3,148 research outputs found
Changes in Testing and Paying for Milk Components as Proposed under the Final Rule of Federal Order Reform: Implications for Dairy Producers
Agricultural and Food Policy, Livestock Production/Industries,
Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization
The spectra of the first galaxies and quasars in the Universe should be
strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by
neutral hydrogen (HI) in the intervening intergalactic medium. However, the
Lyman-alpha line photons emitted by these sources are not eliminated but rather
scatter until they redshift out of resonance and escape due to the Hubble
expansion of the surrounding intergalactic HI. We calculate the resulting
brightness distribution and the spectral shape of the diffuse Lyman-alpha line
emission around high redshift sources, before the intergalactic medium was
reionized. Typically, the Lyman-alpha photons emitted by a source at z=10
scatter over a characteristic angular radius of order 15 arcseconds around the
source and compose a line which is broadened and redshifted by about a thousand
km/s relative to the source. The scattered photons are highly polarized.
Detection of the diffuse Lyman-alpha halos around high redshift sources would
provide a unique tool for probing the neutral intergalactic medium before the
epoch of reionization. On sufficiently large scales where the Hubble flow is
smooth and the gas is neutral, the Lyman-alpha brightness distribution can be
used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3
corrected; new section added on the detectability of Lyman alpha halos;
conclusions update
J06587-5558 -- A Very Unusual Polarised Radio Source
We have found a peculiar radio source in the field of one of the hottest
known clusters of galaxies 1E0657-56. It is slightly extended, highly polarised
(54% at 8.8GHz) and has a very steep spectrum, with alpha ~ -1 at 1.3 GHz,
steepening to ~ -1.5 at 8.8GHz (S \propto nu^alpha). No extragalactic sources
are known with such high integrated polarisation, and sources with spectra as
steep as this are rare. In this paper, we report the unusual properties of the
source J06587-5558 and speculate on its origin and optical identification.Comment: 5 pages, 6 figures, accepted by MNRAS letter
Redshifted 21cm Signatures Around the Highest Redshift Quasars
The Ly-alpha absorption spectrum of the highest redshift quasars indicates
that they are surrounded by giant HII regions, a few Mpc in size. The neutral
gas around these HII regions should emit 21cm radiation in excess of the Cosmic
Microwave Background, and enable future radio telescopes to measure the
transverse extent of these HII regions. At early times, the HII regions expand
with a relativistic speed. Consequently, their measured sizes along the
line-of-sight (via Ly-alpha absorption) and transverse to it (via 21 cm
emission) should have different observed values due to relativistic time-delay.
We show that the combined measurement of these sizes would directly constrain
the neutral fraction of the surrounding intergalactic medium (IGM) as well as
the quasar lifetime. Based on current number counts of luminous quasars at z>6,
an instrument like LOFAR should detect >2 redshifted 21cm shells per field
(with a radius of 11 degrees) around active quasars as bright as those already
discovered by SDSS, and >200 relic shells of inactive quasars per field. We
show that Ly-alpha photons from the quasar are unable to heat the IGM or to
couple the spin and kinetic temperatures of atomic hydrogen beyond the edge of
the HII region. The detection of the IGM in 21cm emission around high redshift
quasars would therefore gauge the presence of a cosmic Ly-alpha background
during the reionization epoch.Comment: 11 pages, 6 figures. Submitted to Ap
Discovery of distant high luminosity infrared galaxies
We have developed a method for selecting the most luminous galaxies detected
by IRAS based on their extreme values of R, the ratio of 60 micron and B-band
luminosity. These objects have optical counterparts that are close to or below
the limits of Schmidt surveys. We have tested our method on a 1079 deg^2 region
of sky, where we have selected a sample of IRAS sources with 60 micron flux
densities greater than 0.2 Jy, corresponding to a redshift limit z~1 for
objects with far-IR luminosities of 10^{13} L_sun. Optical identifications for
these were obtained from the UK Schmidt Telescope plates, using the likelihood
ratio method. Optical spectroscopy has been carried out to reliably identify
and measure the redshifts of six objects with very faint optical counterparts,
which are the only objects with R>100 in the sample. One object is a
hyperluminous infrared galaxy (HyLIG) at z=0.834. Of the remaining, fainter
objects, five are ultraluminous infrared galaxies (ULIGs) with a mean redshift
of 0.45, higher than the highest known redshift of any non-hyperluminous ULIG
prior to this study. High excitation lines reveal the presence of an active
nucleus in the HyLIG, just as in the other known infrared-selected HyLIGs. In
contrast, no high excitation lines are found in the non-hyperluminous ULIGs. We
discuss the implications of our results for the number density of HyLIGs at z<1
and for the evolution of the infrared galaxy population out to this redshift,
and show that substantial evolution is indicated. Our selection method is
robust against the presence of gravitational lensing if the optical and
infrared magnification factors are similar, and we suggest a way of using it to
select candidate gravitationally lensed infrared galaxies.Comment: 6 pages, accepted for publication in A&
NCR366
Sponsored by the Extension services of Illinois, Kansas, Minnesota, Missouri, South Dakota and Wisconsin, in cooperation with Extension Service, USDA, Washington, D.C.Randy D. Shaver & W. Terry Howard, Feeding dairy cows for efficient reproductive performance, University of Wisconsin-Madison, August 1998
Modifications to the Cosmic 21-cm Background Frequency Spectrum by Scattering via electrons in Galaxy Clusters
The cosmic 21-cm background frequency spectrum related to the spin-flip
transition of neutral Hydrogen present during and before the era of
reionization is rich in features associated with physical processes that govern
transitions between the two spin states. The intervening electrons in
foreground galaxy clusters inversely Compton scatter the 21-cm background
spectrum and modify it just as the cosmic microwave background (CMB) spectrum
is modified by inverse-Compton scattering. Towards typical galaxy clusters at
low redshifts, the resulting modification is a few tenths milli-Kelvin
correction to the few tens milli-Kelvin temperature of 21-cm signal relative to
that of the cosmic microwave background black body spectrum. The modifications
are mostly associated with sharp changes in the cosmic 21-cm background
spectrum such as due to the onset of a Lyman- radiation field or
heating of neutral gas. Though low frequency radio interferometers that are now
planned for 21-cm anisotropy measurements are insensitive to the mean 21-cm
spectrum, differential observations of galaxy clusters with these
interferometers can be utilized to indirectly establish global features in the
21-cm frequency spectrum. We discuss the feasibility to detect the spectrum
modified by clusters and find that for upcoming interferometers, while a
detection towards an individual cluster is challenging, one can average signals
over a number of clusters, selected based on the strength of the
Sunyave-Zel'dovich effect at high radio frequencies involving CMB scattering
alone, to establish the mean 21-cm spectrum.Comment: 6 pages, 3 figures, PRD in press; expanded and title changed from v1.
Final version in pres
New Constraints on the Energetics, Progenitor Mass, and Age of the Supernova Remnant G292.0+1.8 Containing PSR J1124-5916
We present spatially resolved spectroscopy of the supernova remnant (SNR)
G292.0+1.8 with the Chandra X-ray observatory. This SNR contains the 135 ms
pulsar, J1124-5916. We apply non-equilibrium ionization (NEI) models to the
data. By comparing the derived abundances with those predicted from
nucleosynthesis models, we estimate a progenitor mass of 30-40 solar masses. We
also derive the intrinsic parameters of the supernova explosion such as its
energy, the age of the SNR, the blast wave velocity, and the swept-up mass. In
the Sedov interpretation, our estimated SNR age of 2,600 years is close to the
pulsar's characteristic age of 2,900 years. This confirms the pulsar/SNR
association and relaxes the need for the pulsar to have a non-canonical value
for the braking index, a large period at birth or a large transverse velocity.
We discuss the properties of the pulsar wind nebula (PWN) in the light of the
Kennel and Coroniti model and estimate the pulsar wind magnetization parameter.
We also report the first evidence for steepening of the power law spectral
index with increasing radius from the pulsar.Comment: 5 pages, 3 figures. To appear in ApJL, Feb 1 2003 (submitted Oct 9
2002, accepted Dec 19 2002
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