10 research outputs found
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Compact Galactic Neutral Hydrogen Clouds in the GALFA-HI Survey
The more sensitive the observations, the more complex the gas in our Galaxy appears. Since the detection of neutral hydrogen in 1951, each new survey has revealed new structure. Using the GALFA-HI survey, we have discovered five populations of compact neutral hydrogen clouds. We began by developing a machine-vision algorithm to identify compact clouds in the GALFA-HI Data Release 1. Based on position, velocity, and linewidth we separated the 1964 identified clouds into five populations: galaxy candidates, high-velocity clouds, cold low-velocity clouds, warm low-velocity clouds, and warm positive low-velocity clouds in the third Galactic quadrant.
We found that the dust properties of the compact clouds support our population definitions. Using both IRAS and the newly released Planck data, we found no dust detections in the high-velocity clouds, or the warm positive low-velocity clouds in the third Galactic quadrant. We claim that the third quadrant clouds are low-velocity halo clouds. The warm low-velocity clouds have a significantly greater dust-to-gas ratio than the cold low-velocity clouds. We interpret this as evidence that the warm clouds have an ionized component not present with the cold clouds, possibly because they are part of the Galactic fountain
SubmilliJansky Transients in Archival Radio Observations
[ABRIDGED] We report the results of a 944-epoch survey for transient sources
with archival data from the Very Large Array spanning 22 years with a typical
epoch separation of 7 days. Observations were obtained at 5 or 8.4 GHz for a
single field of view with a full-width at half-maximum of 8.6' and 5.1',
respectively, and achieved a typical point-source detection threshold at the
beam center of ~300 microJy per epoch. Ten transient sources were detected with
a significance threshold such that only one false positive would be expected.
Of these transients, eight were detected in only a single epoch. Two transients
were too faint to be detected in individual epochs but were detected in
two-month averages. None of the ten transients was detected in longer-term
averages or associated with persistent emission in the deep image produced from
the combination of all epochs. The cumulative rate for the short timescale
radio transients above 370 microJy at 5 and 8.4 GHz is 0.07 < R < 40 deg^-2
yr^-1, where the uncertainty is due to the unknown duration of the transients,
20 min < t_char < 7 days. A two-epoch survey for transients will detect 1.5 +/-
0.4 transient per square degrees above a flux density of 370 microJy. Two
transients are associated with galaxies at z=0.040 and z=0.249. These may be
similar to the peculiar Type Ib/c radio supernova SN 1998bw associated with GRB
980428. Six transients have no counterparts in the optical or infrared (R=27,
Ks=18). The hosts and progenitors of these transients are unknown.Comment: Accepted for ApJ; full quality figures available at
http://astro.berkeley.edu/~gbower/ps/rt.pd
The GALFA-HI Survey: Data Release 1
We present the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey, and
its first full data release (DR1). GALFA-HI is a high resolution (~ 4'), large
area (13000 deg^2), high spectral resolution (0.18 km/s), wide band (-700 <
v_LSR < +700 km/s) survey of the Galactic interstellar medium in the 21-cm line
hyperfine transition of neutral hydrogen conducted at Arecibo Observatory.
Typical noise levels are 80 mK RMS in an integrated 1 km/s channel. GALFA-HI is
a dramatic step forward in high-resolution, large-area Galactic HI surveys, and
we compare GALFA-HI to past, present, and future Galactic HI surveys. We
describe in detail new techniques we have developed to reduce these data in the
presence of fixed pattern noise, gain variation, and inconsistent beam shapes,
and we show how we have largely mitigated these effects. We present our first
full data release, covering 7520 square degrees of sky and representing 3046
hours of integration time, and discuss the details of these data.Comment: Accepted to the ApJ
The GALFA-HI Compact Cloud Catalog
We present a catalog of 1964 isolated, compact neutral hydrogen clouds from
the Galactic Arecibo L-Band Feed Array Survey Data Release One (GALFA-HI DR1).
The clouds were identified by a custom machine-vision algorithm utilizing
Difference of Gaussian kernels to search for clouds smaller than 20'. The
clouds have velocities typically between |VLSR| = 20-400 km/s, linewidths of
2.5-35 km/s, and column densities ranging from 1 - 35 x 10^18 cm^-2. The
distances to the clouds in this catalog may cover several orders of magnitude,
so the masses may range from less than a Solar mass for clouds within the
Galactic disc, to greater than 10^4 Solar Masses for HVCs at the tip of the
Magellanic Stream. To search for trends, we separate the catalog into five
populations based on position, velocity, and linewidth: high velocity clouds
(HVCs); galaxy candidates; cold low velocity clouds (LVCs); warm, low
positive-velocity clouds in the third Galactic Quadrant; and the remaining warm
LVCs. The observed HVCs are found to be associated with previously-identified
HVC complexes. We do not observe a large population of isolated clouds at high
velocities as some models predict. We see evidence for distinct histories at
low velocities in detecting populations of clouds corotating with the Galactic
disc and a set of clouds that is not corotating.Comment: 34 Pages, 9 Figures, published in ApJ (2012, ApJ, 758, 44), this
version has the corrected fluxes and corresponding flux histogram and masse
Compact HI clouds from the GALFA-HI survey
The Galactic Arecibo L-band Feed Array HI (GALFA-HI) survey is mapping the
entire Arecibo sky at 21-cm, over a velocity range of -700 to +700 km/s (LSR),
at a velocity resolution of 0.18 km/s and a spatial resolution of 3.5 arcmin.
The unprecedented resolution and sensitivity of the GALFA-HI survey have
resulted in the detection of numerous isolated, very compact HI clouds at low
Galactic velocities, which are distinctly separated from the HI disk emission.
In the limited area of ~4600 deg surveyed so far, we have detected 96 of
such compact clouds. The detected clouds are cold with a median T
(the kinetic temperature in the case in which there is no non-thermal
broadening) of 300 K. Moreover, these clouds are quite compact and faint, with
median values of 5 arcmin in angular size, 0.75 K in peak brightness
temperature, and cm in HI column density. Most of the
clouds deviate from Galactic rotation at the 20-30 km/s level, and a
significant fraction show evidence for a multiphase medium and velocity
gradients. No counterparts for these clouds were found in other wavebands. From
the modeling of spatial and velocity distributions of the whole compact cloud
population, we find that the bulk of the compact clouds are related to the
Galactic disk, and their distances are likely to be in the range of 0.1 to a
few kpc. We discuss various possible scenarios for the formation and
maintenance of this cloud population and its significance for Galactic ISM
studies.Comment: Accepted for publication in the Astrophysical Journa
A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud
To investigate the fundamental principles of H2 formation in a giant
molecular cloud (GMC), we derive the HI and H2 surface density (Sigma_HI and
Sigma_H2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We
use the far-infrared data from the Improved Reprocessing of the IRAS Survey and
the V-band extinction image provided by the COMPLETE Survey to estimate the
dust column density image of Perseus. In combination with the HI data from the
Galactic Arecibo L-band Feed Array HI Survey and an estimate of the local
dust-to-gas ratio, we then derive the Sigma_H2 distribution across Perseus. We
find a relatively uniform Sigma_HI ~ 6-8 Msun pc^-2 for both dark and
star-forming regions, suggesting a minimum HI surface density required to
shield H2 against photodissociation. As a result, a remarkably tight and
consistent relation is found between Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2.
The transition between the HI- and H2-dominated regions occurs at N(HI)+2N(H2)
~ (8-14) x 10^20 cm^-2. Our findings are consistent with predictions for H2
formation in equilibrium, suggesting that turbulence may not be of primary
importance for H2 formation. However, the importance of a warm neutral medium
for H2 shielding, an internal radiation field, and the timescale of H2
formation still remain as open questions. We also compare H2 and CO
distributions and estimate the fraction of "CO-dark" gas, f_DG ~ 0.3. While
significant spatial variations of f_DG are found, we do not find a clear
correlation with the mean V-band extinction.Comment: updated to match the final version published in April 201
Recommended from our members
Compact Galactic Neutral Hydrogen Clouds in the GALFA-HI Survey
The more sensitive the observations, the more complex the gas in our Galaxy appears. Since the detection of neutral hydrogen in 1951, each new survey has revealed new structure. Using the GALFA-HI survey, we have discovered five populations of compact neutral hydrogen clouds. We began by developing a machine-vision algorithm to identify compact clouds in the GALFA-HI Data Release 1. Based on position, velocity, and linewidth we separated the 1964 identified clouds into five populations: galaxy candidates, high-velocity clouds, cold low-velocity clouds, warm low-velocity clouds, and warm positive low-velocity clouds in the third Galactic quadrant.
We found that the dust properties of the compact clouds support our population definitions. Using both IRAS and the newly released Planck data, we found no dust detections in the high-velocity clouds, or the warm positive low-velocity clouds in the third Galactic quadrant. We claim that the third quadrant clouds are low-velocity halo clouds. The warm low-velocity clouds have a significantly greater dust-to-gas ratio than the cold low-velocity clouds. We interpret this as evidence that the warm clouds have an ionized component not present with the cold clouds, possibly because they are part of the Galactic fountain