199 research outputs found
Simulations of High-Velocity Clouds. I. Hydrodynamics and High-Velocity High Ions
We present hydrodynamic simulations of high-velocity clouds (HVCs) traveling
through the hot, tenuous medium in the Galactic halo. A suite of models was
created using the FLASH hydrodynamics code, sampling various cloud sizes,
densities, and velocities. In all cases, the cloud-halo interaction ablates
material from the clouds. The ablated material falls behind the clouds, where
it mixes with the ambient medium to produce intermediate-temperature gas, some
of which radiatively cools to less than 10,000 K. Using a non-equilibrium
ionization (NEI) algorithm, we track the ionization levels of carbon, nitrogen,
and oxygen in the gas throughout the simulation period. We present
observation-related predictions, including the expected H I and high ion (C IV,
N V, and O VI) column densities on sight lines through the clouds as functions
of evolutionary time and off-center distance. The predicted column densities
overlap those observed for Complex C. The observations are best matched by
clouds that have interacted with the Galactic environment for tens to hundreds
of megayears. Given the large distances across which the clouds would travel
during such time, our results are consistent with Complex C having an
extragalactic origin. The destruction of HVCs is also of interest; the smallest
cloud (initial mass \approx 120 Msun) lost most of its mass during the
simulation period (60 Myr), while the largest cloud (initial mass \approx 4e5
Msun) remained largely intact, although deformed, during its simulation period
(240 Myr).Comment: 20 pages, 13 figures. Accepted for publication in the Astrophysical
Journa
Simulations of High-Velocity Clouds. II. Ablation from High-Velocity Clouds as a Source of Low-Velocity High Ions
In order to determine if the material ablated from high-velocity clouds
(HVCs) is a significant source of low-velocity high ions (C IV, N V, and O VI)
such as those found in the Galactic halo, we simulate the hydrodynamics of the
gas and the time-dependent ionization evolution of its carbon, nitrogen, and
oxygen ions. Our suite of simulations examines the ablation of warm material
from clouds of various sizes, densities, and velocities as they pass through
the hot Galactic halo. The ablated material mixes with the environmental gas,
producing an intermediate-temperature mixture that is rich in high ions and
that slows to the speed of the surrounding gas. We find that the slow mixed
material is a significant source of the low-velocity O VI that is observed in
the halo, as it can account for at least ~1/3 of the observed O VI column
density. Hence, any complete model of the high ions in the halo should include
the contribution to the O VI from ablated HVC material. However, such material
is unlikely to be a major source of the observed C IV, presumably because the
observed C IV is affected by photoionization, which our models do not include.
We discuss a composite model that includes contributions from HVCs, supernova
remnants, a cooling Galactic fountain, and photoionization by an external
radiation field. By design, this model matches the observed O VI column
density. This model can also account for most or all of the observed C IV, but
only half of the observed N V.Comment: 17 pages, 8 figures. Accepted for publication in the Astrophysical
Journa
An XMM-Newton Survey of the Soft X-ray Background. II. An All-Sky Catalog of Diffuse O VII and O VIII Emission Intensities
We present an all-sky catalog of diffuse O VII and O VIII line intensities,
extracted from archival XMM observations. The O VII and O VIII intensities are
typically ~2-11 and <~3 ph/cm^2/s/sr (LU), respectively, although much brighter
intensities were also recorded. Our data set includes 217 directions observed
multiple times by XMM. The time variation of the intensities from such
directions may be used to constrain SWCX models. The O VII and O VIII
intensities typically vary by <~5 and <~2 LU between repeat observations,
although several intensity enhancements of >10 LU were observed. We compared
our measurements with SWCX models. The heliospheric SWCX intensity is expected
to vary with ecliptic latitude and solar cycle. We found that the observed
oxygen intensities generally decrease from solar maximum to solar minimum, both
at high ecliptic latitudes (as expected) and at low ecliptic latitudes (not as
expected). The geocoronal SWCX intensity is expected to depend on the solar
wind proton flux and on the sightline's path through the magnetosheath. The
intensity variations seen in directions that have been observed multiple times
are in poor agreement with the predictions of a geocoronal SWCX model. The
oxygen lines account for ~40-50% of the 3/4 keV X-ray background that is not
due to unresolved AGN, in good agreement with a previous measurement. However,
this fraction is not easily explained by a combination of SWCX emission and
emission from hot plasma in the halo. The line intensities tend to increase
with longitude toward the inner Galaxy, possibly due to an increase in the
supernova rate in that direction or the presence of a halo of accreted material
centered on the Galactic Center. The variation of intensity with Galactic
latitude differs in different octants of the sky, and cannot be explained by a
single simple plane-parallel or constant-intensity halo model. (Abridged)Comment: Accepted for publication in the Astrophysical Journal Supplement
Series. 29 pages (main body of paper) plus 85 pages (full versions of Tables
1, 2, and 4 - these tables will be published as machine-readable tables in
the journal, and appear in abbreviated form in the main body of the paper).
12 figures. v2: Minor corrections, conclusions unaltere
The Origin of the Hot Gas in the Galactic Halo: Confronting Models with XMM-Newton Observations
We compare the predictions of three physical models for the origin of the hot
halo gas with the observed halo X-ray emission, derived from 26 high-latitude
XMM-Newton observations of the soft X-ray background between l=120\degr and
l=240\degr. These observations were chosen from a much larger set of
observations as they are expected to be the least contaminated by solar wind
charge exchange emission. We characterize the halo emission in the XMM-Newton
band with a single-temperature plasma model. We find that the observed halo
temperature is fairly constant across the sky (~1.8e6-2.3e6 K), whereas the
halo emission measure varies by an order of magnitude (~0.0005-0.006 cm^-6 pc).
When we compare our observations with the model predictions, we find that most
of the hot gas observed with XMM-Newton does not reside in isolated extraplanar
supernova remnants -- this model predicts emission an order of magnitude too
faint. A model of a supernova-driven interstellar medium, including the flow of
hot gas from the disk into the halo in a galactic fountain, gives good
agreement with the observed 0.4-2.0 keV surface brightness. This model
overpredicts the halo X-ray temperature by a factor of ~2, but there are a
several possible explanations for this discrepancy. We therefore conclude that
a major (possibly dominant) contributor to the halo X-ray emission observed
with XMM-Newton is a fountain of hot gas driven into the halo by disk
supernovae. However, we cannot rule out the possibility that the extended hot
halo of accreted material predicted by disk galaxy formation models also
contributes to the emission.Comment: 20 pages, 14 figures. New version accepted for publication in ApJ.
Changes include new section discussing systematic errors (Section 3.2),
improved method for characterizing our model spectra (4.2.2), changes to
discussion of other observations (5.1). Note that we can no longer rule out
possibility that extended hot halo of accreted material contributes to
observed halo emission (see 5.2.1
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