Hidden Vela Supercluster Revealed by First Hybrid Redshift & Peculiar Velocity Reconstruction

Abstract

A large fraction of the extragalactic sky is obscured by foreground dust and stars along the plane of the Milky Way, leaving a major gap (~ 20%) in whole-sky maps of large-scale structures -- an incompleteness that is even more severe for peculiar velocity samples. This has long limited an unambiguous interpretation of observed cosmic flows and their connection to the underlying mass-density field. We present a new hybrid reconstruction methodology which combines 65,518 galaxy peculiar velocity distances from the CF4++ catalogue (Courtois2025) with 8283 new galaxy redshifts observed near the southern Galactic plane (|b| <= 10 degrees) Zone of Avoidance. A major advance is the inclusion of 2176 high-sensitivity, interferometric HI redshifts obtained with the SARAO MeerKAT telescope which for the first time provide coverage of the innermost 3degrees-wide strip of the southern ZOA and to unprecedented depth. This hybrid redshifts & peculiar velocities approach yields a substantially revised view of the inferred overdensities in and around the ZOA. In particular, the Vela supercluster emerges as a dominant mass concentration, rivaling the Shapley concentration and exceeding the mass associated with Laniakea and the Great Attractor region. With a total mass of 33.8 10^16 Msol, a characteristic radius of 70 hmpc, and a double core morphology at a distance of 189 hmpc, Vela dominates the mass budget and gravitational influence of the southern Zone of Avoidance. These results provide the most complete and dynamically consistent picture to date of the southern Zone of Avoidance and demonstrate the transformative potential of hybrid reconstruction techniques tailored for the next generation of large-scale surveys.

Figures (12)

• Figure 1: The Galactic $\ell,b$ distribution of: (top) the CF4++ZOA samples along with their measured redshifts; and (bottom) the 8283 ZOA C1-C4 samples introduced in this work (red for H i and blue for optical redshifts). Note the unprecedented dense coverage provided by the seven datasets obtained with SARAO MeerKAT telescope (red circles) along the innermost, nearly fully opaque, ZOA region.
• Figure 2: Comparison of the CF4++ (black) distance moduli as a function of $V_{\rm CMB}$/100 compared to the pseudo distance moduli adopted for the 8283 galaxies in the ZOA datasets (red for H i and blue for optical redshifts).
• Figure 3: Showing the differences in the Vela cosmography for the (left) CF4++, (middle) CF4++ZOA, (right) CF4++ & random-matching density reconstructions. The green points shown in the middle and right panels demonstrate the position of the artificial galaxies added, of the respective case.
• Figure 4: The reconstructed matter density field, averaged over a total width of 23.4 $h^{-1}$ Mpc in the SGZ plane of the superclusters regions that demonstrate the biggest changes with the addition of the ZOA datasets: Columba-Lepus and Vela. The left panel displays results derived from the CF4++ dataset, while the middle panel uses the CF4++ZOA dataset, and the right panel shows the difference between the two reconstructions. Overlaid is the mean velocity field. Black dots represent CF4++ galaxies, and the green dots correspond to newly added ZOA data points located within the averaged region.
• Figure 5: Local superclusters reconstructed using the CF4++ dataset (red dots) expanded by the ZOA dataset (green dots). The green cube provides the 300 $h^{-1}$Mpc scale length and delineates the orientation primarily showing the SGX-SGY plane with +SGZ pointing forwards. Streamlines of the velocity field are coloured according to their gravitational watershed membership. The Vela supercluster emerges as a large single entity gravitational watershed on the left of the image, crossing the ZOA.