1,342 research outputs found
A Reservoir of Ionized Gas in the Galactic Halo to Sustain Star Formation in the Milky Way
Without a source of new gas, our Galaxy would exhaust its supply of gas
through the formation of stars. Ionized gas clouds observed at high velocity
may be a reservoir of such gas, but their distances are key for placing them in
the Galactic halo and unraveling their role. We have used the Hubble Space
Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the
foreground of Galactic stars. We show that iHVCs with 90 < |v_LSR| < 170 km/s
are within one Galactic radius of the sun and have enough mass to maintain star
formation, while iHVCs with |v_LSR|>170 km/s are at larger distances. These may
be the next wave of infalling material.Comment: This paper is part of a set of three papers on circumgalactic gas
observed with COS and STIS on HST, to be published in Science, together with
related papers by Tripp et al. and Tumlinson et al., in the November 18, 2011
edition. This version has not undergone final copyediting. Please see Science
online for the final printed versio
Chandra and Far Ultraviolet Spectroscopic Explorer Observations of z~0 Warm-Hot Gas Toward PKS 2155-304
The X-ray bright z=0.116 quasar PKS 2155-304 is frequently observed as a
Chandra calibration source, with a total of 483 ksec of Low Energy Transmission
Grating (LETG) exposure time accumulated through May 2006. Highly-ionized metal
absorption lines, including numerous lines at z=0 and a putative OVIII K-alpha
line at z=0.055, have been reported in past Chandra studies of this source.
Using all available Chandra LETG spectra and analysis techniques developed for
such z=0 X-ray absorption along other sightlines, we revisit these previous
detections. We detect 4 absorption lines at >3\sigma significance (OVII
K-alpha/beta, OVIII K-alpha, and NeIX K-alpha), with OVII K-alpha being a
7.3\sigma detection. The 1\sigma ranges of z=0 OVII column density and Doppler
parameter are consistent with those derived for Mrk 421 and within 2\sigma of
the Mrk 279 absorption. Temperatures and densities inferred from the relative
OVII and other ionic column densities are found to be consistent with either
the local warm-hot intergalactic medium or a Galactic corona. Unlike the local
X-ray absorbers seen in other sightlines, a link with the low- or high-velocity
far-ultraviolet OVI absorption lines cannot be ruled out. The z=0.055 OVIII
absorption reported by Fang et al. is seen with 3.5\sigma confidence in the
ACIS/LETG spectrum, but no other absorption lines are found at the same
redshift.Comment: 11 pages, 9 figures; minor changes, accepted to Ap
XMM-Newton View of the z>0 Warm-Hot Intergalactic Medium Toward Markarian 421
The recent detection with Chandra of two warm-hot intergalactic medium (WHIM) filaments toward Mrk 421 by Nicastro et al. provides a measurement of the bulk of the "missing baryons" in the nearby universe. Since Mrk 421 is a bright X-ray source, it is also frequently observed by the XMM-Newton Reflection Grating Spectrometer (RGS) for calibration purposes. Using all available archived XMM observations of this source with small pointing offsets ( 29A. The non-detection of the WHIM absorbers by XMM is thus fully consistent with the Chandra measurement
Studying the WHIM Content of the Galaxy Large-Scale Structures along the Line of Sight to H 2356-309
We make use of a 500ks Chandra HRC-S/LETG spectrum of the blazar H2356-309,
combined with a lower S/N spectrum of the same target, to search for the
presence of warm-hot absorbing gas associated with two Large-Scale Structures
(LSSs) crossed by this sightline at z=0.062 (the Pisces-Cetus Supercluster,
PCS) and at z=0.128 ("Farther Sculptor Wall", FSW). No statistically
significant (>=3sigma) individual absorption is detected from any of the strong
He- or H-like transitions of C, O and Ne at the redshifts of the structures.
However we are still able to constrain the physical and geometrical parameters
of the associated putative absorbing gas, by performing joint spectral fit of
marginal detections and upper limits of the strongest expected lines with our
self-consistent hybrid ionization WHIM spectral model. At the redshift of the
PCS we identify a warm phase with logT=5.35_-0.13^+0.07 K and log N_H
=19.1+/-0.2 cm^-2 possibly coexisting with a hotter and less significant phase
with logT=6.9^+0.1_-0.8 K and log N_H=20.1^+0.3_-1.7 cm^-2 (1sigma errors). For
the FSW we estimate logT=6.6_-0.2^+0.1 K and log N_H=19.8_-0.8^+0.4 cm^-2. Our
constraints allow us to estimate the cumulative number density per unit
redshifts of OVII WHIM absorbers. We also estimate the cosmological mass
density obtaining Omega_b(WHIM)=(0.021^+0.031_-0.018) (Z/Z_sun)^-1, consistent
with the mass density of the intergalactic 'missing baryons' for high
metallicities.Comment: 29 pages, 8 figures, 4 tables. Accepted for publication in Ap
A New Method for Obtaining Binary Pulsar Distances and its Implications for Tests of General Relativity
We demonstrate how measuring orbital period derivatives can lead to more
accurate distance estimates and transverse velocities for some nearby binary
pulsars. In many cases this method will estimate distances more accurately than
is possible by annual parallax, as the relative error decreases as t^-5/2.
Unfortunately, distance uncertainties limit the degree to which nearby
relativistic binary pulsars can be used for testing the general relativistic
prediction of orbital period decay to a few percent. Nevertheless, the measured
orbital period derivative of PSR B1534+12 agrees within the observational
uncertainties with that predicted by general relativity if the proper-motion
contribution is accounted for.Comment: 4 pages, latex, uuencoded compressed postscript + source, no figures,
uses aaspptwo.sty and dec.sty, accepted for publication in ApJL, omitted
reference now include
A detailed view of filaments and sheets in the warm-hot intergalactic medium. I. Pancake formation
Numerical simulations predict a considerable fraction of the missing baryons
at redshift z ~ 0 resting in the so called warm-hot intergalactic medium
(WHIM). The filaments and sheets of the WHIM have high temperatures 10^5 - 10^7
K) and a high degree of ionization while having only low to intermediate
densities. The particular physical conditions of the WHIM structures, e.g.
density and temperature profiles, velocity fields, are expected to leave their
special imprint on spectroscopic observations. In order to get further insight
into these conditions, we perform hydrodynamical simulations of the WHIM.
Instead of analyzing large simulations of cosmological structure formation, we
simulate particular well-defined structures and study the impact of different
physical processes as well as of the scale dependencies. We start with the
comprehensive study of the one-dimensional collapse (pancake) and examine the
influence of radiative cooling, heating due to an UV background, and thermal
conduction. We investigate the effect of small scale perturbations given
according to the initial cosmological power spectrum. If the initial
perturbation length scale L exceeds ~ 2 Mpc the collapse leads to shock
confined structures. As a result of radiative cooling and of heating due to an
UV background a relatively cold and dense core forms in the one-dimensional
case. The properties of the core (extension, density, and temperature) are
correlated with L. For larger L the core sizes are more concentrated. Thermal
conduction enhances this trend and may even result in an evaporation of the
core. Our estimates predict that a core may start to evaporate for perturbation
lengths larger than L ~ 30 Mpc. The obtained detailed profiles for density and
temperature for prototype WHIM structures allow for the determination of
possible spectral signatures by the WHIM.Comment: 14 pages, 9 figures, accepted for publication in A&
On the nature of the z=0 X-ray absorbers: I. Clues from an external group
Absorption lines of OVII at redshift zero are observed in high quality
Chandra spectra of extragalactic sightlines. The location of the absorber
producing these lines, whether from the corona of the Galaxy or from the Local
Group or even larger scale structure, has been a matter of debate. Here we
study another poor group like our Local Group to understand the distribution of
column density from galaxy to group scales. We show that we cannot yet rule out
the group origin of z=0 systems. We further argue that the debate over Galactic
vs. extragalactic origin of z=0 systems is premature as they likely contain
both components and predict that future higher resolution observations will
resolve the z=0 systems into multiple components.Comment: Submitted to ApJ
- …