3,032 research outputs found
A STIS Survey for OVI Absorption Systems at 0.12 < z < 0.5 I.: The Statistical Properties of Ionized Gas
We have conducted a systematic survey for intervening OVI absorbers in
available echelle spectra of 16 QSOs at z_QSO = 0.17-0.57. These spectra were
obtained using HST/STIS with the E140M grating. Our search uncovered a total of
27 foreground OVI absorbers with rest-frame absorption equivalent width
W_r(1031) > 25mA. Ten of these QSOs exhibit strong OVI absorbers in their
vicinity. Our OVI survey does not require the known presence of Lya, and the
echelle resolution allows us to identify the OVI absorption doublet based on
their common line centroid and known flux ratio. We estimate the total redshift
survey path, \Delta z, using a series of Monte-Carlo simulations, and find that
\Delta z=1.66, 2.18, and 2.42 for absorbers of strength W_r = 30, 50 and 80mA,
respectively, leading to a number density of dN(W > 50mA)/dz = 6.7 +/- 1.7 and
dN(W > 30mA)/dz = 10.4 +/- 2.2. In contrast, we also measure dN/dz = 27 +/- 9
for OVI absorbers of W_r > 50mA at |\Delta v|< 5000 kms from the background
QSOs. Using the random sample of OVI absorbers with well characterized survey
completeness, we estimate a mean cosmological mass density of the OVI gas
\Omega(OVI)h = 1.7 +/- 0.3 x 10^-7. In addition, we show that <5% of OVI
absorbers originate in underdense regions that do not show a significant trace
of HI. Furthermore, we show that the neutral gas column N(HI) associated with
these OVI absorbers spans nearly five orders of magnitude, and show moderate
correlation with N(OVI). Finally, while the number density of OVI absorbers
varies substantially from one sightline to another, it also appears to be
inversely correlated with the number density of HI absorbers along individual
lines of sight.Comment: 12 pages. ApJ accepte
The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are A Major Reservoir of Galactic Metals
The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of
intergalactic gas, but its mass, heavy element enrichment, and relation to
galaxy properties are poorly constrained by observations. In a survey of the
outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the
Hubble Space Telescope, we detected ubiquitous, large (150 kiloparsec) halos of
ionized oxygen surrounding star-forming galaxies, but we find much less ionized
oxygen around galaxies with little or no star formation. This ionized CGM
contains a substantial mass of heavy elements and gas, perhaps far exceeding
the reservoirs of gas in the galaxies themselves. It is a basic component of
nearly all star-forming galaxies that is removed or transformed during the
quenching of star formation and the transition to passive evolution.Comment: This paper is part of a set of three papers on circumgalactic gas
observed with the Cosmic Origins Spectrograph on HST, to be published in
Science, together with related papers by Tripp et al. and Lehner & Howk, in
the November 18, 2011 edition. This version has not undergone final
copyediting. Please see Science online for the final printed versio
A GLIMPSE into the Nature of Galactic Mid-IR Excesses
We investigate the nature of the mid-IR excess for 31 intermediate-mass stars
that exhibit an 8 micron excess in either the Galactic Legacy Infrared
Mid-Plane Survey Extraordinaire or the Mid-Course Space Experiment using high
resolution optical spectra to identify stars surrounded by warm circumstellar
dust. From these data we determine projected stellar rotational velocities and
estimate stellar effective temperatures for the sample. We estimate stellar
ages from these temperatures, parallactic distances, and evolutionary models.
Using MIPS [24] measurements and stellar parameters we determine the nature of
the infrared excess for 19 GLIMPSE stars. We find that 15 stars exhibit Halpha
emission and four exhibit Halpha absorption. Assuming that the mid-IR excesses
arise in circumstellar disks, we use the Halpha fluxes to model and estimate
the relative contributions of dust and free-free emission. Six stars exhibit
Halpha fluxes that imply free-free emission can plausibly explain the infrared
excess at [24]. These stars are candidate classical Be stars. Nine stars
exhibit Halpha emission, but their Halpha fluxes are insufficient to explain
the infrared excesses at [24], suggesting the presence of a circumstellar dust
component. After the removal of the free-free component in these sources, we
determine probable disk dust temperatures of Tdisk~300-800 K and fractional
infrared luminosities of L(IR)/L(*)~10^-3. These nine stars may be
pre-main-sequence stars with transitional disks undergoing disk clearing. Three
of the four sources showing Halpha absorption exhibit circumstellar disk
temperatures ~300-400 K, L(IR)/L(*)~10^-3, IR colors K-[24]< 3.3, and are warm
debris disk candidates. One of the four Halpha absorption sources has K-[24]>
3.3 implying an optically thick outer disk and is a transition disk candidate.Comment: 17 figures. Accepted for publication in Ap
The Circumgalactic Medium in Massive Halos
This chapter presents a review of the current state of knowledge on the cool
(T ~ 1e4 K) halo gas content around massive galaxies at z ~ 0.2-2. Over the
last decade, significant progress has been made in characterizing the cool
circumgalactic gas in massive halos of Mh ~ 1e12-1e14 Msun at intermediate
redshifts using absorption spectroscopy. Systematic studies of halo gas around
massive galaxies beyond the nearby universe are made possible by large
spectroscopic samples of galaxies and quasars in public archives. In addition
to accurate and precise constraints for the incidence of cool gas in massive
halos, detailed characterizations of gas kinematics and chemical compositions
around massive quiescent galaxies at z ~ 0.5 have also been obtained. Combining
all available measurements shows that infalling clouds from external sources
are likely the primary source of cool gas detected at d >~ 100 kpc from massive
quiescent galaxies. The origin of the gas closer in is currently less certain,
but SNe Ia driven winds appear to contribute significantly to cool gas found at
d < 100 kpc. In contrast, cool gas observed at d <~ 200 kpc from luminous
quasars appears to be intimately connected to quasar activities on parsec
scales. The observed strong correlation between cool gas covering fraction in
quasar host halos and quasar bolometric luminosity remains a puzzle. Combining
absorption-line studies with spatially-resolved emission measurements of both
gas and galaxies is the necessary next step to address remaining questions.Comment: 29 pages, 7 figures, invited review to appear in "Gas Accretion onto
Galaxies", Astrophysics and Space Science Library, eds. A. Fox & R. Dave, to
be published by Springe
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
Multiphase Gas In Galaxy Halos: The OVI Lyman-limit System toward J1009+0713
We have serendipitously detected a strong O VI-bearing Lyman limit system at
z_abs = 0.3558 toward the QSO J1009+0713 (z_em = 0.456) in our survey of
low-redshift galaxy halos with the Hubble Space Telescope's Cosmic Origins
Spectrograph. Its rest-frame equivalent width of W_r = 835 +/- 49 mA is the
highest for an intervening absorber yet detected in any low-redshift QSO
sightline, with absorption spanning 400 km s^-1 in its rest frame. HST/WFC3
images of the galaxy field show that the absorber is associated with two
galaxies lying at 14 and 46 kpc from the QSO line of sight. The bulk of the
absorbing gas traced by H I resides in two strong, blended component groups
that possess a total logN(HI) = 18 - 18.8. The ion ratios and column densities
of C, N, O, Mg, Si, S, and Fe, except the O VI, can be accommodated into a
simple photoionization model in which diffuse, low-metallicity halo gas is
exposed to a photoionizing field from stars in the nearby galaxies that
propagates into the halo at 10% efficiency. We constrain the metallicity firmly
within the range 0.1 - 1 Zsun, and photoionization modeling indirectly
indicates a subsolar metallicity of 0.05 - 0.5 Zsun. The appearance of strong O
VI and nine Mg II components and our review of similar systems in the
literature support the "interface" picture of high-velocity O VI: the total
strength of the O VI shows a positive correlation with the number of detected
components in the low-ionization gas, however the total O VI column densities
still far exceed the values expected from interface models for the number of
detected clouds.Comment: 20 pages, 11 figures, accepted for publication in Ap
High-velocity clouds as streams of ionized and neutral gas in the halo of the Milky Way
High-velocity clouds (HVC), fast-moving ionized and neutral gas clouds found
at high galactic latitudes, may play an important role in the evolution of the
Milky Way. The extent of this role depends sensitively on their distances and
total sky covering factor. We search for HVC absorption in HST high resolution
ultraviolet spectra of a carefully selected sample of 133 AGN using a range of
atomic species in different ionization stages. This allows us to identify
neutral, weakly ionized, or highly ionized HVCs over several decades in HI
column densities. The sky covering factor of UV-selected HVCs with |v_LSR|>90
km/s is 68%+/-4% for the entire Galactic sky. We show that our survey is
essentially complete, i.e., an undetected population of HVCs with extremely low
N(H) (HI+HII) is unlikely to be important for the HVC mass budget. We confirm
that the predominantly ionized HVCs contain at least as much mass as the
traditional HI HVCs and show that large HI HVC complexes have generally ionized
envelopes extending far from the HI contours. There are also large regions of
the Galactic sky that are covered with ionized high-velocity gas with little HI
emission nearby. We show that the covering factors of HVCs with 90<|v_LSR|<170
km/s drawn from the AGN and stellar samples are similar. This confirms that
these HVCs are within 5-15 kpc of the sun. The covering factor of these HVCs
drops with decreasing vertical height, which is consistent with HVCs being
decelerated or disrupted as they fall to the Milky Way disk. The HVCs with
|v_LSR|>170 km/s are largely associated with the Magellanic Stream at b<0 and
its leading arm at b>0 as well as other large known HI complexes. Therefore
there is no evidence in the Local Group that any galaxy shows a population of
HVCs extending much farther away than 50 kpc from its host, except possibly for
those tracing remnants of galaxy interaction.Comment: Submitted to MNRAS (19 pages, 11 figures). Comments are welcom
Gas Accretion in Star-Forming Galaxies
Cold-mode gas accretion onto galaxies is a direct prediction of LCDM
simulations and provides galaxies with fuel that allows them to continue to
form stars over the lifetime of the Universe. Given its dramatic influence on a
galaxy's gas reservoir, gas accretion has to be largely responsible for how
galaxies form and evolve. Therefore, given the importance of gas accretion, it
is necessary to observe and quantify how these gas flows affect galaxy
evolution. However, observational data have yet to conclusively show that gas
accretion ubiquitously occurs at any epoch. Directly detecting gas accretion is
a challenging endeavor and we now have obtained a significant amount of
observational evidence to support it. This chapter reviews the current
observational evidence of gas accretion onto star-forming galaxies.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springer. This chapter includes 22 pages with 7 Figure
Steroids during late preterm labor: better later than never
Review of: Gyamfi-Bannerman C, Thom EA, Blackwell SC, et al; NICHD Maternal-Fetal Medicine Units Network. Antenatal betamethasone for women at risk for late preterm delivery. N Engl J Med. 2016;374:1311-1320.Steroids during late preterm labor: Better later than never. Steroids--even when administered in the last leg of the late preterm period--still reduce the likelihood of respiratory complications in newborns. Practice changer: Use steroids in women at risk of preterm delivery, even if they are 36 weeks, 6 days-pregnant, because steroids may reduce respiratory complications in the newborn with minimal risk for neonatal or maternal complications
The minimum scale of grooving on a recently ruptured limestone fault
AbstractFaults have grooves that are formed by abrasion and wear during slip. Recent observations indicate that this grooving is only a largeâscale feature, indicating brittle behavior has a length scale limit. The connection between this scale and earthquake behavior remains limited because no examples exist from a proven seismogenic fault. Here, we address this problem and analyze differences in this scale between lithologies to further our understanding of the underlying mechanics. This study uses samples from the Mt. Vettoretto fault collected after the Norcia earthquake of 2016. We imaged fault topography with a white light interferometer and 10 ÎŒm resolution structure from motion and then calculated a Monte Carlo version of root mean square roughness. We found a minimum scale of grooving of ~100 ÎŒm. In comparing this fault to the Corona Heights fault, we find that this minimum grooving scale is consistent with predictions based on material properties
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