Galaxies Found by the Shallow Survey

The shallow 21-cm survey of the southern ZOA has been completed, and 107 galaxies with peak HI flux densities about 80 mJy have been cataloged. Refinement of the measurement of their HI characteristics is ongoing, but the objects seem to be normal galaxies. However, of three large multibeam ZOA galaxies imaged in HI with the ATCA, two were seen to break up into complexes of HI suggestive of tidally-interacting systems (Staveley-Smith et al. 1998) Continued follow-up synthesis observations are planned to investigate the frequency of these interacting systems in this purely HI-selected sample.

Most of the galaxies are within

, which is about the redshift limit for detection of normal spirals of this shallow phase of the survey. As the deep survey continues, spirals at higher velocities will be recovered. The effective depth of the shallow survey is not quite sufficient to recover large numbers of galaxies which might be associated with the Great Attractor (but see Juraszek et al. this volume.) However, a striking feature becomes apparent with the addition of the ZOA galaxies. An enormous filament, which crosses the ZOA twice, is clearly evident when these ZOA data are displayed along with optically-known galaxies above and below the plane within

(Fig. 2.) This structure snakes over through the southern sky. Taking a mean distance of 30h^-1 Mpc, this implies a linear size of

Mpc, with thickness of Mpc or less.

Also, note the relative emptiness of the Local Void. Three hidden galaxies found on a boundary of the Void (l ) lie at

. Two of these objects were also recovered by the Dwingeloo Obscured Galaxies Survey (Rivers et al. this volume.) The positions and redshifts of these objects are consistent with their being members of the cluster at this location proposed by Roman et al. (1998).

Figure 2: Galaxies within v

. The open circles mark galaxies between

, triangles for

, and dots for those with

. High latitude data are taken from the literature (LEDA). Also plotted are galaxies from deep optical ZOA surveys (Kraan-Korteweg et al. this volume.) The galaxies discovered by the multibeam shallow ZOA survey fill in the lowest galactic latitudes, where optical surveys fail.

Of the 107 objects found, 28 have counterparts in the NASA/IPAC Extragalactic Database (NED) with matching positions and redshifts. Optical absorption, estimated from the Galactic dust data of Schlegel et al. (1998), ranges from A = 1 to more than 60 mag at the positions of the 107 galaxies, and is patchy over the survey area. No objects lying behind more than about 6 mag of obscuration have confirmed counterparts in NED, as expected.

The shallow multibeam HI survey connects structures all the way across the ZOA within

for the first time. The ongoing, deep ZOA survey will have sufficient sensitivity to connect structures at higher redshifts.

While 14 of the 107 galaxies lie within

and are therefore fairly nearby, all of the newly-discovered objects have peak HI flux densities an order of magnitude or more lower than the Circinus galaxy. Thus, it seems our census of the most dynamically important, HI-rich nearby galaxies is now complete, at least for those objects with velocities offset from Galactic HI. Simulations are currently being devised to investigate our sensitivity to HI galaxies whose signals lie within the frequency range of the Milky Way's HI. This will be done by embedding artificial HI signals of varying strength, width, position, and frequency, into real data cubes. Then, an experienced HI galaxy finder (PH) will examine the cubes without previous knowledge of the locations of the fake galaxies. In this way, we hope to quantify better this remaining blind spot of the HI search method.

© Copyright Astronomical Society of Australia 1997