94 research outputs found
Competition between Pressure and Gravity Confinement in Lyman-Alpha Forest Observations
A break in the distribution function of Ly clouds (at a typical
redshift of ) has been reported by Petitjean et al. (1993). This feature
is what would be expected from a transition between pressure confinement and
gravity confinement (as predicted in Charlton, Salpeter, and Hogan (1993)). The
column density at which the feature occurs has been used to determine the
external confining pressure, , which could be
due to a hot, intergalactic medium. For models that provide a good fit to the
data, the contribution of the gas in clouds to is small. The specific
shape of the distribution function at the transition (predicted by models to
have a non-monotonic slope) can serve as a diagnostic of the distribution of
dark matter around Ly forest clouds, and the present data already
eliminate certain models.Comment: 10 pages plain TeX, 2 figures available upon request, submitted to
ApJ Letters, PSU-jc-
Galaxies as Fluctuations in the Ionizing Background Radiation at Low Redshift
Some Lyman continuum photons are likely to escape from most galaxies, and
these can play an important role in ionizing gas around and between galaxies,
including gas that gives rise to Lyman alpha absorption. Thus the gas
surrounding galaxies and in the intergalactic medium will be exposed to varying
amounts of ionizing radiation depending upon the distances, orientations, and
luminosities of any nearby galaxies. The ionizing background can be
recalculated at any point within a simulation by adding the flux from the
galaxies to a uniform quasar contribution. Normal galaxies are found to almost
always make some contribution to the ionizing background radiation at redshift
zero, as seen by absorbers and at random points in space. Assuming that about 2
percent of ionizing photons escape from a galaxy like the Milky Way, we find
that normal galaxies make a contribution of at least 30 to 40 percent of the
assumed quasar background. Lyman alpha absorbers with a wide range of neutral
column densities are found to be exposed to a wide range of ionization rates,
although the distribution of photoionization rates for absorbers is found to be
strongly peaked. On average, less highly ionized absorbers are found to arise
farther from luminous galaxies, while local fluctuations in the ionization rate
are seen around galaxies having a wide range of properties.Comment: 10 pages, 8 figures, references added, clarified explanation of first
two equation
The Ingram Vessel 38CT204: Intensive Survey & Excavation of an Upland Rivercraft at Cheraw, South Carolina
In 1993 and 1994 the Underwater Archaeology Division of the South Carolina Institute of Archaeology and Anthropology conducted an intensive survey of the remains of a small, wooden hulled craft in the Great Pee Dee River near Cheraw, South Carolina. The project was sponsored in part by the Cheraw Historical Society and partially funded by a grant from the South Carolina Humanities Council. The Ingram Vessel (38CT204), named after its discoverer Miller Ingram, lay overturned and largely buried beneath the river sediments and protected by a large mushroom-shaped rock just upstream of the site. The site was partially excavated and the hull remains mapped in situ. The investigation revealed a shallow draught, keeled vessel, built entirely of Southern Yellow Pine. The site is tentatively dated to the late 18th-early 19th century. Overall dimensions are estimated to have been approximately 15.5m (50ft, l0 in) in length, with a maximum beam of 4.6m (15ft, lin). This report details the research on the site and places the vessel within a regional maritime historical context. The vessel is, to date, the only ship-built hull excavated in an uplands context near the head of navigation of a South Carolina river.https://scholarcommons.sc.edu/archanth_books/1198/thumbnail.jp
Galaxy Selection and Clustering and Lyman alpha Absorber Identification
The effects of galaxy selection on our ability to constrain the nature of
weak Ly\alpha absorbers at low redshift are explored. Current observations
indicate the existence of a population of gas-rich, low surface brightness
(LSB) galaxies, and these galaxies may have large cross sections for Ly\alpha
absorption. Absorption arising in LSB galaxies may be attributed to HSB
galaxies at larger impact parameters from quasar lines of sight, so that the
observed absorption cross sections of galaxies may seem unreasonably large.
Thus it is not possible to rule out scenarios where LSB galaxies make
substantial contributions to Ly\alpha absorption using direct observations.
Less direct tests, where observational selection effects are taken into account
using simulations, should make it possible to determine the nature of Ly\alpha
absorbers by observing a sample of ~100 galaxies around quasar lines of sight
with well-defined selection criteria. Such tests, which involve comparing
simulated and observed plots of the unidentified absorber fractions and
absorbing galaxy fractions versus impact parameter, can distinguish between
scenarios where absorbers arise in particular galaxies and those where
absorbers arise in gas tracing the large scale galaxy distribution. Care must
be taken to minimize selection effects even when using these tests. Results
from such tests are likely to depend upon the limiting neutral hydrogen column
density. While not enough data are currently available to make a strong
conclusion about the nature of moderately weak absorbers, evidence is seen that
such absorbers arise in gas that is around or between galaxies that are often
not detected in surveys.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journa
Comparing Galaxies and Lyman Alpha Absorbers at Low Redshift
A scenario is explored in which Lyman alpha absorbers at low redshift arise
from lines of sight through extended galaxy disks, including those of dwarf and
low surface brightness galaxies. A population of galaxies is simulated based
upon observed distributions of galaxy properties, and the gas disks are modeled
using pressure and gravity confinement. Some parameter values are ruled out by
comparing simulation results with the observed galaxy luminosity function, and
constraints may be made on the absorbing cross sections of galaxies. Simulation
results indicate that it is difficult to match absorbers with particular
galaxies observationally since absorption typically occurs at high impact
parameters (>200 kpc) from luminous galaxies. Low impact parameter absorption
is dominated by low luminosity dwarfs. A large fraction of absorption lines is
found to originate from low surface brightness galaxies, so that the absorbing
galaxy is likely to be misidentified. Low redshift Lyman alpha absorber counts
can easily be explained by moderately extended galaxy disks when low surface
brightness galaxies are included, and it is easily possible to find a scenario
which is consistent with observed the galaxy luminosity function, with low
redshift Lyman limit absorber counts, and with standard nucleosynthesis
predictions of the baryon density, Omega_Baryon.Comment: 17 pages, 8 figures, accepted to the Astrophysical Journa
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