Discovery of a supercluster in the ZOA in Vela

Abstract

We report the discovery of a potentially major supercluster that extends across the Galactic Plane in the constellation of Vela, at a mean recessional velocity of ~18,000 km/s. Recent multi-object spectroscopic observations of this Vela Supercluster (VSCL), using AAOmega+2dF and the Southern African Large Telescope, confirm an extended galaxy overdensity in the Zone of Avoidance (ZOA) located where residual bulk flows predict a considerable mass excess. We present a preliminary analysis of ~4,500 new spectroscopic galaxy redshifts obtained in the ZOA centred on the Vela region (l=272.5+-20 deg, b=0+-10 deg). The presently sparsely-sampled dataset traces an overdensity that covers 25 deg in Galactic longitude on either side of the Plane, suggesting an extent of 25 deg x 20 deg, corresponding to ~115 x 90 $h_{70}$ Mpc at the supercluster redshift. In redshift space, the overdensity appears to consist of two merging wall-like structures, interspersed with clusters and groups. Both the velocity histogram and the morphology of the multi-branching wall structure are consistent with a supercluster classification. $K_s^o$ galaxy counts show an enhancement of ~1.2 over the survey area for galaxies brighter than $M_K^*$ at the VSCL distance, and a galaxy overdensity of $\delta=0.50\rm{-}0.77$ within a photometric redshift shell around the VSCL, when compared to various Two-Micron All-Sky Survey samples. Taking account of selection effects, the VSCL is estimated to contribute $v_\rm{LG} \gtrsim 50$ km/s to the motion of the Local Group.