HI Galaxy Searches in the ZOA

In March 1997, a systematic blind HI survey began in the most opaque part of the southern Milky Way (

;

) with the Multibeam (MB) receiver (13 beams in the focal plane array) at the 64m Parkes telescope. The ZOA is being surveyed along constant Galactic latitudes in 23 contiguous fields of length

. The ultimate goal is 25 scans per field where adjacent strips will be offset in latitude by

for homogeneous sampling. With an effective integration time of 25 min/beam we obtain a 3 detection limit of 25mJy. The survey covers a velocity range of

km with a channel spacing of 13.2 km per channel, and will be sensitive to normal spiral galaxies well beyond the Great Attractor region.

So far, a shallow survey based on 2 out of the foreseen 25 driftscan passages has been analyzed (cf. Kraan-Korteweg, Koribalski & Juraszek 1998, Henning etal. 1999). 107 galaxies were catalogued with peak HI-flux densities of

80 mJy (

 mJy after Hanning smoothing) and their detection show no dependence on Galactic latitude, nor the amount of foreground obscuration through which they have been detected.

Four cubes centered on the Great Attractor region (

,

) of the full-sensitivity survey have been analyzed (Juraszek etal. 1999) and uncovered 236 galaxies above the detection level of 25 mJy. 70% of the detections had no previous identification.

In the left panel of Fig. 4, a sky distribution centered on the GA region displays all galaxies with redshifts

 km . Next to redshifts from the literature (circles; LEDA), redshifts from the follow-up observations of Kraan-Korteweg and collaborators in the Hy/Ant-Crux-GA ZOA surveys (dashed area) are plotted. They clearly reveal the prominence of the cluster A3627 at

 km , Kraan-Korteweg etal. 1996) close to the core of the GA region at

 km ) as predicted by Kolatt etal. (1995). Adding now the new detections from the systematic blind HI MB-ZOA survey (box), we for the first time can trace structures all the way across the Milky Way. The new picture seems to suggest that the GA overdensity is a ``great-wall'' like structure starting at the Pavo cluster, having its core at the A3627 cluster and then bending over towards shorter longitudes across the ZOA.

Figure: A sky distribution (left) and redshift cone (right) for galaxies with v<10000 km in the GA region. Circles mark redshifts from the literature (LEDA), squares redshifts from the optical galaxy search in the Hy/Ant-Crux-GA regions (outlined on left panel) and crosses detections in the full-sensitivity HI MB-ZOA survey (box).

This becomes even clearer in the right panel of Fig. 4 where the galaxies are displayed in a redshift cone out to

 km for the longitude range

analyzed so far of the MB full-sensitivity data. The A3627 cluster is clearly the most massive galaxy cluster uncovered by the various surveys in the GA region and therefore the most likely candidate for the previously unidentified but predicted density-peak at the bottom of the potential well of the GA overdensity.

Finding a hitherto uncharted further cluster of galaxies at the heart of the GA would have serious implications for our current understanding of this massive overdensity in the local Universe. Various indications suggest that PKS1343-601, the second brightest extragalactic radio source in the southern sky (f_20cm = 79 Jy, McAdam 1991), might form the center of yet another highly obscured rich cluster, particularly as it also shows significant X-ray emission (cf. Kraan-Korteweg & Woudt 1999 for further details). At (

, this radio galaxy lies behind an obscuration layer of about 12 magnitudes of extinction in the B-band, hence optical surveys are ineffective. Still, West & Tarenghi (1989) observed this source and identified it - with an extinction-corrected diameter of

and a recession velocity of v = 3872 km - as a giant elliptical galaxy. Giant ellipticals generally reside at the cores of clusters.

Interestingly enough, the HI MB survey does uncover a significant excess of galaxies at this position in velocity space (cf. Fig. 4). However, we do not see a ``finger of God'', the characteristic signature of a cluster in redshift space. Could it be that too many central cluster galaxies are missed by the HI observations because spiral galaxies generally avoid the cores of clusters? The existence of this possible cluster still remains a mystery. Meanwhile, this prospective cluster has been imaged in the near infrared (Woudt etal. in progress), where extinction effects are less severe compared to the optical, and which should uncover early-type cluster members if they are there. The forthcoming results should then unambiguously settle the question whether another cluster forms part of the GA supercluster.

© Copyright Astronomical Society of Australia 1997