146 research outputs found
Evidence for Correlated Titanium and Deuterium Depletion in the Galactic ISM
Current measurements indicate that the deuterium abundance in diffuse
interstellar gas varies spatially by a factor of ~4 among sightlines extending
beyond the Local Bubble. One plausible explanation for the scatter is the
variable depletion of D onto dust grains. To test this scenario, we have
obtained high signal-to-noise, high resolution profiles of the refractory ion
TiII along seven Galactic sightlines with D/H ranging from 0.65 to 2.1x10^-5.
These measurements, acquired with the recently upgraded Keck/HIRES
spectrometer, indicate a correlation between Ti/H and D/H at the >95% c.l.
Therefore, our observations support the interpretation that D/H scatter is
associated with differential depletion. We note, however, that Ti/H values
taken from the literature do not uniformly show the correlation. Finally, we
identify significant component-to-component variations in the depletion levels
among individual sightlines and discuss complications arising from this
behavior.Comment: 4 pages; Accepted to Astrophysical Journal Letter
O VI and Multicomponent H I Absorption Associated with a Galaxy Group in the Direction of PG0953+415: Physical Conditions and Baryonic Content
We report the discovery of an O VI absorption system at z(abs) = 0.14232 in a
high resolution FUV spectrum of PG0953+415 obtained with the Space Telescope
Imaging Spectrograph (STIS). Both lines of the O VI 1032, 1038 doublet and
multicomponent H I Lya absorption are detected, but the N V doublet and the
strong lines of C II and Si III are not apparent. We examine the ionization
mechanism of the O VI absorber and find that while theoretical considerations
favor collisional ionization, it is difficult to observationally rule out
photoionization. If the absorber is collisionally ionized, it may not be in
equilibrium due to the rapid cooling of gas in the appropriate temperature
range. Non-equilibrium collisionally ionized models are shown to be consistent
with the observations. A WIYN survey of galaxy redshifts near the sight line
has revealed a galaxy at a projected distance of 395 kpc separated by ~130 km/s
from this absorber, and three additional galaxies are found within 130 km/s of
this redshift with projected separations ranging from 1.0 Mpc to 3.0 Mpc.
Combining the STIS observations of PG0953+415 with previous high S/N GHRS
observations of H1821+643, we derive a large number of O VI absorbers per unit
redshift, dN/dz ~20. We use this sample to obtain a first estimate of the
cosmological mass density of the O VI systems at z ~ 0. If further observations
confirm the large dN/dz derived for the O VI systems, then these absorbers
trace a significant reservoir of baryonic matter at low redshift.Comment: Accepted for publication in Ap.J., vol. 542 (Oct. 10, 2000
A Comparison of Absorption and Emission Line Abundances in the Nearby Damped Lyman-alpha Galaxy SBS 1543+593
We have used the Space Telescope Imaging Spectrograph (STIS) aboard HST to
measure a sulfur abundance of [S/H] = -0.41 +/-0.06 in the interstellar medium
(ISM) of the nearby damped Lyman-alpha (DLA) absorbing galaxy SBS 1543+593. A
direct comparison between this QSO absorption line abundance on the one hand,
and abundances measured from HII region emission line diagnostics on the other,
yield the same result: the abundance of sulfur in the neutral ISM is in good
agreement with that of oxygen measured in an HII region 3 kpc away. Our result
contrasts with those of other recent studies which have claimed
order-of-magnitude differences between HI (absorption) and HII (emission)
region abundances. We also derive a nickel abundance of [Ni/H] < -0.81, some
three times less than that of sulfur, and suggest that the depletion is due to
dust, although we cannot rule out an over-abundance of alpha-elements as the
cause of the lower metallicity. It is possible that our measure of [S/H] is
over-estimated if some SII arises in ionized gas; adopting a plausible star
formation rate for the galaxy along the line of sight, and a measurement of the
CII* 1335.7 absorption line detected from SBS 1543+593, we determine that the
metallicity is unlikely to be smaller than we derive by more than 0.25 dex. We
estimate that the cooling rate of the cool neutral medium is log [l_c (ergs
s^{-1} H atom^{-1})] = -27.0, the same value as that seen in the high redshift
DLA population.Comment: 31 pages; accepted for publication in the Ap
Revealing the Warm-Hot Intergalactic Medium with OVI Absorption
Hydrodynamic simulations of growth of cosmic structure suggest that 30-50% of
the total baryons at z=0 may be in a warm-hot intergalactic medium (WHIM) with
temperatures ~10^5-10^7K. The O VI \lambda \lambda 1032, 1038 absorption line
doublet in the FUV portion of QSO spectra provides an important probe of this
gas. Utilizing recent hydrodynamic simulations, it is found that there should
be ~5 O VI absorption lines per unit redshift with equivalent widths >= 35 mA,
decreasing rapidly to ~0.5 per unit redshift at >= 350 mA. About 10% of the
total baryonic matter or 20-30% of the WHIM is expected to be in the O VI
absorption line systems with equivalent width >= 20 mA; the remaining WHIM gas
may be too hot or have too low metallicity to be detected in O VI. We find that
the simulation results agree well with observations with regard to the line
abundance and total mass contained in these systems. Some of the O VI systems
are collisionally ionized and some are photoionized, but most of the mass is in
the collisionally ionized systems. We show that the gas that produces the O VI
absorption lines does not reside in virialized regions such as galaxies,
groups, or clusters of galaxies, but rather has an overdensity of 10-40 times
the average density. These regions form a somewhat connected network of
filaments. The typical metallicity of these regions is 0.1-0.3Zsun.Comment: accepted to ApJ Letters; full color Figure 1 may be obtained at
http://astro.princeton.edu/~cen/PROJECTS/p2/p2.html (at the bottom of the
page
Measurements of the f-Values of the Resonance Transitions of Ni II at 1317.217A and 1370.132A
We have retrieved high-resolution UV spectra of 69 hot stars from the HST
archive and determined the strengths of the interstellar Ni II absorption
features at 1317.217A. We then compared them to absorptions from either the
transitions at 1741.553A (covered in the spectra of 21 of the stars) or the one
at 1370.132A (seen for the remaining 48 stars). All spectra were recorded by
the either the E140M, E140H, or E230H gratings of STIS. By comparing the
strengths of the two lines in each spectrum and evaluating a weighted average
of all such comparisons, we have found that the f-value of the 1317A line is
1.34 +/-0.019 times the one at 1741A and 0.971 +/-0.014 times that of the one
at 1370A. We adopt as a comparison standard an experimentally determined
f-value for the 1741A line (known to 10% accuracy), so that f(1317A) = 0.0571
+/-0.006. It follows from this f-value and our measured line-strength ratios
that f(1370A) = 0.0588 +/-0.006. As an exercise to validate our methodology, we
compared the 1317A transition to another Ni II line at 1454.842A and arrived at
an f-value for the latter that is consistent with a previously measured
experimental value to within the expected error.Comment: 14 pages, 4 figures, accepted for publication in the Astrophysical
Journa
21-cm H I emission from the Damped Lyman-alpha absorber SBS 1543+593
We detect 21-cm emission from the Low Surface Brightness (LSB) galaxy SBS
1543+593, which gives rise to a Damped Lyman-alpha (DLA) absorption line in the
spectrum of the background QSO HS 1543+5921 (z=0.807). We obtain an accurate
measure of the velocity of the H I gas in the LSB galaxy, v=2868 km/s, and
derive a mass of 1.3e9 solar masses. We compare this value with limits obtained
towards two other z~0.1 DLA systems, and show that SBS 1543+593 would not have
been detected. Hence LSB galaxies similar to SBS 1543+593 can be responsible
for DLA systems at even modest redshifts without being detectable from their
21-cm emission.Comment: 4 pages, 1 figure, accepted for publication in A&
The O VI Absorbers Toward PG0953+415: High Metallicity, Cosmic-Web Gas Far From Luminous Galaxies
The spectrum of the low-redshift QSO PG0953+415 shows two strong, intervening
O VI absorption systems. To study the nature of these absorbers, we have used
the Gemini Multiobject Spectrograph to conduct a deep spectroscopic galaxy
redshift survey in the 5' x 5' field centered on the QSO. This survey is fully
complete for r' < 19.7 and is 73% complete for r' < 21.0. We find three
galaxies at the redshift of the higher-z O VI system (z = 0.14232) including a
galaxy at projected distance rho = 155 kpc. We find no galaxies in the Gemini
field at the redshift of the lower-z O VI absorber (z = 0.06807), which
indicates that the nearest galaxy is more than 195 kpc away or has L < 0.04 L*.
Previous shallower surveys covering a larger field have shown that the z =
0.06807 O VI absorber is affiliated with a group/filament of galaxies, but the
nearest known galaxy has rho = 736 kpc. The z = 0.06807 absorber is notable for
several reasons. The absorption profiles reveal simple kinematics indicative of
quiescent material. The H I line widths and good alignment of the H I and metal
lines favor photoionization and, moreover, the column density ratios imply a
high metallicity: [M/H] = -0.3 +/- 0.12. The z = 0.14232 O VI system is more
complex and less constrained but also indicates a relatively high metallicity.
Using galaxy redshifts from SDSS, we show that both of the PG0953+415 O VI
absorbers are located in large-scale filaments of the cosmic web. Evidently,
some regions of the web filaments are highly metal enriched. We discuss the
origin of the high-metallicity gas and suggest that the enrichment might have
occurred long ago (at high z).Comment: Submitted for publication in the Astrophysical Journal Letters. Figs.
1 and 2 compressed for astro-ph. High-resolution version available at
http://www.astro.umass.edu/~tripp/astro/qualitypreps/pg0953tripp.pd
- …