275 research outputs found
Dark Matter Halo Environment for Primordial Star Formation
We study the statistical properties (such as shape and spin) of high-z halos
likely hosting the first (PopIII) stars with cosmological simulations including
detailed gas physics. In the redshift range considered () the
average sphericity is , and for more than 90% of halos the
triaxiality parameter is , showing a clear preference for
oblateness over prolateness. Larger halos in the simulation tend to be both
more spherical and prolate: we find and , with and at z = 11.
The spin distributions of dark matter and gas are considerably different at
, with the baryons rotating slower than the dark matter. At lower
redshift, instead, the spin distributions of dark matter and gas track each
other almost perfectly, as a consequence of a longer time interval available
for momentum redistribution between the two components. The spin of both the
gas and dark matter follows a lognormal distribution, with a mean value at z=16
of , virtually independent of halo mass. This is in good
agreement with previous studies. Using the results of two feedback models (MT1
and MT2) by McKee & Tan (2008) and mapping our halo spin distribution into a
PopIII IMF, we find that at high- the IMF closely tracks the spin lognormal
distribution. Depending on the feedback model, though, the distribution can be
centered at (MT1) or (MT2). At later
times, model MT1 evolves into a bimodal distribution with a second prominent
peak located at as a result of the non-linear relation between
rotation and halo mass. We conclude that the dark matter halo properties might
be a key factor shaping the IMF of the first stars.Comment: 10 pages, 6 figures, accepted for publication in MNRA
Contradiction between strong lensing statistics and a feedback solution to the cusp/core problem
Standard cosmology has many successes on large scales, but faces some
fundamental difficulties on small, galactic scales. One such difficulty is the
cusp/core problem. High resolution observations of the rotation curves for dark
matter dominated low surface brightness (LSB) galaxies imply that galactic dark
matter halos have a density profile with a flat central core, whereas N-body
structure formation simulations predict a divergent (cuspy) density profile at
the center. It has been proposed that this problem can be resolved by stellar
feedback driving turbulent gas motion that erases the initial cusp. However,
strong gravitational lensing prefers a cuspy density profile for galactic
halos. In this paper, we use the most recent high resolution observations of
the rotation curves of LSB galaxies to fit the core size as a function of halo
mass, and compare the resultant lensing probability to the observational
results for the well defined combined sample of the Cosmic Lens All-Sky Survey
(CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS). The
lensing probabilities based on such density profiles are too low to match the
observed lensing in CLASS/JVAS. High baryon densities in the galaxies that
dominate the lensing statistics can reconcile this discrepancy, but only if
they steepen the mass profile rather than making it more shallow. This places
contradictory demands upon the effects of baryons on the central mass profiles
of galaxies.Comment: 8 pages, 3 figures. Largely improved compared with the version 1 to
reflect the referees' reports, conclusions unchanged. Published in Research
in Astronomy and Astrophysics (RAA
Population III stars and the Long Gamma Ray Burst rate
Because massive, low-metallicity population III (PopIII) stars may produce
very powerful long gamma-ray bursts (LGRBs), high-redshift GRB observations
could probe the properties of the first stars. We analyze the correlation
between early PopIII stars and LGRBs by using cosmological
N-body/hydrodynamical simulations, which include detailed chemical evolution,
cooling, star formation, feedback effects and the transition between PopIII and
more standard population I/II (PopII/I) stars. From the Swift observed rate of
LGRBs, we estimate the fraction of black holes that will produce a GRB from
PopII/I stars to be in the range 0.028<f_{GRB}<0.140, depending on the assumed
upper metallicity of the progenitor. Assuming that as of today no GRB event has
been associated to a PopIII star, we estimate the upper limit for the fraction
of LGRBs produced by PopIII stars to be in the range 0.006<f_{GRB}<0.022. When
we apply a detection threshold compatible with the BAT instrument, we find that
the expected fraction of PopIII GRBs (GRB3) is ~10% of the full LGRB population
at z>6, becoming as high has 40% at z>10. Finally, we study the properties of
the galaxies hosting our sample of GRB3. We find that the average metallicity
of the galaxies hosting a GRB3 is typically higher than the critical
metallicity used to select the PopIII stars, due to the efficiency in polluting
the gas above such low values. We also find that the highest probability of
finding a GRB3 is within galaxies with a stellar mass <10^7 Msun, independently
from the redshift.Comment: 8 pages,3 figures. Submitted to MNRAS, revised version after
referee's comment
Effect of metallicity on the gravitational-wave signal from the cosmological population of compact binary coalescences
Recent studies on stellar evolution have shown that the properties of compact
objects strongly depend on the metallicity of the environment in which they
were formed. Using some very simple assumptions on the metallicity of the
stellar populations, we explore how this property affects the unresolved
gravitational-wave background from extragalactic compact binaries. We obtained
a suit of models using population synthesis code, estimated the
gravitational-wave background they produce, and discuss its detectability with
second- (advanced LIGO, advanced Virgo) and third- (Einstein Telescope)
generation detectors. Our results show that the background is dominated by
binary black holes for all considered models in the frequency range of
terrestrial detectors, and that it could be detected in most cases by advanced
LIGO/Virgo, and with Einstein Telescope with a very high signal-to-noise ratio.
The observed peak in a gravitational wave spectrum depends on the metallicity
of the stellar population.Comment: 9 pages, 5 figures, accepted to A&
Feedback from the IR Background in the Early Universe
It is commonly believed that the earliest stages of star-formation in the
Universe were self-regulated by global radiation backgrounds - either by the
ultraviolet Lyman-Werner (LW) photons emitted by the first stars (directly
photodissociating H_2), or by the X-rays produced by accretion onto the black
hole (BH) remnants of these stars (heating the gas but catalyzing H_2
formation). Recent studies have suggested that a significant fraction of the
first stars may have had low masses (a few M_sun). Such stars do not leave BH
remnants and they have softer spectra, with copious infrared (IR) radiation at
photon energies around 1eV. Similar to LW and X-ray photons, these photons have
a mean-free path comparable to the Hubble distance, building up an early IR
background. Here we show that if soft-spectrum stars, with masses of a few
M_sun, contributed more than 1% of the UV background (or their mass fraction
exceeded 90%), then their IR radiation dominated radiative feedback in the
early Universe. The feedback is different from the UV feedback from high-mass
stars, and occurs through the photo-detachment of H^- ions, necessary for
efficient H_2 formation. Nevertheless, we find that the baryon fraction which
must be incorporated into low-mass stars in order to suppress H_2-cooling is
only a factor of few higher than for high-mass stars.Comment: Accepted for publication in MNRAS (Letters). 5 pages with 2 figure
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