2,325 research outputs found
Radio Afterglows of Gamma-Ray Bursts and Hypernovae at High Redshift, and their Potential for 21-cm Absorption Studies
We investigate the radio afterglows of gamma-ray bursts (GRBs) and hypernovae
(HNe) at high redshifts and quantify their detectability, as well as their
potential usefulness for 21 cm absorption line studies of the intergalactic
medium (IGM) and intervening structures. We examine several sets of source and
environment model parameters that are physically plausible at high redshifts.
The radio afterglows of GRBs would be detectable out to z ~ 30, while the
energetic HNe could be detectable out to z ~ 20 even by the current Very Large
Array (VLA). We find that the 21 cm absorption line due to the diffuse neutral
IGM is difficult to detect even by the proposed Square Kilometer Array (SKA),
except for highly energetic sources. We also find that the 21 cm line due to
collapsed gas clouds with high optical depth may be detected on rare occasions.Comment: 36 pages, 12 figures, 1 table, accepted for publication in Ap
An upper limit on anomalous dust emission at 31 GHz in the diffuse cloud [LPH96]201.663+1.643
[LPH96]201.663+1.643, a diffuse H{\sc ii} region, has been reported to be a
candidate for emission from rapidly spinning dust grains. Here we present
Cosmic Background Imager (CBI) observations at 26-36 GHz that show no evidence
for significant anomalous emission. The spectral index within the CBI band, and
between CBI and Effelsberg data at 1.4/2.7 GHz, is consistent with optically
thin free-free emission. The best-fitting temperature spectral index from 2.7
to 31 GHz, , is close to the theoretical value,
for K. We place an upper limit of 24% ~ (2\sigma)
for excess emission at 31 GHz as seen in a 6\arcmin FWHM beam. Current
spinning dust models are not a good fit to the spectrum of LPH96. No polarized
emission is detected in the CBI data with an upper limit of 2% on the
polarization fraction.Comment: 5 pages, 3 figures, submitted to ApJ
Toward Empirical Constraints on the Global Redshifted 21 cm Brightness Temperature During the Epoch of Reionization
Preliminary results are presented from a simple, single-antenna experiment
designed to measure the all-sky radio spectrum between 100 and 200 MHz. The
system used an internal comparison-switching scheme to reduce non-smooth
instrumental contaminants in the measured spectrum to 75 mK. From the
observations, we place an initial upper limit of 450 mK on the relative
brightness temperature of the redshifted 21 cm contribution to the spectrum due
to neutral hydrogen in the intergalactic medium (IGM) during the epoch of
reionization, assuming a rapid transition to a fully ionized IGM at a redshift
of 8. With refinement, this technique should be able to distinguish between
slow and fast reionization scenarios. To constrain the duration of reionization
to dz > 2, the systematic residuals in the measured spectrum must be reduced to
3 mK.Comment: Submitted to ApJ. 9 pages including 6 figure
Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization
A number of radio interferometers are currently being planned or constructed
to observe 21 cm emission from reionization. Not only will such measurements
provide a detailed view of that epoch, but, since the 21 cm emission also
traces the distribution of matter in the Universe, this signal can be used to
constrain cosmological parameters at 6 < z < 20. The sensitivity of an
interferometer to the cosmological information in the signal may depend on how
precisely the angular dependence of the 21 cm 3-D power spectrum can be
measured. Utilizing an analytic model for reionization, we quantify all the
effects that break the spherical symmetry of the 3-D 21 cm power spectrum and
produce physically motivated predictions for this power spectrum. We find that
upcoming observatories will be sensitive to the 21 cm signal over a wide range
of scales, from larger than 100 to as small as 1 comoving Mpc. We consider
three methods to measure cosmological parameters from the signal: (1) direct
fitting of the density power spectrum to the signal, (2) using only the
velocity field fluctuations in the signal, (3) looking at the signal at large
enough scales such that all fluctuations trace the density field. With the
foremost method, the first generation of 21 cm observations should moderately
improve existing constraints on cosmological parameters for certain
low-redshift reionization scenarios, and a two year observation with the second
generation interferometer MWA5000 can improve constraints on Omega_w, Omega_m
h^2, Omega_b h^2, Omega_nu, n_s, and alpha_s. If the Universe is substantially
ionized by z = 12 or if spin temperature fluctuations are important, we show
that it will be difficult to place competitive constraints on cosmological
parameters with any of the considered methods.Comment: 20 pages, 12 figures, accepted by Ap
OH Zeeman Magnetic Field Detections Toward Five Supernova Remnants Using the VLA
We have observed the OH (1720 MHz) line in five galactic SNRs with the VLA to
measure their magnetic field strengths using the Zeeman effect. We detected all
12 of the bright ( mJy) OH (1720 MHz) masers previously detected
by Frail et al. (1996) and Green et al. (1997) and measured significant
magnetic fields (i.e. ) in ten of them. Assuming that the
``thermal'' Zeeman equation can be used to estimate for OH
masers, our estimated fields range from 0.2 to 2 mG. These magnetic field
strengths are consistent with the hypothesis that ambient molecular cloud
magnetic fields are compressed via the SNR shock to the observed values.
Magnetic fields of this magnitude exert a considerable influence on the
properties of the cloud with the magnetic pressures ( erg
cm) exceeding the pressure in the ISM or even the thermal pressure of
the hot gas interior to the remnant. This study brings the number of galactic
SNRs with OH (1720 MHz) Zeeman detections to ten.Comment: 23 pages, 14 figures, accepted to ApJ, for higher resolution images
of Figs 4,11, and 12 see http://www.pa.uky.edu/~brogan/brog_publ.htm
Radio Signatures of HI at High Redshift: Mapping the End of the ``Dark Ages''
The emission of 21-cm radiation from a neutral intergalactic medium (IGM) at
high redshift is discussed in connection with the thermal and ionization
history of the universe. The physical mechanisms that make such radiation
detectable against the cosmic microwave background include Ly_alpha coupling of
the hydrogen spin temperature to the kinetic temperature of the gas and
preheating of the IGM by the first generation of stars and quasars. Three
different signatures are investigated in detail: (a) the fluctuations in the
redshifted 21-cm emission induced by the gas density inhomogeneities that
develop at early times in cold dark matter (CDM) dominated cosmologies; (b) the
sharp absorption feature in the radio sky due to the rapid rise of the Ly_alpha
continuum background that marks the birth of the first UV sources in the
universe; and (c) the 21-cm emission and absorption shells that are generated
on several Mpc scales around the first bright quasars. Future radio
observations with projected facilities like the Giant Metrewave Radio Telescope
and the Square Kilometer Array may shed light on the power spectrum of density
fluctuations at z>5, and map the end of the "dark ages", i.e. the transition
from the post-recombination universe to one populated with radiation sources.Comment: LateX, 19 pages, 5 figures, significantly revised version to be
published in the Ap
VLA Observations of the "Eye of the Tornado"- the High Velocity \HII Region G357.63-0.06
The unusual supernova remnant candidate G357.7-0.1 and the compact source
G357.63-0.06 have been observed with the Very Large Array at 1.4 and 8.3 GHz.
The H92 line (8.3 GHz) was detected from the compact source with a
surprising velocity of about -210 km/s indicating that this source is an \HII
region, is most likely located at the Galactic center, and is unrelated to the
SNR. The \HI absorption line (1.4 GHz) data toward these sources supports this
picture and suggests that G357.7-0.1 lies farther away than the Galactic
center.Comment: Latex, 14 pages including 4 figures. Accepted to A
Spatial Correlation Function of X-ray Selected AGN
We present a detailed description of the first direct measurement of the
spatial correlation function of X-ray selected AGN. This result is based on an
X-ray flux-limited sample of 219 AGN discovered in the contiguous 80.7 deg^2
region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at
the 4 sigma level at comoving scales in the interval r = 5-60 h^-1 Mpc. Fitting
the data with a power law of slope gamma=1.8, we find a correlation length of
r_0 = 7.4 (+1.8, -1.9) h^-1 Mpc (Omega_M=0.3, Omega_Lambda=0.7). The median
redshift of the AGN contributing to the signal is z_xi=0.22. This clustering
amplitude implies that X-ray selected AGN are spatially distributed in a manner
similar to that of optically selected AGN. Furthermore, the ROSAT NEP
determination establishes the local behavior of AGN clustering, a regime which
is poorly sampled in general. Combined with high-redshift measures from optical
studies, the ROSAT NEP results argue that the AGN correlation strength
essentially does not evolve with redshift, at least out to z~2.2. In the local
Universe, X-ray selected AGN appear to be unbiased relative to galaxies and the
inferred X-ray bias parameter is near unity, b_X~1. Hence X-ray selected AGN
closely trace the underlying mass distribution. The ROSAT NEP AGN catalog,
presented here, features complete optical identifications and spectroscopic
redshifts. The median redshift, X-ray flux, and X-ray luminosity are z=0.41,
f_X=1.1*10^-13 cgs, and L_X=9.2*10^43 h_70^-2 cgs (0.5-2.0 keV), respectively.
Unobscured, type 1 AGN are the dominant constituents (90%) of this soft X-ray
selected sample of AGN.Comment: 17 pages, 8 figures, accepted for publication in ApJ, a version with
high-resolution figures is available at
http://www.eso.org/~cmullis/papers/Mullis_et_al_2004b.ps.gz, a
machine-readable version of the ROSAT NEP AGN catalog is available at
http://www.eso.org/~cmullis/research/nep-catalog.htm
Issues for the Next Generation of Galaxy Surveys
I argue that the weight of the available evidence favours the conclusions
that galaxies are unbiased tracers of mass, the mean mass density (excluding a
cosmological constant or its equivalent) is less than the critical Einstein-de
Sitter value, and an isocurvature model for structure formation offers a viable
and arguably attractive model for the early assembly of galaxies. If valid
these conclusions complicate our work of adding structure formation to the
standard model for cosmology, but it seems sensible to pay attention to
evidence.Comment: 14 pages, 3 postscript figures, uses rspublic.st
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