1,014 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
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
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
Hubble-COS Observations of Galactic High-Velocity Clouds: Four AGN Sight Lines through Complex C
We report ultraviolet spectra of Galactic high-velocity clouds (HVCs) in
Complex C, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space
Telescope (HST), together with new 21-cm spectra from the Green Bank Telescope.
The wide spectral coverage and higher S/N, compared to previous HST spectra,
provide better velocity definition of the HVC absorption, additional ionization
species, and improved abundances in this halo gas. Complex C has a metallicity
of 0.1-0.3 solar and a wide range of ions, suggesting dynamical and thermal
interactions with hot gas in the Galactic halo. Spectra in the COS
medium-resolution G130M (1133-1468 A) and G160M (1383-1796 A) gratings detect
ultraviolet absorption lines from 8 elements in low ionization stages (O I, N
I, C II, S II, Si II, Al II, Fe II, P II) and 3 elements in intermediate and
high-ionization states (Si III, Si IV, C IV, N V). Our four AGN sight lines
toward Mrk 817, Mrk 290, Mrk 876, and PG1259+593 have high-velocity H I and O
VI column densities, log N_HI = 19.39-20.05 and log N_OVI = 13.58-14.10, with
substantial amounts of kinematically associated photoionized gas. The high-ion
abundance ratios are consistent with cooling interfaces between photoionized
gas and collisionally ionized gas: N(C IV)/N(O VI) = 0.3-0.5, N(Si IV)/N(O VI)
= 0.05-0.11, N(N V)/N(O VI) = 0.07-0.13, and N(Si IV)/N(Si III) = 0.2.Comment: 43 pages, 11 figures (appearing in ApJ, Sept 1, 2011
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
Absorption Line Studies in the Halo
Significant progress has been made over the last few years to explore the
gaseous halo of the Milky Way by way of absorption spectroscopy. I review
recent results on absorption line studies in the halo using various
instruments, such as the Far Ultraviolet Spectroscopic Explorer, the Space
Telescope Imaging Spectrograph, and others. The new studies imply that the
infall of low-metallicity gas, the interaction with the Magellanic Clouds, and
the Galactic Fountain are responsible for the phenomenon of the intermediate-
and high-velocity clouds in the halo. New measurements of highly-ionized gas in
the vicinity of the Milky Way indicate that these clouds are embedded in a
corona of hot gas that extends deep into the intergalactic space.Comment: 7 pages, 1 figure; Invited review at the conference "How does the
Galaxy work ?", Granada/Spain, June 200
The Spitzer Space Telescope First-Look Survey: Neutral Hydrogen Emission
The Spitzer Space Telescope (formerly SIRTF) extragalactic First-Look Survey
covered about 5 square degrees centered on J2000 17:18 +59:30 in order to
characterize the infrared sky with high sensitivity. We used the 100-m Green
Bank Telescope to image the 21cm Galactic HI emission over a 3x3 degree field
covering this position with an effective angular resolution of 9.8 arcmin and a
velocity resolution of 0.62 km/s. In the central square degree of the image the
average column density is N(HI) = 2.5 x 10^{20} cm-2 with an rms fluctuation of
0.3 x 10^{20}. The Galactic HI in this region has a very interesting structure.
There is a high-velocity cloud, several intermediate-velocity clouds (one of
which is probably part of the Draco nebula), and narrow-line low velocity
filaments. The HI emission shows a strong and detailed correlation with dust.
Except for the high-velocity cloud, all features in the HI map have
counterparts in an E(B-V) map derived from infrared data. Relatively high
E(B-V)/N(HI) ratios in some directions suggest the presence of molecular gas.
The best diagnostic of such regions is the peak HI line brightness temperature,
not the total N(HI): directions where Tb > 12 K have E(B-V)/N(HI) significantly
above the average value. The data corrected for stray radiation have been
released via the Web.Comment: Accepted for publication in the Astronomical Journal, April 2005. 25
pages includes 11 figures. The data and higher resolution figures are
available from http::/www.cv.nrao.edu/fls_gb
Distances to Galactic high-velocity clouds. I. Cohen Stream, complex GCP, cloud g1
The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are
tracers of energetic processes in and around the Milky Way. Clouds with
near-solar metallicity about one kpc above the disk trace the circulation of
material between disk and halo (the Galactic Fountain). The Magellanic Stream
consists of gas tidally extracted from the SMC, tracing the dark matter
potential of the Milky Way. Several other HVCs have low-metallicity and appear
to trace the continuing accretion of infalling intergalactic gas. These
assertions are supported by the metallicities (0.1 to 1 solar) measured for
about ten clouds in the past decade. Direct measurements of distances to HVCs
have remained elusive, however. In this paper we present four new distance
brackets, using VLT observations of interstellar \CaII H and K absorption
toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7
kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8
to 15.1 kpc for complex GCP (also known as the Smith Cloud or HVC40-15+100 and
with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated
with the return flow of the Fountain in the Perseus Arm, and 1.8 to 3.8 kpc for
cloud g1, which appears to be in the outflow phase of the Fountain. Our
measurements further demonstrate that the Milky Way is accreting substantial
amounts of gaseous material, which influences the Galaxy's current and future
dynamical and chemical evolution.Comment: Accepted by Ap
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