1,006 research outputs found
The Nature of Radio Emission from Distant Galaxies: The 1.4 GHz Observations
We have conducted a deep radio survey with the Very Large Array at 1.4 GHz of
a region containing the Hubble Deep Field. This survey overlaps previous
observations at 8.5 GHz allowing us to investigate the radio spectral
properties of microjansky sources to flux densities greater than 40 Jy at
1.4 GHz and greater than 8 Jy at 8.5 GHz. A total of 371 sources have been
catalogued at 1.4 GHz as part of a complete sample within 20\arcmin ~of the
HDF. The differential source count for this region is only marginally
sub-Euclidean and is given by
srJy. Above about 100 Jy the radio source count is
systematically lower in the HDF as compared to other fields. We conclude that
there is clustering in our radio sample on size scales of 1\arcmin - 40\arcmin
\.
The 1.4 GHz selected sample shows that the radio spectral indices are
preferentially steep () and the sources are
moderately extended with average angular size = 1.8\arcsec . Optical
identification with disk-type systems at 0.5-0.8 suggests that
synchrotron emission, produced by supernovae remnants, is powering the radio
emission in the majority of sources. The 8.5 GHz sample contains primarily
moderately flat spectrum sources (), with less than
15% inverted. We argue that we may be observing an increased fraction of
optically thin bremsstrahlung over synchrotron radiation in these distant
star-forming galaxies.Comment: 31 pages, 5 tables, 10 figures, to appear in Jan. 2000 Ap
The Radial Extent and Warp of the Ionized Galactic Disk. II. A Likelihood Analysis of Radio-Wave Scattering Toward the Anticenter
We use radio-wave scattering data to constrain the distribution of ionized
gas in the outer Galaxy. Like previous models, our model for the H II disk
includes parameters for the radial scale length and scale height of the H II,
but we allow the H II disk to warp and flare. Our model also includes the
Perseus arm. We use a likelihood analysis on 11 extragalactic sources and 7
pulsars. Scattering in the Perseus arm is no more than 60% of the level
contributed by spiral arms in the inner Galaxy, equivalent to a 1 GHz
scattering diameter of 1.5 mas. Our analysis favors an unwarped, nonflaring
disk with a 1 kpc scale height, though this may reflect the non-uniform and
coarse coverage provided by the available data. The lack of a warp indicates
that VLBI observations near 1 GHz with an orbiting station having baseline
lengths of a few Earth diameters will not be affected by interstellar
scattering at Galactic latitudes |b| ~ 15 degrees. The radial scale length is
15--20 kpc, but the data cannot distinguish between a gradual decrease in the
electron density and a truncated distribution. We favor a truncated one,
because we associate the scattering with massive star formation, which is also
truncated near 20 kpc. The distribution of electron density turbulence
decreases more rapidly with Galactocentric distance than does the hydrogen
distribution. Alternate ionizing and turbulent agents---the intergalactic
ionizing flux and satellite galaxies passing through the disk---do not
contribute significantly to scattering. We cannot exclude the possibility that
a largely ionized, but quiescent disk extends to >~ 100 kpc, similar to that
for some Ly-alpha absorbers.Comment: 34 pages, LaTeX2e with AASTeX aaspp4 macro, 9 figures in 9 PostScript
files, accepted for publication in Ap
The VLA Survey of the Chandra Deep Field South: I. Overview of the Radio Data
We report 20 and 6 cm VLA deep observations of the CDF-S including the
Extended CDF-S. We discuss the radio properties of 266 cataloged radio sources,
of which 198 are above a 20 cm completeness level reaching down to 43 microJy
at the center of the field. Survey observations made at 6 cm over a more
limited region covers the original CDF-S to a comparable level of sensitivity
as the 20 cm observations.
Of 266 cataloged radio sources, 52 have X-ray counterparts in the CDF-S and a
further 37 in the E-CDF-S area not covered by the 1 Megasecond exposure. Using
a wide range of material, we have found optical or infrared counterparts for
254 radio sources, of which 186 have either spectroscopic or photometric
redshifts (Paper II). Three radio sources have no apparent counterpart at any
other wavelength. Measurements of the 20 cm radio flux density at the position
of each CDF-S X-ray source detected a further 30 radio sources above a
conservative 3-sigma detection limit.
X-ray and sub-mm observations have been traditionally used as a measure of
AGN and star formation activity, respectively. These new observations probe the
faint end of both the star formation and radio galaxy/AGN population, as well
as the connection between the formation and evolution of stars and SMBHs. Both
of the corresponding gravitational and nuclear fusion driven energy sources can
lead to radio synchrotron emission. AGN and radio galaxies dominate at high
flux densities. Although emission from star formation becomes more prominent at
the microjansky levels reached by deep radio surveys, even for the weakest
sources, we still find an apparent significant contribution from low luminosity
AGN as well as from star formation.Comment: Accpted for publication in the Astrophysical Journal supplements with
3 tables and 18 figure
Evolution of Structure in the Intergalactic Medium and the Nature of the Ly-alpha Forest
We have performed a detailed statistical study of the evolution of structure
in a photoionized intergalactic medium (IGM) using analytical simulations to
extend the calculation into the mildly non-linear density regime found to
prevail at z = 3. Our work is based on a simple fundamental conjecture: that
the probability distribution function of the density of baryonic diffuse matter
in the universe is described by a lognormal (LN) random field. The LN field has
several attractive features and follows plausibly from the assumption of
initial linear Gaussian density and velocity fluctuations at arbitrarily early
times. Starting with a suitably normalized power spectrum of primordial fluc-
tuations in a universe dominated by cold dark matter (CDM), we compute the
behavior of the baryonic matter, which moves slowly toward minima in the dark
matter potential on scales larger than the Jeans length. We have computed two
models that succeed in matching observations. One is a non-standard CDM model
with Omega=1, h=0.5 and \Gamma=0.3, and the other is a low density flat model
with a cosmological constant(LCDM), with Omega=0.4, Omega_Lambda=0.6 and h=.65.
In both models, the variance of the density distribution function grows with
time, reaching unity at about z=4, where the simulation yields spectra that
closely resemble the Ly-alpha forest absorption seen in the spectra of high z
quasars. The calculations also successfully predict the observed properties of
the Ly-alpha forest clouds and their evolution from z=4 down to at least z=2,
assuming a constant intensity for the metagalactic UV background over this
redshift range. However, in our model the forest is not due to discrete clouds,
but rather to fluctuations in a continuous intergalactic medium. (This is an
abreviated abstract; the complete abstract is included with the manuscript.)Comment: Wrong Fig. 10 is corrected. Our custom made postscript is available
at ftp://hut4.pha.jhu.edu/incoming/igm, or contact Arthur Davidsen
([email protected]) for nice hardcopies; accepted for publication in Ap
Computerized cyclic voltammetric detection after HPLC of the antineoplastic agents etoposide, teniposide, adriamycin and its metabolite adriamycinol in urine samples
A computerized electrochemical detection system for application
after HPLC, provided with a cyclic voltammetric oxidative and
reductive module, is described for the on-line qualitative determination
of electroactive antineoplastic agents and metabolites in urine
samples, collected from cancer patients, following intravenous
administration
Panoramic Views of the Cygnus Loop
We present a complete atlas of the Cygnus Loop supernova remnant in the light
of [O III] (5007), H alpha, and [S II] (6717, 6731). Despite its shell-like
appearance, the Cygnus Loop is not a current example of a Sedov-Taylor blast
wave. Rather, the optical emission traces interactions of the supernova blast
wave with clumps of gas. The surrounding interstellar medium forms the walls of
a cavity through which the blast wave now propagates, including a nearly
complete shell in which non-radiative filaments are detected. The Cygnus Loop
blast wave is not breaking out of a dense cloud, but is instead running into
confining walls. The interstellar medium dominates not only the appearance of
the Cygnus Loop but also the continued evolution of the blast wave. If this is
a typical example of a supernova remnant, then global models of the
interstellar medium must account for such significant blast wave deceleration.Comment: 28 pages AAS Latex, 28 black+white figures, 6 color figures. To be
published in The Astrophysical Journal Supplement Serie
High-Latitude HI in the Low Surface Brightness Galaxy UGC7321
From the analysis of sensitive HI 21-cm line observations, we find evidence
for vertically extended HI emission (|z|<~2.4 kpc) in the edge-on, low surface
brightness spiral galaxy UGC7321. Three-dimensional modelling suggests that the
HI disk of UGC7321 is both warped and flared, but that neither effect can fully
reproduce the spatial distribution and kinematics of the highest z-height gas.
We are able to model the high-latitude emission as an additional HI component
in the form of a ``thick disk'' or ``halo'' with a FWHM~3.3 kpc. We find
tentative evidence that the vertically extended gas declines in rotational
velocity as a function of z, although we are unable to completely rule out
models with constant V(z). In spite of the low star formation rate of UGC7321,
energy from supernovae may be sufficient to sustain this high-latitude gas.
However, alternative origins for this material, such as slow, sustained infall,
cannot yet be excluded.Comment: to appear in the August 20 Astrophysical Journal; 17 pages; version
with full resolution figures available at
http://cfa-www.harvard.edu/~lmatthew
Relaxation to thermal equilibrium in the self-gravitating sheet model
We revisit the issue of relaxation to thermal equilibrium in the so-called
"sheet model", i.e., particles in one dimension interacting by attractive
forces independent of their separation. We show that this relaxation may be
very clearly detected and characterized by following the evolution of order
parameters defined by appropriately normalized moments of the phase space
distribution which probe its entanglement in space and velocity coordinates.
For a class of quasi-stationary states which result from the violent relaxation
of rectangular waterbag initial conditions, characterized by their virial ratio
R_0, we show that relaxation occurs on a time scale which (i) scales
approximately linearly in the particle number N, and (ii) shows also a strong
dependence on R_0, with quasi-stationary states from colder initial conditions
relaxing much more rapidly. The temporal evolution of the order parameter may
be well described by a stretched exponential function. We study finally the
correlation of the relaxation times with the amplitude of fluctuations in the
relaxing quasi-stationary states, as well as the relation between temporal and
ensemble averages.Comment: 37 pages, 24 figures; some additional discussion of previous
literature and other minor modifications, final published versio
A Catalogue of Field Horizontal Branch Stars Aligned with High Velocity Clouds
We present a catalogue of 430 Field Horizontal Branch (FHB) stars, selected
from the Hamburg/ESO Survey (HES), which fortuitously align with high column
density neutral hydrogen (HI) High-Velocity Cloud (HVC) gas. These stars are
ideal candidates for absorption-line studies of HVCs, attempts at which have
been made for almost 40 years with little success. A parent sample of 8321 HES
FHB stars was used to extract HI spectra along each line-of-sight, using the HI
Parkes All-Sky Survey. All lines-of-sight aligned with high velocity HI
emission with peak brightness temperatures greater than 120mK were examined.
The HI spectra of these 430 probes were visually screened and cross-referenced
with several HVC catalogues. In a forthcoming paper, we report on the results
of high-resolution spectroscopic observations of a sample of stars drawn from
this catalogue.Comment: 7 pages, 4 figures. ApJS accepted. Full catalogue and all online-only
images available at
http://astronomy.swin.edu.au/staff/cthom/catalogue/index.htm
Hierarchical clustering and formation of power-law correlation in 1-dimensional self-gravitating system
The process of formation of fractal structure in one-dimensional
self-gravitating system is examined numerically. It is clarified that
structures created in small spatial scale grow up to larger scale through
clustering of clusters, and form power-law correlation.Comment: 9pages,4figure
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