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

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To investigate the fundamental principles of H 2 formation in a giant molecular cloud, we derive the Hi and H 2 surface density (Σ Hi and Σ 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 Σ H2 distribution across Perseus. We find a relatively uniform Σ Hi ∼ 6-8 M pc −2 for both dark and star-forming regions, suggesting a minimum Hi surface density required to shield H 2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Σ H2 /Σ Hi and Σ Hi + Σ H2 . The transition between the Hi-and H 2 -dominated regions occurs at N (Hi) + 2N (H 2 ) ∼ (8-14) × 10 20 cm −2 . Our findings are consistent with predictions for H 2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H 2 formation. However, the importance of warm neutral medium for H 2 shielding, internal radiation field, and the timescale of H 2 formation still remain as open questions. We also compare H 2 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.

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 400 km s −1 , line widths of 2.5-35 km s −1 , and column densities ranging from 1 to 35 × 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 disk, to greater than 10 4 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.

M33 contains a large number of emission nebulae identified as supernova remnants (SNRs) based on the high [S ii]:Hα ratios characteristic of shocked gas. Using Chandra data from the ChASeM33 survey with a 0.35-2 keV sensitivity of ∼2 × 10 34 erg s −1 , we have detected 82 of 137 SNR candidates, yielding confirmation of (or at least strongly support for) their SNR identifications. This provides the largest sample of remnants detected at optical and X-ray wavelengths in any galaxy, including the Milky Way. A spectral analysis of the seven X-ray brightest SNRs reveals that two, G98-31 and G98-35, have spectra that appear to indicate enrichment by ejecta from core-collapse supernova explosions. In general, the X-ray-detected SNRs have soft X-ray spectra compared to the vast majority of sources detected along the line of sight to M33. It is unlikely that there are any other undiscovered thermally dominated X-ray SNRs with luminosities in excess of ∼4 × 10 35 erg s −1 in the portions of M33 covered by the ChASeM33 survey. We have used a combination of new and archival optical and radio observations to attempt to better understand why some objects are detected as X-ray sources and others are not. We have also developed a morphological classification scheme for the optically identified SNRs and discussed the efficacy of this scheme as a predictor of X-ray detectability. Finally, we have compared the SNRs found in M33 to those that have been observed in the Galaxy and the Magellanic Clouds. There are no close analogs of Cas A, Kepler's SNR, Tycho's SNR, or the Crab Nebula in the regions of M33 surveyed, but we have found an X-ray source with a power-law spectrum coincident with a small-diameter radio source that may be the first pulsar-wind nebula recognized in M33.