6 research outputs found
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