Parkes H I observations of galaxies behind the southern Milky Way

Abstract

As part of our programme to map the large-scale distribution of galaxies behind the southern Milky Way, we observed 314 opticallyselected, partially-obscured galaxies in the Zone of Avoidance (ZOA) in the Crux and Great Attractor (GA) regions. An additional 29 galaxies were observed in the Vela ZOA survey region (because of the small numbers they are not discussed any further). The observations were conducted with the Parkes 64 m (210 ft) radio telescope, in a single-pixel pointed mode, reaching an rms noise level of typically 2−6 mJy over the velocity search range of 400 < v < 10 500 km s −1 . A total of 162 galaxies were detected (plus 14 galaxies in the Vela region). The detection rate is slightly higher than for the Hydra/Antlia region (52% versus 45%) observed in the same way. This can be explained by the prominence of the GA overdensity in the survey regions, which leads to a relatively higher fraction of nearby galaxies. It is also evident from the quite narrow velocity distribution (largely confined to 3000−6000 km s −1 ) and deviates significantly from the expectation of a uniform galaxy distribution for the given sensitivity and velocity range. No systematic differences were found between detections and non-detections, in terms of latitude, foreground extinction, or environment, except for the very central part of the rich Norma cluster, where hardly any galaxies were detected. A detailed investigation of the H i content of the galaxies reveals strong H i deficiency at the core of the Norma cluster (within about a 0.4 Abell radius), similar to what has been found in the Coma cluster. The redshifts obtained by this observing technique result in a substantial reduction of the so-called redshift ZOA. This is obvious when analysing the large-scale structure of the new H i data in combination with data from other (optical) ZOA redshift surveys. The lower latitude detections provide further evidence of the extension of the Norma Wall, across the ZOA, in particular its bending towards the Cen-Crux clusters above the Galactic plane at slightly higher redshift, rather than a straight continuation towards the Centaurus clusters.

The Parkes H I Zone of Avoidance Survey

“…J1222-57. -Schröder et al (2009) have detected this galaxy in the OFF beam and associated it with WKK 0969 (which lies d 10.2 sep = ¢ to the southeast of our detection) as the only known galaxy in the area, while at d 6.2 sep = ¢ we find a galaxy that is not listed in the literature as the more likely counterpart.…”

“…ii. We have cross-checked our detections with the nondetections in the deeper (but pointed) Parkes survey of optically selected galaxies in the southern ZOA by Kraan-Korteweg et al (2002) and Schröder et al (2009) and found no false positives (with the caveat that a pointed survey has a different selection function than a blind survey).…”

“…We have also compared our detections with the deeper, pointed Parkes survey of southern ZOA galaxies by Kraan-Korteweg et al (2002) and Schröder et al (2009), which has a typical rms of 2-6 mJy (hereafter referred to as the PKS sample). We have 39 detections in common.…”

“…An early fully sampled, blind, but shallow H I survey (Koribalski et al 2004) has delineated local large-scale structures at low Galactic latitudes. Further, pointed 21cm observations of partially obscured galaxies have revealed largescale structures in selected regions (Kraan-Korteweg et al 2002;Schröder et al 2009), but the current work covers the entire southern ZOA, over 1800 deg 2 , in a blind, deep, and complete fashion. The remainder of the great circle of the ZOA in the northern hemisphere is now being surveyed by the Arecibo (Henning et al 2010;McIntyre et al 2015) and Effelsberg (Kerp et al 2011) radio telescopes.…”

The 2MASS Redshift Survey in the Zone of Avoidance

“…One of us (R.K.K.) contributed previously-unpublished measurements for 39 objects and we found redshifts in NED (NASA/IPAC Extragalactic Database) for five galaxies (Crook et al 2007;Guzzo et al 2009;Schröder et al 2009), leaving 997 objects to be observed.…”