2,217 research outputs found
Dwarf Sphs/First-galaxies connection
I analyze the properties of the first galaxies in cosmological simulations
with radiative feedback. Preliminary results indicate similarities with the
observed properties of the bulk of dwarf spheroidal galaxies (dSphs) in the
Local Group and Andromeda. I briefly discuss observational tests that could
help in understanding the impact of a population of small primordial objects on
the cosmic evolution.Comment: 6 pages, 3 figures. To appear in the proceedings of "The IGM/Galaxy
Connection - The Distribution of Baryons at z=0", ed. M. Putman & J.
Rosenber
Rayleigh-Taylor Instability at Ionization Fronts: Perturbation Analysis
The linear growth rate of the Rayleigh-Taylor instability (RTI) at ionization
fronts is investigated via perturbation analysis in the limit of incompressible
fluids. In agreement with previous numerical studies is found that absorption
of ionizing radiation inside the HII region due to hydrogen recombinations
suppresses the growth of instabilities. In the limit of a large density
contrast at the ionization front the RTI growth rate has the simple analytical
solution n=-nur+(nur^2+gk)^(1/2), where nur is the hydrogen recombination rate
inside the HII region, k is the perturbation's wavenumber and g is the
effective acceleration in the frame of reference of the front. Therefore, the
growth of surface perturbations with wavelengths lambda >> lambda_{cr} = 2\pi
g/nur^2 is suppressed by a factor (lambda_{cr}/4lambda)^(1/2) with respect to
the non-radiative incompressible RTI. Implications on stellar and black hole
feedback are briefly discussed.Comment: Accepted for publication in MNRAS. 9 page
The First Galaxies and the Likely Discovery of their Fossils in the Local Group
In cold dark matter cosmologies, small mass halos outnumber larger mass halos
at any redshift. However, the lower bound for the mass of a galaxy is unknown,
as are the typical luminosity of the smallest galaxies and their numbers in the
universe. The answers depend on the extent to which star formation in the first
population of small mass halos may be suppressed by radiative feedback loops
operating over cosmological distance scales. If early populations of dwarf
galaxies did form in significant number, their relics should be found today in
the Local Group. These relics have been termed "fossils of the first galaxies".
This paper is a review that summarizes our ongoing efforts to simulate and
identify these fossils around the Milky Way and Andromeda.
It is widely believed that reionization of the intergalactic medium would
have stopped star formation in the fossils of the first galaxies. Thus, they
should be among the oldest objects in the Universe. However, here we dispute
this idea and discuss a physical mechanism whereby relatively recent episodes
of gas accretion and star formation would be produced in some fossils of the
first galaxies. We argue that fossils may be characterized either by a single
old population of stars or by a bimodal star formation history. We also propose
that the same mechanism could turn small mass dark halos formed before
reionization into gas-rich but starless "dark galaxies".
We believe that current observational data support the thesis that a fraction
of the new ultra-faint dwarfs recently discovered in the Local Group are in
fact fossils of the first galaxies.Comment: Invited review/tutorial paper, 18 pages, 11 figures. Accepted to
Advances in Astronomy, special issue on "Dwarf-Galaxy Cosmology
X-ray Preionisation Powered by Accretion on the First Black Holes. II: Cosmological Simulations and Observational Signatures
We use cosmological simulations to study the X-ray ionisation and heating of
the intergalactic medium by an early population of accreting black holes. By
considering observational constraints from the X-ray background, we find an
upper limit for the optical depth to Thompson scattering tau_e~0.17. The
redshifted soft X-ray background from these early sources produces:(i) fully
ionised atomic hydrogen in the low density intergalactic medium before redshift
z~7 (consequently stellar reionisation is characterised by an instantaneous
overlap phase of HII regions),(ii) a second HeII reionisation at z~3 and (iii)
heats the intergalactic medium to near 10000 K at low redshifts. The typical
luminosity in the soft X-ray band of the galaxies hosting the black holes is
about one order of magnitude below the sensitivity limit of the Chandra deep
field. About a thousand of these sources may be present per square arcmin of
the sky, producing detectable fluctuations. Few rarer objects could be luminous
enough to be visible in the Chandra deep field. XEUS and Con-X satellites will
be able to detect more of these sources that, if radio loud, could be used to
study the 21cm forest in absorption.A signature of an early X-ray preionisation
is the production of secondary CMB anisotropies on small angular scales. We
find that in these models the power spectrum of temperature fluctuations
increases with decreasing angular scale (dT~16 muK at 1arcsec scales), while
for stellar reionisation scenarios the power decreases on smaller scales. We
also show that the redshifted 21 cm radiation from neutral hydrogen can be
marginally detected in emission at redshifts 7<z<12. At a redshift of about
z~30 a stronger and narrower (in redshift space) signal in absorption against
the CMB, that is peculiar to these models,could be detectable.[abridged]Comment: 14 pages including 14 figures and 2 tables. Accepted for publication
in MNRA
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