122 research outputs found
Dwarf Spheroidal Satellite Formation in a Reionized Local Group
Dwarf spheroidal satellite galaxies have emerged a powerful probe of
small-scale dark matter clustering and of cosmic reionization. They exhibit
structural and chemical continuity with dwarf irregular galaxies in the field
and with spheroidal galaxies in high-density environments. By combining
empirical constraints derived for star formation at low gas column densities
and metallicities in the local universe with a model for dark matter and
baryonic mass assembly, we provide an analytical description of how the dwarf
spheroidals acquired their stellar content. Their progenitors formed stars
until the gas content, initially reduced from the cosmic average by the thermal
pressure of the reionized intergalactic medium, was finally ram pressure
stripped during the progenitors' accretion on to the host galaxy. Dwarf
spheroidal satellites of differing luminosities seem to share very similar most
massive progenitor histories that reach thresholds for gas cooling by atomic
line emission at epochs at which the Lagrangian volume of the Local Group
should have been reionized. We hypothesize that dwarf spheroidals formed the
bulk of their stars in partially rotationally supported HI disks in a reionized
universe. This model provides an explanation for the "common mass scale"
relation and reproduces the empirical luminosity-size and
luminosity-metallicity relations. Explosive feedback phenomena, such as
outflows driven by the concerted action of supernovae, need not have been
significant in the dwarf spheroidals' formation. We further speculate that the
true pre-reionization fossils should exhibit a structure distinct from that of
the dwarf spheroidals, e.g., in the form of dense isolated or nuclear star
clusters.Comment: 18 pages, 7 figures, MNRAS, in pres
Efficient Cosmological Parameter Estimation from Microwave Background Anisotropies
We revisit the issue of cosmological parameter estimation in light of current
and upcoming high-precision measurements of the cosmic microwave background
power spectrum. Physical quantities which determine the power spectrum are
reviewed, and their connection to familiar cosmological parameters is
explicated. We present a set of physical parameters, analytic functions of the
usual cosmological parameters, upon which the microwave background power
spectrum depends linearly (or with some other simple dependence) over a wide
range of parameter values. With such a set of parameters, microwave background
power spectra can be estimated with high accuracy and negligible computational
effort, vastly increasing the efficiency of cosmological parameter error
determination. The techniques presented here allow calculation of microwave
background power spectra times faster than comparably accurate direct
codes (after precomputing a handful of power spectra). We discuss various
issues of parameter estimation, including parameter degeneracies, numerical
precision, mapping between physical and cosmological parameters, and systematic
errors, and illustrate these considerations with an idealized model of the MAP
experiment.Comment: 22 pages, 12 figure
Cluster Merger Shock Constraints on Particle Acceleration and Nonthermal Pressure in the Intracluster Medium
X-ray observations of galaxy cluster merger shocks can be used to constrain
nonthermal processes in the intracluster medium (ICM). The presence of
nonthermal pressure components in the ICM, as well as the shock acceleration of
particles and their escape, all affect shock jump conditions in distinct ways.
Therefore, these processes can be constrained using X-ray surface brightness
and temperature maps of merger shock fronts. Here we use these observations to
place constraints on particle acceleration efficiency in intermediate Mach
number (M ~ 2-3) shocks and explore the potential to constrain the contribution
of nonthermal components (e.g., cosmic rays, magnetic field, and turbulence) to
ICM pressure in cluster outskirts. We model the hydrodynamic jump conditions in
merger shocks discovered in the galaxy clusters A520 (M ~ 2) and 1E 0657-56 (M
~ 3) using a multifluid model comprised of a thermal plasma, a nonthermal
plasma, and a magnetic field. Based on the published X-ray spectroscopic data
alone, we find that the fractional contribution of cosmic rays accelerated in
these shocks is lower than about 10% of the shock downstream pressure. Current
observations do not constrain the fractional contribution of nonthermal
components to the pressure of the undisturbed shock upstream. Future X-ray
observations, however, have the potential to either detect particle
acceleration in these shocks through its effect on the shock dynamics, or to
place a lower limit on the nonthermal pressure contributions in the undisturbed
ICM. We briefly discuss implications for models of particle acceleration in
collisionless shocks and the estimates of galaxy cluster masses derived from
X-ray and Sunyaev-Zel'dovich effect observations.Comment: 10 pages, 4 figures, comments welcom
The First Galaxies: Assembly under Radiative Feedback from the First Stars
We investigate how radiative feedback from the first stars affects the
assembly of the first dwarf galaxies. We perform cosmological zoomed SPH
simulations of a dwarf galaxy assembling inside a halo of virial mass 10^9
solar at z = 10. The simulations follow the non-equilibrium chemistry/cooling
of primordial gas and the conversion of the gas into metal-free stars. To
quantify the radiative feedback, we compare a simulation in which stars emit
both molecular hydrogen dissociating and hydrogen/helium ionizing radiation
with a simulation in which stars emit only dissociating radiation, and with a
simulation in which stars remain dark. Photodissociation and -ionization exert
a strong negative feedback on the assembly of the simulated galaxy. Gas
condensation is strongly impeded, and star formation is strongly suppressed in
comparison with the simulation in which stars remain dark. The feedback on the
gas implies a suppression of the central dark matter densities in the minihalo
progenitor by factors of up to a few, which is a significant deviation from the
singular isothermal density profile characterizing the dark matter distribution
in the absence of radiative feedback. The evolution of gas densities, star
formation rates, and the distribution of dark matter becomes insensitive to the
inclusion of dissociating radiation in the late stages of the minihalo
assembly, and it becomes insensitive to the inclusion of ionizing radiation
once the minihalo turns into an atomically cooling galaxy. The formation of an
extended disk inside the dwarf galaxy is a robust outcome not affected by the
inclusion of radiation. We estimate that dwarf galaxies such as simulated here
will be among the faintest galaxies the upcoming James Webb Space Telescope
will detect. Our conclusions are subject to our neglect of feedback from
supernovae and chemical enrichment as well as to cosmic variance. [abridged]Comment: 25 pages (including 5 pages appendix), 13 figures. Accepted for
publication in Ap
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