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. $K_s<sup>o$ galaxy counts show an enhancement of ~1.2 over the survey area for galaxies brighter than $M_K<sup>*$ 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.

• The paper confirms and characterizes the Vela Supercluster, a massive galaxy overdensity previously hidden in the Zone of Avoidance, using extensive spectroscopic data from AAOmega+2dF and SALT.
• Utilizing approximately 4500 new galaxy redshifts, the study locates the supercluster at a mean redshift of ~18,000, describing its elongated, wall-like morphology spanning ~115x90 h70 Mpc and quantifying its galaxy overdensity.
• The discovery highlights the Vela Supercluster's significant gravitational potential, which contributes substantially to the Local Group's motion and helps address discrepancies in local bulk flow analyses.

Discovery of the Vela Supercluster: An Examination of Galactic Overdensity in the Zone of Avoidance

The paper presents a detailed investigation into the Vela Supercluster (VSCL), a previously uncharted galaxy supercluster located in the Zone of Avoidance (ZOA). Using extensive spectroscopic data, the study confirms the presence of a massive galaxy overdensity that transcends the Galactic Plane, shedding light on cosmic structures that have significant gravitational implications for local celestial dynamics.

Spectroscopic Observations and Data Acquisition

The research utilizes data from AAOmega+2dF and the Southern African Large Telescope, focusing on a segment of the sky within Galactic coordinates $l = 272\fdg5 \pm 20\degr$ and $b = 0\degr \pm 10\degr$. Through multi-object spectroscopic observations, approximately 4500 new galaxy redshifts were obtained, enabling the identification of an extended galaxy overdensity at a mean redshift of $\sim$18,000. The VSCL is characterized by an elongated morphology, consisting of two merging wall-like structures dotted with clusters and groups.

Morphological Characteristics and Overdensity Assessment

The VSCL's structural properties align with those of known superclusters, embodying a prolate, wall-like configuration with embedded clusters characteristic of a supercluster classification. The angular extent of the VSCL is significant, covering $25\degr \times 20\degr$, which corresponds to roughly $\sim$115$\times$90~$h_{70}$~Mpc when considering its redshift distance.

Quantitative analysis reveals a galaxy overdensity in the VSCL with enhancements of $1.17$ to $1.25$ in galaxy counts compared to other survey areas, pinpointing an overdensity $\delta=0.50$--$0.77$ within a specific photometric redshift shell. Such results underscore the VSCL's potential as a gravitational influencer on local cosmological flow fields.

Implications of the Vela Supercluster

The discovery and characterization of the VSCL bear significant implications for both theoretical and observational cosmology. The VSCL's gravitational potential likely contributes to residual bulk flows and influences local group dynamics, contributing at least $v_\mathrm{LG} \ga 50$ \ to the motion of the Local Group, analogous in impact to the renowned Shapley Supercluster. This influence is paramount in addressing discrepancies in local bulk flow analyses.

Future Prospects

Further investigations into the VSCL are necessary to comprehend fully its mass distribution, extent, and dynamic influence on the nearby universe. Upcoming systematic surveys and new methods, such as those offered by forthcoming instruments like Taipan and innovative 21 cm neutral hydrogen line studies like SKA, are poised to further clarify and solidify our understanding of this significant cosmic structure.

In conclusion, the exploration of the Vela Supercluster unveils a pivotal structure within the Zone of Avoidance, unveiling yet another piece in the intricate web of the cosmic large-scale structure. As astronomical observational techniques advance, unearthing such hidden galactic entities will continue to be crucial in refining our understanding of universal dynamics and large-scale cosmic architecture.