An obscured galaxy redshift survey with FLAIR

Abstract

A problem for studies of large scale structures in nearby space (cz < 10,000 km/s) is the presence of the Zone of Avoidance which is so large and wide on the sky that potentially important clusters and voids remain undetected. A prime example was the Ophiuchus cluster discovered by Wakamatsu and Malkan (1981) as a heavily obscured cD cluster close to the Galactic centre region (l=0.5deg, b=+9.5deg). It is the 2nd brightest X-ray cluster after Perseus. A hidden galaxy survey was performed by visually searching ESO/SERC Sky Survey (R and J) copy films of the region centred at l=355deg, b=+10deg, finding more than 4000 galaxies in 6 fields. Several irregular clusters adjacent to Ophiuchus were found forming a supercluster which may be connected to the Hercules supercluster by a wall structure parallel to the local supergalactic plane (Wakamatsu et al. 1994). In front of this supercluster, an `Ophiuchus Void' is suggested (cz = 4,500 km/s). The Ophiuchus supercluster at cz=8,500 km/s is similar to the Hercules supercluster (cz=11,000 km/s), and extends north toward the latter supercluster. We have used FLAIR, the fibre-spectroscopy system on the UKST (Parker,1996) to study the bridge region between the two superclusters which covers (16:00 <= alpha <= 17:20, -25deg <= delta <= +2.5deg) and a void region, 1.5 hour west of a declination zone (-30deg <= delta <= -15deg. FLAIR is well matched to the number-density (~3/sq.deg) and magnitude limit (B<=17.0) of the survey galaxies. The region, mostly above b ~ 15deg, has star densities low enough for FLAIR use without severe crowding or contamination problems. So far 1500 redshifts for obscured galaxies have been obtained. The Ophiuchus supercluster extends at least one field north towards the Hercules supercluster and is surrounded by a diffuse, extended halo ~20deg = 30h^-1 Mpc across. A new sparse `Libra' supercluster candidate is also detected at cz=9,000km/s, one field south of the southern edge of the Hercules supercluster. A wall structure is clearly suggested between this and the Ophiuchus supercluster. The proposed `Ophiuchus - Hercules Wall' formed by a local void in front of, and another behind the Ophiuchus and Libra superclusters, may form a structure as large as the Great Wall both in apparent size (>70deg) and physically (100h^-1 Mpc). These two walls cross perpendicularly near Abell 2199-the northern edge of Hercules supercluster. Any `true' 3-D orthogonality between the Ophiuchus - Hercules Wall and the Great Wall may be crucial for understanding 0.1c scale structure whilst this local contrast of galaxy distributions may strongly affect our estimation of the Local Group motion relative to the Microwave background.