Steven J. Gibson

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.

A High-resolution Study of the H I-H2 Transition across the Perseus Molecular Cloud

To investigate the fundamental principles of H2 formation in a giant molecular cloud, we derive the H I and H2 surface density (?H I and ?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 H I data from the Galactic Arecibo L-band Feed Array H I Survey and an estimate of the local dust-to-gas ratio, we then derive the ?H2 distribution across Perseus. We find a relatively uniform ?H I ~ 6-8 M ??pc?2 for both dark and star-forming regions, suggesting a minimum H I surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between ?H2/?H I and ?H I + ?H2. The transition between the H I- and H2-dominated regions occurs at N(H I) + 2N(H2) ~ (8-14)?? 1020?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.

The GALFA-H I 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. The clouds were identified by a custom machine-vision algorithm utilizing the difference of Gaussian kernels to search for clouds smaller than 20. The clouds have velocities typically between |V LSR| =20 and 400xa0kmxa0s–1, line widths of 2.5-35xa0kmxa0s–1, and column densities ranging from 1 to 35 × 1018xa0cm–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 disk, to greater than 104 M ☉ for high-velocity clouds (HVCs) at the tip of the Magellanic Stream. To search for trends, we separate the catalog into five populations based on position, velocity, and line width: 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 disk and a set of clouds that is not corotating.

SPLASH: the Southern Parkes Large-Area Survey in Hydroxyl – first science from the pilot region

ABSTRACT SPLASH (the Southern Parkes Large-Area Survey in Hydroxyl) is a sensitive, unbiasedand fully-sampled survey of the Southern Galactic Plane and Galactic Centre in allfour ground-state transitions of the hydroxyl (OH) radical. The survey provides a deepcensus of 1612-, 1665-, 1667- and 1720-MHz OH absorption and emission from theGalactic ISM, and is also an unbiased search for maser sources in these transitions.We present here rst results from the SPLASH pilot region, which covers Galacticlongitudes 334 x0e to 344 and latitudes 2 . Di use OH is widely detected in all fourtransitions, with optical depths that are always small (averaged over the Parkes beam),and with departures from LTE common even in the 1665- and 1667-MHz main lines.To a 3˙sensitivity of ˘ 30 mK, we nd no evidence of OH envelopes extending beyondthe CO-bright regions of molecular cloud complexes, and conclude that the similarityof the OH excitation temperature and the level of the continuum background is atleast partly responsible for this. We detect masers and maser candidates in all fourtransitions, approximately 50 per cent of which are new detections. This implies thatSPLASH will produce a substantial increase in the known population of ground-stateOH masers in the Southern Galactic Plane.Key words: Galaxy: disc, ISM: molecules, masers, radio lines: ISM, surveys

COMPACT H I CLOUDS FROM THE GALFA-H I SURVEY

The Galactic Arecibo L-band Feed Array H I (GALFA-H I) survey is mapping the entire Arecibo sky at 21xa0cm, over a velocity range of –700 to +700xa0kmxa0s–1 (LSR), at a velocity resolution of 0.18xa0kmxa0s–1, and a spatial resolution of 3.5xa0arcmin. The unprecedented resolution and sensitivity of the GALFA-H I survey have resulted in the detection of numerous isolated, very compact H I clouds at low Galactic velocities, which are distinctly separated from the H I disk emission. In the limited area of ~4600xa0deg2 surveyed so far, we have detected 96 such compact clouds. The detected clouds are cold with a median T k,max (the kinetic temperature in the case in which there is no non-thermal broadening) of 300xa0K. Moreover, these clouds are quite compact and faint, with median values of 5xa0arcmin in angular size, 0.75xa0K in peak brightness temperature, and 5 × 1018xa0cm–2 in H I column density. Most of the clouds deviate from Galactic rotation at the 20-30xa0kmxa0s–1 level, and a significant fraction show evidence for a multiphase medium and velocity gradients. No counterparts for these clouds were found in other wave bands. 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 fewxa0kpc. We discuss various possible scenarios for the formation and maintenance of this cloud population and its significance for Galactic interstellar medium studies.

GASKAP-The Galactic ASKAP Survey

A survey of the Milky Way disk and the Magellanic System at the wavelengths of the 21-cm atomic hydrogen (H i) line and three 18-cm lines of the OH molecule will be carried out with the Australian Square Kilometre Array Pathfinder telescope. The survey will study the distribution of H i emission and absorption with unprecedented angular and velocity resolution, as well as molecular line thermal emission, absorption, and maser lines. The area to be covered includes the Galactic plane (|b| < 10°) at all declinations south of δ = +40°, spanning longitudes 167° through 360° to 79° at b = 0°, plus the entire area of the Magellanic Stream and Clouds, a total of 13 020 deg2. The brightness temperature sensitivity will be very good, typically σT≃ 1 K at resolution 30 arcsec and 1 km s−1. The survey has a wide spectrum of scientific goals, from studies of galaxy evolution to star formation, with particular contributions to understanding stellar wind kinematics, the thermal phases of the interstellar medium, the interaction between gas in the disk and halo, and the dynamical and thermal states of gas at various positions along the Magellanic Stream.

A peculiar H i cloud near the distant globular cluster Pal 4

We present 21-cm observations of four Galactic globular clusters, as part of the on-going Galactic Arecibo L-band Feed Array (GALFA) Hxa0i survey at Arecibo. We discovered a peculiar Hxa0i cloud in the vicinity of the distant (109 kpc) cluster Palxa04, and discuss its properties and likelihood of association with the cluster. We conclude that an association of the Hxa0i cloud and Palxa04 is possible, but that a chance coincidence between Palxa04 and a nearby compact high-velocity cloud cannot be ruled out altogether. New, more stringent upper limits were derived for the other three clusters: Mxa03, NGCxa05466, and Palxa013. We briefly discuss the fate of globular cluster gas and the interaction of compact clouds with the Galactic halo gas.

CHIMPS: the 13CO/C18O (J = 3 → 2) Heterodyne Inner Milky Way Plane Survey

We present the

Synthetic CO, H2 and H i surveys of the second galactic quadrant, and the properties of molecular gas

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H I SHELLS AND SUPERSHELLS IN THE I-GALFA H I 21 cm LINE SURVEY. I. FAST-EXPANDING H I SHELLS ASSOCIATED WITH SUPERNOVA REMNANTS

CO/C