2,777 research outputs found
Detecting the Earliest Galaxies Through Two New Sources of 21cm Fluctuations
The first galaxies that formed at a redshift ~20-30 emitted continuum photons
with energies between the Lyman-alpha and Lyman limit wavelengths of hydrogen,
to which the neutral universe was transparent except at the Lyman-series
resonances. As these photons redshifted or scattered into the Lyman-alpha
resonance they coupled the spin temperature of the 21cm transition of hydrogen
to the gas temperature, allowing it to deviate from the microwave background
temperature. We show that the fluctuations in the radiation emitted by the
first galaxies produced strong fluctuations in the 21cm flux before the
Lyman-alpha coupling became saturated. The fluctuations were caused by biased
inhomogeneities in the density of galaxies, along with Poisson fluctuations in
the number of galaxies. Observing the power-spectra of these two sources would
probe the number density of the earliest galaxies and the typical mass of their
host dark matter halos. The enhanced amplitude of the 21cm fluctuations from
the era of Lyman-alpha coupling improves considerably the practical prospects
for their detection.Comment: 11 pages, 7 figures, ApJ, published. Normalization fixed in top
panels of Figures 4-
Emission from Bow Shocks of Beamed Gamma-Ray Bursts
Beamed gamma-ray burst (GRB) sources produce a bow shock in their gaseous
environment. The emitted flux from this bow shock may dominate over the direct
emission from the jet for lines of sight which are outside the angular radius
of the jet emission, theta. The event rate for these lines of sight is
increased by a factor of 260*(theta/5_degrees)^{-2}. For typical GRB
parameters, we find that the bow shock emission from a jet with half-angle of
about 5 degrees is visible out to tens of Mpc in the radio and hundreds of Mpc
in the X-rays. If GRBs are linked to supernovae, studies of peculiar supernovae
in the local universe should reveal this non-thermal bow shock emission for
weeks to months following the explosion.Comment: ApJ, submitted, 15 pages, 3 figure
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
Observing GRBs with EXIST
We describe the Energetic X-ray Imaging Survey Telescope EXIST, designed to carry out a sensitive all-sky survey in the 10 keV – 600 keV band. The primary goal of EXIST is to find black holes in the local and distant universe. EXIST also traces cosmic star formation via gamma-ray bursts and gamma-ray lines from radioactive elements ejected by supernovae and novae
Measuring the Virial Temperature of Galactic Halos Through Electron Scattering of Quasar Emission Lines
Semi-analytic models of galaxy formation postulate the existence of
virialized gaseous halos around galaxies at high redshifts. A small fraction of
the light emitted by any high-redshift quasar is therefore expected to scatter
off the free electrons in the halo of its host galaxy. The broadening of the
scattered emission lines of the quasar can be used to measure the temperature
of these electrons. For gas in virial equilibrium, the velocity width of the
scattered line profile is larger by a factor of (m_p/m_e)^{1/2}=43 than the
velocity dispersion of the host galaxy and reaches >10,000 km/s for the massive
galaxies and groups in which bright quasars reside. In these systems, the
scattered width exceeds the intrinsic width of the quasar lines and hence can
be used to measure the virial temperature of the quasar host. The high degree
of polarization of the scattered radiation can help filter out the extended
scattered light from the central emission by the quasar and its host galaxy.
The signal-to-noise ratio of the spectral broadening can be enhanced by
matching the full spectrum of the scattered radiation to a template of the
unscattered quasar spectrum. Although the central fuzz around low-redshift
quasars is dominated by starlight, the fuzz around high-redshift quasars might
be dominated by scattering before galaxies have converted most of their gas
reservoirs into stars.Comment: 10 pages, 1 figures, submitted to ApJ Letter
Disabled Beggars in Addis Ababa
This study brings together qualitative and quantitative data to better understand the lives of people with disabilities who beg in Ethiopia. It sets out to provide an initial understanding of the lives of disabled beggars with particular emphasis on determining social and economic factors, and sequences of events or patterns of behaviour that are common to people with disabilities who now work as beggars. Chapter 3 provides an overview of the demographic characteristics of the survey respondents, including their education and vocational training levels and work history. Additional attention was directed to identifying possible areas of intervention that might sever the links between disability and poverty. The study yielded an intriguing set of results that identify: the complex set of issues with which disabled beggars grapple; a series of points where targeted intervention by governments, UN agencies, NGOs and disabled people's organizations could help break the on-going cycle of disability and poverty; and choices that lead some men and women with disabilities to beg. The study contains recommendations for policy, programming and areas for further research
Dynamical Mass Estimates of Large-Scale Filaments in Redshift Surveys
We propose a new method to measure the mass of large-scale filaments in
galaxy redshift surveys. The method is based on the fact that the mass per unit
length of isothermal filaments depends only on their transverse velocity
dispersion. Filaments that lie perpendicular to the line of sight may therefore
have their mass per unit length measured from their thickness in redshift
space. We present preliminary tests of the method and find that it predicts the
mass per unit length of filaments in an N-body simulation to an accuracy of
~35%. Applying the method to a select region of the Perseus-Pisces supercluster
yields a mass-to-light ratio M/L_B around 460h in solar units to within a
factor of two. The method measures the mass-to-light ratio on length scales of
up to 50h^(-1) Mpc and could thereby yield new information on the behavior of
the dark matter on mass scales well beyond that of clusters of galaxies.Comment: 21 pages, LaTeX with 6 figures included. Submitted to Ap
Formation of the First Supermassive Black Holes
We consider the physical conditions under which supermassive black holes
could have formed inside the first galaxies. Our SPH simulations indicate that
metal-free galaxies with a virial temperature ~10^4 K and with suppressed H2
formation (due to an intergalactic UV background) tend to form a binary black
hole system which contains a substantial fraction (>10%) of the total baryonic
mass of the host galaxy. Fragmentation into stars is suppressed without
substantial H2 cooling. Our simulations follow the condensation of ~5x10^6
M_sun around the two centers of the binary down to a scale of < 0.1pc. Low-spin
galaxies form a single black hole instead. These early black holes lead to
quasar activity before the epoch of reionization. Primordial black hole
binaries lead to the emission of gravitational radiation at redshifts z>10 that
would be detectable by LISA.Comment: 11 pages, 9 figures, revised version, ApJ in press (October 10, 2003
- …