HIPASS High-Velocity Clouds: Properties of the Compact and Extended Populations

Abstract

A catalog of Southern anomalous-velocity HI clouds at Decl. < +2 • is presented. This catalog is based on data from the HI Parkes All-Sky Survey (HIPASS) reprocessed with the minmed5 procedure (Putman et al. 2002;Putman 2000), and searched with the -2high-velocity cloud finding algorithm described by de Heij et al. (2001). The improved sensitivity (5σ: ∆T B = 0.04 K), resolution (15. ′ 5), and velocity range (−500 < V LSR < +500 km s −1 ) of the HIPASS data, results in a substantial increase in the number of individual clouds (1956, as well as 41 galaxies) compared to what was known from earlier Southern data. The method of cataloging the anomalous-velocity objects is described, and a catalog of key cloud parameters, including velocity, angular size, peak column density, total flux, position angle, and degree of isolation, is presented. The data are characterized into several classes of anomalous-velocity HI emission. Most high-velocity emission features, HVCs, have a filamentary morphology and are loosely organized into large complexes extending over tens of degrees. In addition, 179 compact and isolated anomalous-velocity objects, CHVCs, are identified based on their size and degree of isolation. 25% of the CHVCs originally classified by Braun & Burton (1999) are reclassified based on the HIPASS data. The properties of all the highvelocity emission features and only the CHVCs are investigated, and distinct similarities and differences are found. Both populations have typical HI masses of ∼ 4.5 D 2 kpc M ⊙ and have similar slopes for their column density and flux distributions. On the other hand, the CHVCs appear to be clustered and the population can be broken up into three spatially distinct groups, while the entire population of clouds is more uniformly distributed with a significant percentage aligned with the the Magellanic Stream. The median velocities are V GSR = −38 km s −1 for the CHVCs and −30 km s −1 for all of the anomalous-velocity clouds. Based on the catalog sizes, high-velocity features cover 19% of the southern sky, and CHVCs cover 1%.

GASS: The Parkes Galactic All-Sky Survey

“…2. Broad high-velocity profiles are not likely to be genuine since we know from HIPASS observations that this population of high-velocity clouds does not exist (Putman et al 2002). Inspection of the profiles with such components has demonstrated that these Gaussians are frequently caused by baseline uncertainties near the profile edges and therefore this criterion somewhat extends, but partly duplicates our second criterion.…”

Disk evaporation in a planetary nebula

“…It may be related to a nearby catalogued high velocity cloud, HVC 002.9-06.2-088 (Putman et al 2002), of the same velocity, because the emission at this velocity seems to extend over quite some distance. The distance to this cloud or complex is not known, but as M 2-29 is clearly located behind it, we suggest that it is situated relatively close to the Galactic centre.…”

An absorption-selected survey of neutral gas in the Milky Way halo

“…New H i data obtained with EBHIS and GASS revealed that complexes like the Galactic center negative (GCN) are strongly fragmented into several warm and compact clumps if observed with higher resolution. Furthermore, numerous isolated and compact HVCs can be seen all over the sky (Braun & Burton 1999;Putman et al 2002;de Heij et al 2002). The symbols in Fig.…”