398 research outputs found
Too small to form a galaxy: How the UV background determines the baryon fraction
The cosmic ultraviolet background (UVB) heats the intergalactic medium (IGM),
as a result the gas in dark matter halos below a certain mass is too hot to
cool within a Hubble time. The UVB effectively suppresses the formation of
dwarf galaxies. Using high resolution cosmological hydrodynamical simulations
we show that photo heating leads to small baryon fractions in halos below ~
6x10^9 h^{-1}M_sun, independent of the cosmic environment. The simulations are
carried out assuming a homogeneous UVB with flux densities as given by Haardt &
Madau (1996). A halo may stop to condense gas significantly after the universe
is reionised, namely when its mass falls below the characteristic mass scale
set by the photo heating. Assuming a spherical halo model we derive this
characteristic mass analytically and identify the main mechanisms that prevent
the gas from cooling in small halos. The theoretically derived characteristic
mass is smaller than the one obtained from observations. Increasing the energy
per ionising photon by a factor between four and eight would be sufficient to
reconcile both. This is equivalent to an average temperature of the IGM of ~
10^4 K. In this sense the faint end of the luminosity function may serve as a
calorimeter for the IGM.Comment: To appear in Proceedings of IAU Symp #244, "Dark Galaxies and Lost
Baryons", June 2007, 5 pages including 3 figure
Modelling Baryon Acoustic Oscillations with Perturbation Theory and Stochastic Halo Biasing
In this work we investigate the generation of mock halo catalogues based on
perturbation theory and nonlinear stochastic biasing with the novel
PATCHY-code. In particular, we use Augmented Lagrangian Perturbation Theory
(ALPT) to generate a dark matter density field on a mesh starting from Gaussian
fluctuations and to compute the peculiar velocity field. ALPT is based on a
combination of second order LPT (2LPT) on large scales and the spherical
collapse model on smaller scales. We account for the systematic deviation of
perturbative approaches from N-body simulations together with halo biasing
adopting an exponential bias model. We then account for stochastic biasing by
defining three regimes: a low, an intermediate and a high density regime, using
a Poisson distribution in the intermediate regime and the negative binomial
distribution to model over-dispersion in the high density regime. Since we
focus in this study on massive halos, we suppress the generation of halos in
the low density regime. The various nonlinear and stochastic biasing
parameters, and density thresholds (five) are calibrated with the large
BigMultiDark N-body simulation to match the power spectrum of the corresponding
halo population. Our mock catalogues show power spectra, both in real- and
redshift-space, which are compatible with N-body simulations within about 2% up
to k ~ 1 h Mpc^-1 at z = 0.577 for a sample of halos with the typical BOSS
CMASS galaxy number density. The corresponding correlation functions are
compatible down to a few Mpc. We also find that neglecting over-dispersion in
high density regions produces power spectra with deviations of 10% at k ~ 0.4 h
Mpc^-1. These results indicate the need to account for an accurate statistical
description of the galaxy clustering for precise studies of large-scale
surveys.Comment: 5 pages, 4 figure
On the shape of dark matter halos from MultiDark Planck simulations
The halo shape plays a central role in determining important observational
properties of the haloes such as mass, concentration and lensing
cross-sections. The triaxiality of lensing galaxy clusters has a substantial
impact on the distribution of the largest Einstein radii, while weak lensing
techniques are sensitive to the intrinsic halo ellipticity. In this work, we
provide scaling relations for the shapes of dark matter haloes as a function of
mass (peak height) and redshift over more than four orders of magnitude in halo
masses, namely from to M. We have
analysed four dark matter only simulations from the MultiDark cosmological
simulation suite with more than 56 billion particles within boxes of 4.0, 2.5,
1.0 and 0.4 Gpc size assuming \textit{Planck} cosmology. The dark
matter haloes have been identified in the simulations using the {\sc rockstar}
halo finder, which also determines the axis ratios in terms of the
diagonalization of the inertia tensor. In order to infer the shape for a
hypothetical halo of a given mass at a given redshift, we provide fitting
functions to the minor-to-major and intermediate-to-major axis ratios as a
function of the peak height.Comment: Accepted for publication in MNRAS (14 pages, 13 figures). The
ROCKSTAR outputs used in this paper are available at
https://www.cosmosim.org/cms/simulations/data
Dwarf galaxies in voids: Suppressing star formation with photo-heating
We study structure formation in cosmological void regions using
high-resolution hydrodynamical simulations. Despite being significantly
underdense, voids are populated abundantly with small dark matter halos which
should appear as dwarf galaxies if their star formation is not suppressed
significantly. We here investigate to which extent the cosmological
UV-background photo-evaporates baryons out of halos of dwarf galaxies, and
thereby limits their cooling and star formation rates. Assuming a Haardt &
Madau UV-background with reionisation at redshift z=6, our samples of simulated
galaxies show that halos with masses below a characteristic mass of M_c(z=0) =
6.5 x 10^9 h^{-1} M_sun are baryon-poor, but in general not completely empty,
because baryons that are in the condensed cold phase or are already locked up
in stars resist evaporation. In halos with mass M < M_c, we find that
photo-heating suppresses further cooling of gas. The redshift and UV-background
dependent characteristic mass M_c(z) can be understood from the equilibrium
temperature between heating and cooling at a characteristic overdensity of
\delta ~ 1000. If a halo is massive enough to compress gas to this density
despite the presence of the UV background, gas is free to `enter' the condensed
phase and cooling continues in the halo, otherwise it stalls. By analysing the
mass accretion histories of dwarf galaxies in voids, we show that they can
build up a significant amount of condensed mass at early times before the epoch
of reionisation. Later on, the amount of mass in this phase remains roughly
constant, but the masses of the dark matter halos continue to increase.
(abridged)Comment: revised version as accepted by MNRAS, 15 pages, 15 figures, new
simulation results and a significantly extended discussion have been include
Accurate mass and velocity functions of dark matter halos
-body cosmological simulations are an essential tool to understand the
observed distribution of galaxies. We use the MultiDark simulation suite, run
with the Planck cosmological parameters, to revisit the mass and velocity
functions. At redshift , the simulations cover four orders of magnitude in
halo mass from with 8,783,874 distinct halos and 532,533
subhalos. The total volume used is 515 Gpc, more than 8 times larger
than in previous studies. We measure and model the halo mass function, its
covariance matrix w.r.t halo mass and the large scale halo bias. With the
formalism of the excursion-set mass function, we explicit the tight
interconnection between the covariance matrix, bias and halo mass function. We
obtain a very accurate ( level) model of the distinct halo mass function.
We also model the subhalo mass function and its relation to the distinct halo
mass function. The set of models obtained provides a complete and precise
framework for the description of halos in the concordance Planck cosmology.
Finally, we provide precise analytical fits of the maximum velocity
function up to redshift to push for the development of halo occupation
distribution using . The data and the analysis code are made publicly
available in the \textit{Skies and Universes} database.Comment: Corresponding data is available at the Skies and Universes data base:
http://projects.ift.uam-csic.es/skies-universe
Is WMAP3 normalization compatible with the X-Ray cluster abundance?
We present the mass and X-ray temperature functions derived from a sample of
more than 15,000 galaxy clusters of the MareNostrum Universe cosmological SPH
simulations. In these simulations, we follow structure formation in a cubic
volume of 500/h Mpc on a side assuming cosmological parameters consistent with
either the first or third year WMAP data and gaussian initial conditions. We
compare our numerical predictions with the most recent observational estimates
of the cluster X-ray temperature functions and find that the low normalization
cosmological model inferred from the 3 year WMAP data results is barely
compatible with the present epoch X-ray cluster abundances. We can only
reconcile the simulations with the observational data if we assume a
normalization of the Mass-Temperature relation which is a factor of 2.5--3
smaller than our non-radiative simulations predict. This deviation seems to be
too large to be accounted by the effects of star formation or cooling in the
ICM, not taken into account in these simulations.Comment: 4 pages, 3 figures. Accepted for publication in The Astrophysical
Journal Letter
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