feedback and metal enrichment in cosmological SPH simulations I. A model for chemical enrichment

We discuss a model for treating chemical enrichment by SNII and SNIa explosions in simulations of cosmological structure formation. Our model includes metal-dependent radiative cooling and star formation in dense collapsed gas clumps. Metals are returned into the diffuse interstellar medium by star particles using a local SPH smoothing kernel. A variety of chemical abundance patterns in enriched gas arise in our treatment owing to the different yields and lifetimes of SNII and SNIa progenitor stars. In the case of SNII chemical production, we adopt metal-dependent yields. Because of the sensitive dependence of cooling rates on metallicity, enrichment of galactic haloes with metals can in principle significantly alter subsequent gas infall and the build up of the stellar components. Indeed, in simulations of isolated galaxies we find that a consistent treatment of metal-dependent cooling produces 25% more stars outside the central region than simulations with a primordial cooling function. In the highly-enriched central regions, the evolution of baryons is however not affected by metal cooling, because here the gas is always dense enough to cool. A similar situation is found in cosmological simulations because we include no strong feedback processes which could spread metals over large distances and mix them into unenriched diffuse gas. We demonstrate this explicitly with test simulations which adopt super-solar cooling functions leading to large changes both in the stellar mass and in the metal distributions. We also find that the impact of metallicity on the star formation histories of galaxies may depend on their particular evolutionary history. Our results hence emphasise the importance of feedback processes for interpreting the cosmic metal enrichment.Comment: 15 pages, 15 figures, MNRAS, modified to match published versio

The Role of Clustering of Sub-Clumps in Bright Elliptical Galaxy Formation from a Low-Spin Seed Galaxy

We reveal the role of clustering of sub-clumps, which is expected in the cold dark matter (CDM) universe, in forming a bright elliptical galaxy (BEG) from a low-spin seed galaxy. This can be done by comparing the evolution of a low-spin seed galaxy including small-scale density fluctuations expected in the CDM universe (Model 1) with that of a completely uniform one (Model 2), using numerical experiments. We show that Model 2 cannot reproduce the properties of BEGs and forms a disk which is too compact and too bright due to the conservation of the initial-small angular momentum. In Model 1 clustering of the sub-clumps caused by initial small-scale density fluctuations leads to angular momentum transfer from the baryon component to the dark matter and consequently a nearly spherical system supported by random motions is formed. Moreover the collisionless property of the stars formed in the sub-clumps prevents the dissipative contraction of the system, leading to a large measured half-light radius. As a result, the end-product is quite well reproduces the observed properties of BEGs, such as the de Vaucouleurs light-profile, typical color and metallicity gradients, the large half-light radius, the small ratio of the rotational velocity to the velocity dispersion (V/\sigma). We conclude that the clustering of sub-clumps, i.e., the hierarchical clustering, plays a crucial role in the formation of BEGs from a low-spin seed galaxy.Comment: 10pages, 10figures, accepted for publication in the Astrophysical Journal. Low resolution figures included; high resolution version available at http://www.astr.tohoku.ac.jp/~kawata/research/papers.htm