465 research outputs found
Stability of the Forward/Reverse Shock System Formed by the Impact of a Relativistic Fireball on an Ambient Medium
We analyze the stability of a relativistic double (forward/reverse) shock
system which forms when the fireball of a Gamma-Ray Burst (GRB) impacts on the
surrounding medium. We find this shock system to be stable to linear global
perturbations for either a uniform or a wind (r^{-2}) density profile of the
ambient medium. For the wind case, we calculate analytically the frequencies of
the normal modes which could modulate the early short-term variability of GRB
afterglows. We find that perturbations in the double shock system could induce
oscillatory fluctuations in the observed flux on short (down to seconds) time
scales during the early phase of an afterglow.Comment: ApJ, submitted, 26 pages, 5 figure
H_2 Cooling of Primordial Gas Triggered by UV Irradiation
We investigate the formation of molecular hydrogen (H_2) in a primordial H+He
gas cloud irradiated by a power-law UV flux. We find that at high densities (>1
cm^{-3}) and low temperatures (<10^4 K), the background radiation enhances the
formation of H_2 and results in molecular cooling dominating over
photoionization heating. This process could accelerate the collapse and
fragmentation of dense objects at high redshifts.Comment: 13 pages, uuencoded compressed Postscript, 3 figures include
Cosmological Origin of the Stellar Velocity Dispersions in Massive Early-Type Galaxies
We show that the observed upper bound on the line-of-sight velocity
dispersion of the stars in an early-type galaxy, sigma<400km/s, may have a
simple dynamical origin within the LCDM cosmological model, under two main
hypotheses. The first is that most of the stars now in the luminous parts of a
giant elliptical formed at redshift z>6. Subsequently, the stars behaved
dynamically just as an additional component of the dark matter. The second
hypothesis is that the mass distribution characteristic of a newly formed dark
matter halo forgets such details of the initial conditions as the stellar
"collisionless matter" that was added to the dense parts of earlier generations
of halos. We also assume that the stellar velocity dispersion does not evolve
much at z<6, because a massive host halo grows mainly by the addition of
material at large radii well away from the stellar core of the galaxy. These
assumptions lead to a predicted number density of ellipticals as a function of
stellar velocity dispersion that is in promising agreement with the Sloan
Digital Sky Survey data.Comment: ApJ, in press (2003); matches published versio
Imprint of Inhomogeneous Reionization on the Power Spectrum of Galaxy Surveys at High Redshifts
We consider the effects of inhomogeneous reionization on the distribution of
galaxies at high redshifts. Modulation of the formation process of the ionizing
sources by large scale density modes makes reionization inhomogeneous and
introduces a spread to the reionization times of different regions with the
same size. After sources photo-ionize and heat these regions to a temperature
\ga 10^4K at different times, their temperatures evolve as the ionized
intergalactic medium (IGM) expands. The varying IGM temperature makes the
minimum mass of galaxies spatially non-uniform with a fluctuation amplitude
that increases towards small scales. These scale-dependent fluctuations modify
the shape of the power spectrum of low-mass galaxies at high redshifts in a way
that depends on the history of reionization. The resulting distortion of the
primordial power spectrum is significantly larger than changes associated with
uncertainties in the inflationary parameters, such as the spectral index of the
scalar power spectrum or the running of the spectral index. Future surveys of
high-redshift galaxies will offer a new probe of the thermal history of the IGM
but might have a more limited scope in constraining inflation.Comment: 8 pages, 5 figures, replaced to match version accepted by Ap
Destruction of Molecular Hydrogen During Cosmological Reionization
We investigate the ability of primordial gas clouds to retain molecular
hydrogen (H_2) during the initial phase of the reionization epoch. We find that
before the Stromgren spheres of the individual ionizing sources overlap, the UV
background below the ionization threshold is able to penetrate large clouds and
suppress their H_2 abundance. The consequent lack of H_2 cooling could prevent
the collapse and fragmentation of clouds with virial temperatures T_vir < 10^4
K (or masses 10^8 Msun [(1+z_vir)/10]^{-3/2}). This negative feedback on
structure-formation arises from the very first ionizing sources, and precedes
the feedback due to the photoionization heating.Comment: 14 pages, uuencoded compressed Postscript, 4 figures included. To
appear in Ap
The Expected Rate of Gamma-Ray Burst Afterglows In Supernova Searches
We predict the rate at which Gamma-Ray Burst (GRB) afterglows should be
detected in supernova searches as a function of limiting flux. Although GRB
afterglows are rarer than supernovae, they are detectable at greater distances
because of their higher intrinsic luminosity. Assuming that GRBs trace the
cosmic star formation history and that every GRB gives rise to a bright
afterglow, we find that the average detection rate of supernovae and afterglows
should be comparable at limiting magnitudes brighter than K=18. The actual rate
of afterglows is expected to be somewhat lower since only a fraction of all
gamma-ray selected GRBs were observed to have associated afterglows. However,
the rate could also be higher if the initial gamma-ray emission from GRB
sources is more beamed than their late afterglow emission. Hence, current and
future supernova searches can place strong constraints on the afterglow
appearance fraction and the initial beaming angle of GRB sources.Comment: 13 pages, submitted to ApJ
The Treaty Power
It seems particularly appropriate and timely that the Federalist Society should address the subject of the treaty power. For I can think of no area of separation of powers law in which recent executive practice has moved further from the original intent of the framers. Judge Sofaer has presented you with one version of how the treaty power has evolved over our Nation\u27s 200-year constitutional history. Let me present a somewhat less sanguine, neo-Federalist account of what has happened to the treaty power over the last two centuries, and during the Reagan administration in particular
Is Double Reionization Physically Plausible?
Recent observations of z~6 quasars and the cosmic microwave background imply
a complex history to cosmic reionization. Such a history requires some form of
feedback to extend reionization over a long time interval, but the nature of
the feedback and how rapidly it operates remain highly uncertain. Here we focus
on one aspect of this complexity: which physical processes can cause the global
ionized fraction to evolve non-monotonically with cosmic time? We consider a
range of mechanisms and conclude that double reionization is much less likely
than a long, but still monotonic, ionization history. We first examine how
galactic winds affect the transition from metal-free to normal star formation.
Because the transition is actually spatially inhomogeneous and temporally
extended, this mechanism cannot be responsible for double reionization given
plausible parameters for the winds. We next consider photoheating, which causes
the cosmological Jeans mass to increase in ionized regions and hence suppresses
galaxy formation there. In this case, double reionization requires that small
halos form stars efficiently, that the suppression from photoheating is strong
relative to current expectations, and that ionizing photons are preferentially
produced outside of previously ionized regions. Finally, we consider H_2
photodissociation, in which the buildup of a soft ultraviolet background
suppresses star formation in small halos. This can in principle cause the
ionized fraction to temporarily decrease, but only during the earliest stages
of reionization. Finally, we briefly consider the effects of some of these
feedback mechanisms on the topology of reionization.Comment: 13 pages, 5 figures, in press at ApJ (reorganized significantly but
major conclusions unchanged
The Photo-Evaporation of Dwarf Galaxies During Reionization
During the period of reionization the Universe was filled with a cosmological
background of ionizing radiation. By that time a significant fraction of the
cosmic gas had already been incorporated into collapsed galactic halos with
virial temperatures below about 10000 K that were unable to cool efficiently.
We show that photoionization of this gas by the fresh cosmic UV background
boiled the gas out of the gravitational potential wells of its host halos. We
calculate the photoionization heating of gas inside spherically symmetric dark
matter halos, and assume that gas which is heated above its virial temperature
is expelled. In popular Cold Dark Matter models, the Press-Schechter halo
abundance implies that about 50-90% of the collapsed gas was evaporated at
reionization. The gas originated from halos below a threshold circular velocity
of 10-15 km/s. The resulting outflows from the dwarf galaxy population at
redshifts 5-10 affected the metallicity, thermal and hydrodynamic state of the
surrounding intergalactic medium. Our results suggest that stellar systems with
a velocity dispersion below about 10 km/s, such as globular clusters or the
dwarf spheroidal galaxies of the Local Group, did not form directly through
cosmological collapse at high redshifts.Comment: 29 pages, 7 PostScript figures, accepted for ApJ. Final version,
revised due to referee comments. Figures 6 & 7 have been corrected for a
small numerical erro
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