426 research outputs found
The Stellar Initial Mass Function at the Epoch of Reionization
I provide estimates of the ultraviolet and visible light luminosity density
at z~6 after accounting for the contribution from faint galaxies below the
detection limit of deep Hubble and Spitzer surveys. I find the rest-frame
V-band luminosity density is a factor of ~2-3 below the ultraviolet luminosity
density at z~6. This implies that the maximal age of the stellar population at
z~6, for a Salpeter initial mass function, and a single, passively evolving
burst, must be <100 Myr. If the stars in z~6 galaxies are remnants of the
star-formation that was responsible for ionizing the intergalactic medium,
reionization must have been a brief process that was completed at z<7. This
assumes the most current estimates of the clumping factor and escape fraction
and a Salpeter slope extending up to 200 M_{\sun} for the stellar initial mass
function (IMF; dN/dM \propto M^{\alpha}, \alpha=-2.3). Unless the ratio of the
clumping factor to escape fraction is less than 60, a Salpeter slope for the
stellar IMF and reionization redshift higher than 7 is ruled out. In order to
maintain an ionized intergalactic medium from redshift 9 onwards, the stellar
IMF must have a slope of \alpha=-1.65 even if stars as massive as ~200 M_{\sun}
are formed. Correspondingly, if the intergalactic medium was ionized from
redshift 11 onwards, the IMF must have \alpha~-1.5. The range of stellar mass
densities at z~6 straddled by IMFs which result in reionization at z>7 is
1.3+/-0.4\times10^{7} Msun/Mpc^3.Comment: 25 pages, 2 tables, 6 figures, ApJ, in press, v680 n
Metal and molecule cooling in simulations of structure formation
Cooling is the main process leading to the condensation of gas in the dark
matter potential wells and consequently to star and structure formation. In a
metal-free environment, the main available coolants are H, He, H and HD;
once the gas is enriched with metals, these also become important in defining
the cooling properties of the gas. We discuss the implementation in Gadget-2 of
molecular and metal cooling at temperatures lower that , following
the time dependent properties of the gas and pollution from stellar evolution.
We have checked the validity of our scheme comparing the results of some test
runs with previous calculations of cosmic abundance evolution and structure
formation, finding excellent agreement. We have also investigated the relevance
of molecule and metal cooling in some specific cases, finding that inclusion of
HD cooling results in a higher clumping factor of the gas at high redshifts,
while metal cooling at low temperatures can have a significant impact on the
formation and evolution of cold objects.Comment: 9 pages, plus appendices. Revised version. MNRAS accepte
How does gas cool in DM halos?
In order to study the process of cooling in dark-matter (DM) halos and assess
how well simple models can represent it, we run a set of radiative SPH
hydrodynamical simulations of isolated halos, with gas sitting initially in
hydrostatic equilibrium within Navarro-Frenk-White (NFW) potential wells. [...]
After having assessed the numerical stability of the simulations, we compare
the resulting evolution of the cooled mass with the predictions of the
classical cooling model of White & Frenk and of the cooling model proposed in
the MORGANA code of galaxy formation. We find that the classical model predicts
fractions of cooled mass which, after about two central cooling times, are
about one order of magnitude smaller than those found in simulations. Although
this difference decreases with time, after 8 central cooling times, when
simulations are stopped, the difference still amounts to a factor of 2-3. We
ascribe this difference to the lack of validity of the assumption that a mass
shell takes one cooling time, as computed on the initial conditions, to cool to
very low temperature. [...] The MORGANA model [...] better agrees with the
cooled mass fraction found in the simulations, especially at early times, when
the density profile of the cooling gas is shallow. With the addition of the
simple assumption that the increase of the radius of the cooling region is
counteracted by a shrinking at the sound speed, the MORGANA model is also able
to reproduce for all simulations the evolution of the cooled mass fraction to
within 20-50 per cent, thereby providing a substantial improvement with respect
to the classical model. Finally, we provide a very simple fitting function
which accurately reproduces the cooling flow for the first ~10 central cooling
times. [Abridged]Comment: 15 pages, accepted by MNRA
Simulating the formation of a proto-cluster at z~2
We present results from two high-resolution hydrodynamical simulations of
proto-cluster regions at z~2.1. The simulations have been compared to
observational results for the socalled Spiderweb galaxy system, the core of a
putative proto-cluster region at z = 2.16, found around a radio galaxy. The
simulated regions have been chosen so as to form a poor cluster with M200~10^14
h-1 Msun (C1) and a rich cluster with M200~2x10^15 h-1 Msun (C2) at z = 0. The
simulated proto-clusters show evidence of ongoing assembly of a dominating
central galaxy. The stellar mass of the brightest cluster galaxy (BCG) of the
C2 system is in excess with respect to observational estimates for the
Spiderweb galaxy, with a total star formation rate which is also larger than
indicated by observations. We find that the projected velocities of galaxies in
the C2 cluster are consistent with observations, while those measured for the
poorer cluster C1 are too low compared to the observed velocities. We argue
that the Spiderweb complex resemble the high-redshift progenitor of a rich
galaxy cluster. Our results indicate that the included supernovae feedback is
not enough to suppress star formation in these systems, supporting the need of
introducing AGN feedback. According to our simulations, a diffuse atmosphere of
hot gas in hydrostatic equilibrium should already be present at this redshift,
and enriched at a level comparable to that of nearby galaxy clusters. The
presence of this gas should be detectable with future deep X-ray observations.Comment: 6 pages, 4 figures, accepted for publication in MNRAS (Letters
Lyman Alpha Emitter Evolution in the Reionization Epoch
Combining cosmological SPH simulations with a previously developed Lyman
Alpha production/transmission model and the Early Reionization Model (ERM,
reionization ends at redshift z~7), we obtain Lyman Alpha and UV Luminosity
Functions (LFs) for Lyman Alpha Emitters (LAEs) for redshifts between 5.7 and
7.6. Matching model results to observations at z~5.7 requires escape fractions
of Lyman Alpha, f_alpha=0.3, and UV (non-ionizing) continuum photons, f_c=0.22,
corresponding to a color excess, E(B-V)=0.15. We find that (i) f_c increases
towards higher redshifts, due the decreasing mean dust content of galaxies,
(ii) the evolution of f_alpha/f_c hints at the dust content of the ISM becoming
progressively inhomogeneous/clumped with decreasing redshift. The clustering
photoionization boost is important during the initial reionization phases but
has little effect on the Lyman Alpha LF for a highly ionized IGM. Halo
(stellar) masses are in the range 10.0 < \log M_h < 11.8 (8.1 < \log M_* <
10.4) with M_h \propto M_*^{0.64}. The star formation rates are between 3-120
solar masses per year, mass-weighted mean ages are greater than 20 Myr at all
redshifts, while the mean stellar metallicity increases from Z=0.12 to 0.22
solar metallicity from z~7.6 to z~5.7; both age and metallicity positively
correlate with stellar mass. The brightest LAEs are all characterized by large
star formation rates and intermediate ages (~200 Myr), while objects in the
faint end of the Lyman Alpha LF show large age and star formation rate spreads.
With no more free parameters, the Spectral Energy Distributions of three LAE at
z~5.7 observed by Lai et al. (2007) are well reproduced by an intermediate age
(182-220 Myr) stellar population and the above E(B-V) value.Comment: 13 pages, 9 figures, accepted to MNRA
X-MAS2: Study Systematics on the ICM Metallicity Measurements
(Abridged)The X-ray measurements of the ICM metallicity are becoming more
frequent due to the availability of powerful X-ray telescope with excellent
spatial and spectral resolutions. The information which can be extracted from
the measurements of the alpha-elements, like Oxygen, Magnesium and Silicon with
respect to the Iron abundance is extremely important to better understand the
stellar formation and its evolutionary history. In this paper we investigate
possible source of bias connected to the plasma physics when recovering metal
abundances from X-ray spectra. To do this we analyze 6 simulated galaxy
clusters processed through the new version of our X-ray MAp Simulator, which
allows to create mock XMM-Newton EPIC MOS1 and MOS2 observations. By comparing
the spectroscopic results to the input values we find that: i) Fe is recovered
with high accuracy for both hot (T>3 keV) and cold (T<2 keV) systems; at
intermediate temperatures, however, we find a systematic overestimate which
depends on the number counts; ii) O is well recovered in cold clusters, while
in hot systems its measure may overestimate by a factor up to 2-3; iii) Being a
weak line, the measurement of Mg is always difficult; despite of this, for cold
systems (T<2 keV) we do not find any systematic behavior, while for very hot
systems (T>5 keV) the spectroscopic measurement may be strongly overestimated
up to a factor of 4; iv) Si is well recovered for all the clusters in our
sample. We investigate in detail the nature of the systematic effects and
biases found. We conclude that they are mainly connected with the
multi-temperature nature of the projected observed spectra and to the intrinsic
limitation of the XMM-Newton EPIC spectral resolution that does not always
allow to disentangle among the emission lines produced by different elements.Comment: (e.g.: 17 pages, 8 figures, accepted for publication in the
Astrophysical Journal, updated discussion to match published version-new
section:6.3
Properties of the galaxy population in hydrodynamical simulations of clusters
We present a study of the galaxy population predicted by hydrodynamical
simulations for a set of 19 galaxy clusters based on the GADGET-2 Tree+SPH
code. These simulations include gas cooling, star formation, a detailed
treatment of stellar evolution and chemical enrichment, as well as SN energy
feedback in the form of galactic winds. We compute the spectro-photometric
properties of the simulated galaxies. All simulations have been performed for
two choices of the stellar initial mass function: a standard Salpeter IMF, and
a top-heavier IMF. Several of the observational properties of the galaxy
population in nearby clusters are reproduced fairly well by simulations. A
Salpeter IMF is successful in accounting for the slope and the normalization of
the color-magnitude relation for the bulk of the galaxy population. Simulated
clusters have a relation between mass and optical luminosity which generally
agrees with observations, both in normalization and slope. We find that
galaxies are generally bluer, younger and more star forming in the cluster
outskirts, thus reproducing the observational trends. However, simulated
clusters have a total number of galaxies which is significantly smaller than
the observed one, falling short by about a factor 2-3. Finally, the brightest
cluster galaxies are always predicted to be too massive and too blue, when
compared to observations, due to gas overcooling in the core cluster regions,
even in the presence of a rather efficient SN feedback.Comment: 15 pages, 17 figures, to appear in MNRA
The impact of feedback on the low-redshift intergalactic medium
We analyse the evolution of the properties of the low-redshift Intergalactic Medium (IGM) using high-resolution hydrodynamic simulations that include a detailed chemical evolution model. We focus on the effects that two different forms of energy feedback, strong galactic winds driven by supernova explosion and Active Galactic Nuclei (AGN) powered by gas accretion onto super-massive black holes (BHs), have on the thermo- and chemo-dynamical properties of of the low redshift IGM. We find that feedback associated to winds (W) and BHs leave distinct signatures in both the chemical and thermal history of the baryons, especially at redshift z<3 [..] We present results for the enrichment in terms of mass and metallicity distributions for the WHIM phase, both as a function of density and temperature. Finally, we compute the evolution of the relative abundances between different heavy elements, namely Oxygen, Carbon and Iron. While both C/O and O/Fe evolve differently at high redshifts for different feedback models, their values are similar at z=0 [..]. The sensitivity of WHIM properties on the implemented feedback scheme could be important both for discriminating between different feedback physics and for detecting the WHIM with future far-UV and X-ray telescopes
Reproducing the entropy structure in galaxy groups
We carry out a comparison between observations and hydrodynamic simulations
of entropy profiles of groups and clusters of galaxies. We use the Tree+SPH
GADGET code to simulate four halos of sizes in the M_500 = 1.0 - 16.e13 h^-1
Msun range, corresponding to poor groups up to Virgo-like clusters. We
concentrate on the effect of introducing radiative cooling, star formation, and
a variety of non-gravitational heating schemes on the entropy structure and the
stellar fraction. We show that all the simulations result in a correct entropy
profile for the Virgo-like cluster. With the heating energy budget of ~0.7
keV/particle injected at z_h=3, we are also able to reproduce the entropy
profiles of groups. We obtain the flat entropy cores as a combined effect of
preheating and cooling, while we achieve the high entropy at outskirts by
preheating. The resulting baryon fraction locked into stars is in the 25-30%
range, compared to 35-40% in the case of no preheating. Heating at higher
redshift, z_h=9, strongly delays the star-formation, but fails to produce a
sufficiently high specific entropy.Comment: 5 page, A&A in pres
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