A sub-resolution multiphase interstellar medium model of star formation and SNe energy feedback

We present a new multi-phase sub-resolution model for star formation and feedback in SPH numerical simulations of galaxy formation. Our model, called MUPPI (MUlti-Phase Particle Integrator), describes each gas particle as a multi-phase system, with cold and hot gas phases, coexisting in pressure equilibrium, and a stellar component. Cooling of the hot tenuous gas phase feeds the cold gas phase. Stars are formed out of molecular gas with a given efficiency, which scales with the dynamical time of the cold phase. Our prescription for star formation is not based on imposing the Schmidt-Kennicutt relation, which is instead naturally produced by MUPPI. Energy from supernova explosions is deposited partly into the hot phase of the gas particles, and partly to that of neighboring particles. Mass and energy flows among the different phases of each particle are described by a set of ordinary differential equations which we explicitly integrate for each gas particle, instead of relying on equilibrium solutions. This system of equations also includes the response of the multi-phase structure to energy changes associated to the thermodynamics of the gas. We apply our model to two isolated disk galaxy simulations and two spherical cooling flows. MUPPI is able to reproduce the Schmidt-Kennicutt relation for disc galaxies. It also reproduces the basic properties of the inter-stellar medium in disc galaxies, the surface densities of cold and molecular gas, of stars and of star formation rate, the vertical velocity dispersion of cold clouds and the flows connected to the galactic fountains. Quite remarkably, MUPPI also provides efficient stellar feedback without the need to include a scheme of kinetic energy feedback. [abridged]Comment: 23 pages, 26 figures, MNRAS accepte

Faint Lyman-Break galaxies as a crucial test for galaxy formation models

It has recently been shown that galaxy formation models within the LambdaCDM cosmology predict that, compared to the observed population, small galaxies (with stellar masses < 10^{11} M_sun) form too early, are too passive since z ~ 3 and host too old stellar populations at z=0. We then expect an overproduction of small galaxies at z > 4 that should be visible as an excess of faint Lyman-break galaxies. To check whether this excess is present, we use the MORGANA galaxy formation model and GRASIL spectro-photometric + radiative transfer code to generate mock catalogues of deep fields observed with HST-ACS. We add observational noise and the effect of Lyman-alpha emission, and perform color-color selections to identify Lyman-break galaxies. The resulting mock candidates have plausible properties that closely resemble those of observed galaxies. We are able to reproduce the evolution of the bright tail of the luminosity function of Lyman-break galaxies (with a possible underestimate of the number of the brightest i-dropouts), but uncertainties and degeneracies in dust absorption parameters do not allow to give strong constraints to the model. Besides, our model shows a clear excess with respect to observations of faint Lyman-break galaxies, especially of z_{850} ~ 27 V-dropouts at z ~ 5. We quantify the properties of these "excess" galaxies and discuss the implications: these galaxies are hosted in dark matter halos with circular velocities in excess of 100 km s^{-1}, and their suppression may require a deep re-thinking of stellar feedback processes taking place in galaxy formation.Comment: 17 pages, 13 figures, 1 table; accepted for publication by MNRA

The Degeneracy of Galaxy Formation Models

We develop a new formalism for modeling the formation and evolution of galaxies within a hierarchical universe. Similarly to standard semi-analytical models we trace galaxies inside dark-matter merger-trees. The formalism includes treatment of feedback, star-formation, cooling, smooth accretion, gas stripping in satellite galaxies, and merger-induced star bursts. However, unlike in other models, each process is assumed to have an efficiency which depends only on the host halo mass and redshift. This allows us to describe the various components of the model in a simple and transparent way. By allowing the efficiencies to have any value for a given halo mass and redshift, we can easily encompass a large range of scenarios. To demonstrate this point, we examine several different galaxy formation models, which are all consistent with the observational data. Each model is characterized by a different unique feature: cold accretion in low mass haloes, zero feedback, stars formed only in merger-induced bursts, and shutdown of star-formation after mergers. Using these models we are able to examine the degeneracy inherent in galaxy formation models, and look for observational data that will help to break this degeneracy. We show that the full distribution of star-formation rates in a given stellar mass bin is promising in constraining the models. We compare our approach in detail to the semi-analytical model of De Lucia & Blaizot. It is shown that our formalism is able to produce a very similar population of galaxies once the same median efficiencies per halo mass and redshift are being used. We provide a public version of the model galaxies on our web-page, along with a tool for running models with user-defined parameters. Our model is able to provide results for a 62.5 h^{-1} Mpc box within just a few seconds.Comment: Accepted for publication in MNRAS. Fig 6 & 7 corrected. For the project page which allows running your own model, see http://www.mpa-garching.mpg.de/galform/sesam