Local Structures as Revealed by HIPASS

The HI Parkes All-Sky Survey (HIPASS) gives an unprecedented view of the local large-scale structures in the southern sky. I will review the results from the HIPASS Bright Galaxy Catalog (BGC, Koribalski et al. 2004) and the first version of the deep catalog (HICAT, Meyer et al. 2004) with particular emphasis on galaxy structures across the Zone of Avoidance. Some previously hardly noticed galaxy groups stand out quite distinctively in the HI sky distribution and several large-scale structures are seen for the first time. Because HI surveys preferentially detect late-type spiral and dwarf galaxies, their distribution is less clustered than that of optically selected galaxies. HI surveys also significantly enhance the connectivity of large-scale filaments by filling in gaps populated by gas-rich galaxies. 1. The HIPASS Galaxy Catalogs The HIPASS Bright Galaxy Catalog (BGC, Koribalski et al. 2004) contains the 1000 HI-brightest galaxies in the southern sky as obtained from the HI Parkes All-Sky Survey. These were selected by their HI peak flux density (Speak ∼> 116 mJy) measured in the spatially integrated HIPASS spectrum. We note that a subset of ∼500 galaxies is complete in integrated HI flux density (FHI ∼> 25 Jy km s). The HIPASS BGC is highly reliable (S/N ∼> 9) and contains accurate HI properties of all sources as well as optical identifications were available. Sources without cataloged optical (or infrared) counterparts, i.e. ∼10% of the BGC, fall into two categories: (1) gas-rich galaxies in regions of high Galactic extinction or high stellar density, and (2) late-type dwarf and low surface brightness (LSB) galaxies. The latter are generally missed in optical catalogs because they are small in size or rather faint, but they are easily identified given the HI position. For details of the newly cataloged galaxies in the HIPASS BGC, 70% of which lie in the Zone of Avoidance (|b| < 10), see Ryan-Weber et al. (2001). We found only one extragalactic HI cloud, HIPASS J0731–69 (MHI ∼ 10 9 M⊙), located at a projected distance of 180 kpc from the asymmetric spiral galaxy NGC 2442 (Ryder et al. 2001). Fig. 1 shows the distribution of all sources in the HIPASS BGC and HICAT, divided into velocity bins of ∆v = 1000 km s. Fig. 2 shows the histogram of local group velocities in both catalogs. The deep HIPASS catalog (HICAT) contains 4315 HI sources, including a large fraction of the BGC. For the first time, we can trace local large-scale structures over the whole southern sky, unobscured by dust or foreground stars. A beautiful network of filaments, loops, groups and voids is revealed, creating a cellular web that gradually changes with redshift.