646 research outputs found
Infrared Search for Young Stars in HI High-velocity Clouds
We have searched the IRAS Point Source Catalog and HIRES maps for young
stellar objects (YSOs) in the direction of five \HI high-velocity clouds
(HVCs). In agreement with optical searches in the halo, no evidence was found
for extensive star-forming activity inside the high-latitude HVCs.
Specifically, we have found no signs of star formation or YSOs in the direction
of the A IV cloud or in the very-high-velocity clouds HVC~110-7-465 and
HVC~114-10-440. We have identified only one young star in the direction of the
M~I.1 cloud, which shows almost perfect alignment with a knot of \HI emission.
Because of the small number of early-type stars observed in the halo, the
probability for such a positional coincidence is low; thus, this young star
appears to be physically associated with the M~I.1 cloud. We have also
identified a good YSO candidate in the \HI shell-like structure observed in the
core region of the low-latitude cloud complex H (HVC~131+1-200). This region
could be a supernova remnant with several other YSO candidates formed along the
shock front produced by the explosion. In agreement with recent theoretical
estimates, these results point to a low but significant star-formation rate in
intermediate and high Galactic latitude HVCs. For M~I.1 in particular, we
estimate that the efficiency of the star-formation process is M(YSO)/M(\HI)\ga
10^{-4}-10^{-3} by mass. Such efficiency is sufficient to account for (a) the
existence of the few young blue stars whose ages imply that they were born in
the Galactic halo, and (b) the nonprimordial metallicities inferred for some
HVCs if their metal content proves to be low.Comment: 9 pages, 4 JPEG figures. PostScript figures available from author
Dependence of Gas Phase Abundances in the ISM on Column Density
Sightlines through high- and intermediate-velocity clouds allow measurements
of ionic gas phase abundances, A, at very low values of HI column density,
N(HI). Present observations cover over 4 orders of magnitude in N(HI).
Remarkably, for several ions we find that the A vs N(HI) relation is the same
at high and low column density and that the abundances have a relatively low
dispersion (factors of 2-3) at any particular N(HI). Halo gas tends to have
slightly higher values of A than disk gas at the same N(HI), suggesting that
part of the dispersion may be attributed to the environment. We note that the
dispersion is largest for NaI; using NaI as a predictor of N(HI) can lead to
large errors. Important implications of the low dispersions regarding the
physical nature of the ISM are: (a) because of clumping, over sufficiently long
pathlengths N(HI) is a reasonable measure of the_local_ density of_most_ of the
H atoms along the sight line; (b) the destruction of grains does not mainly
take place in catastrophic events such as strong shocks, but is a continuous
function of the mean density; (c) the cycling of the ions becoming attached to
grains and being detached must be rapid, and the two rates must be roughly
equal under a wide variety of conditions; (d) in gas that has a low average
density the attachment should occur within denser concentrations
A FUSE survey of high-latitude Galactic molecular hydrogen
Measurements of molecular hydrogen (H_2) column densities are presented for
the first six rotational levels (J=0 to 5) for 73 extragalactic targets
observed with FUSE. All of these have a final signal-to-noise ratio larger than
\snlimit, and are located at galactic latitude |b|>20 deg. The individual
observations were calibrated with the FUSE calibration pipeline CalFUSE version
2.1 or higher, and then carefully aligned in velocity. The final velocity
shifts for all the FUSE segments are listed. H_2 column densities or limits are
determined for the 6 lowest rotational (J) levels for each HI component in the
line of sight, using a curve-of-growth approach at low column densities ~16.5),
and Voigt-profile fitting at higher column densities. Detections include 73
measurements of low-velocity H_2 in the Galactic Disk and lower Halo. Eight
sightlines yield non-detections for Galactic H_2. The measured column densities
range from log N(H_2)=14 to log N(H_2)=20. Strong correlations are found
between log N(H_2) and T_01, the excitation temperature of the H_2, as well as
between log N(H_2) and the level population ratios (log (N(J')/N(J))). The
average fraction of nuclei in molecular hydrogen (f(H_2)) in each sightline is
calculated; however, because there are many HI clouds in each sightline, the
physics of the transition from HI to H_2 can not be studied. Detections also
include H2 in 16 intermediate-velocity clouds in the Galactic Halo (out of 35
IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm
of the Magellanic Stream), although 19 high-velocity clouds are intersected;
this strongly suggests that dust is rare or absent in these objects. Finally,
there are five detections of H_2 in external galaxies.Comment: Accepted for ApJ Supplement. Note: figs 7 and 8 not included because
astro-ph rejects them as too bi
High-resolution 21-cm observations of low-column density gas clumps in the Milky Way halo
We study the properties of low-column density gas clumps in the halo of the
Milky Way based on high-resolution 21-cm observations.
Using interferometric data from the WSRT and the VLA we study HI emission at
low-, intermediate- and high radial velocities along four lines of sight
towards quasars. Along these sightlines we previously detected weak CaII and
NaI absorbers in their optical spectra.
The analysis of the high-resolution HI data reveals the presence of several
compact and cold clumps of neutral gas at velocities similar to the optical
absorption. The clumps have narrow HI line widths in the range of 1.8 to 13
km/s, yielding upper limits for the kinetic temperature of the gas of 70 to
3700 K. The neutral gas has low HI column densities in the range of 5E18 to
3E19 1/cm^2. All clumps have angular sizes of only a few arcminutes.
Our high-resolution 21-cm observations indicate that many of the CaII and NaI
absorbers seen in our optical quasar spectra are associated with low-column
density HI clumps at small angular scales. This suggests that next to the
massive, high-column density neutral gas clouds in the halo (the common 21-cm
LVCs, IVCs, and HVCs) there exists a population of low-mass, neutral gas
structures in the halo that remain mostly unseen in the existing 21-cm all-sky
surveys of IVCs and HVCs. The estimated thermal gas pressures of the detected
HI clumps are consistent with what is expected from theoretical models of gas
in the inner and outer Milky Way halo.Comment: 12 pages, 7 figure
A Catalogue of Field Horizontal Branch Stars Aligned with High Velocity Clouds
We present a catalogue of 430 Field Horizontal Branch (FHB) stars, selected
from the Hamburg/ESO Survey (HES), which fortuitously align with high column
density neutral hydrogen (HI) High-Velocity Cloud (HVC) gas. These stars are
ideal candidates for absorption-line studies of HVCs, attempts at which have
been made for almost 40 years with little success. A parent sample of 8321 HES
FHB stars was used to extract HI spectra along each line-of-sight, using the HI
Parkes All-Sky Survey. All lines-of-sight aligned with high velocity HI
emission with peak brightness temperatures greater than 120mK were examined.
The HI spectra of these 430 probes were visually screened and cross-referenced
with several HVC catalogues. In a forthcoming paper, we report on the results
of high-resolution spectroscopic observations of a sample of stars drawn from
this catalogue.Comment: 7 pages, 4 figures. ApJS accepted. Full catalogue and all online-only
images available at
http://astronomy.swin.edu.au/staff/cthom/catalogue/index.htm
High-resolution imaging of compact high-velocity clouds (II)
We have imaged five compact high-velocity clouds in HI with arcmin angular-
and km/s spectral-resolution using the WSRT. Supplementary total-power data,
which is fully sensitive to both the cool and warm components of HI, is
available for comparison for all the sources, albeit with angular resolutions
that vary from 3' to 36'. The fractional HI flux in compact CNM components
varies from 4% to 16% in our sample. All objects have at least one local peak
in the CNM column which exceeds about 10^19 cm^-2 when observed with arcmin
resolution. It is plausible that a peak column density of 1-2x10^19 cm^-2 is a
prerequisite for the long-term survival of these sources. One object in our
sample, CHVC120-20-443 (Davies' cloud), lies in close projected proximity to
the disk of M31. This object is characterized by exceptionally broad linewidths
in its CNM concentrations (more than 5 times greater than the median value).
These CNM concentrations lie in an arc on the edge of the source facing the M31
disk, while the diffuse HI component of this source has a position offset in
the direction of the disk. All of these attributes suggest that CHVC120-20-443
is in a different evolutionary state than most of the other CHVCs which have
been studied. Similarly broad CNM linewidths have only been detected in one
other object, CHVC111-07-466, which also lies in the Local Group barycenter
direction and has the most extreme radial velocity known. A distinct
possibility for Davies' cloud seems to be physical interaction of some type
with M31. The most likely form of this interaction might be the ram-pressure or
tidal- stripping by either one of M31's visible dwarf companions, M32 or
NGC205, or else by a dark companion with an associated HI condensation.Comment: 12 pages, 11 (low res.) png figs, accepted for pub. in A&
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