111 research outputs found
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
Can the Near-IR Fluctuations Arise from Known Galaxy Populations?
Spatial Fluctuations in the Cosmic Infrared Background have now been measured
out to sub-degree scales showing a strong clustering signal from unresolved
sources. We attempt to explain these measurement by considering faint galaxy
populations at z<6 as the underlying sources for this signal using 233 measured
UV, optical and NIR luminosity functions (LF) from a variety of surveys
covering a wide range of redshifts. We populate the lightcone and calculate the
total emission redshifted into the near-IR bands in the observer frame and
recover the observed optical and near-IR galaxy counts to a good accuracy.
Using a halo model for the clustering of galaxies with an underlying LCDM
density field, we find that fluctuations from known galaxy populations are
unable to account for the large scale CIB clustering signal seen by HST/NICMOS,
Spitzer/IRAC and AKARI/IRC and continue to diverge out to larger angular
scales. Our purely empirical reconstruction shows that known galaxy populations
are not responsible for the bulk of the fluctuation signal seen in the
measurements and suggests an unknown population of very faint and highly
clustered sources dominating the signal.Comment: 3 pages, 2 figures. To appear in proceedings of First Stars IV
meeting (Kyoto, Japan; 2012
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