Exploring the Zone of Avoidance: hidden structures

Title: Revealing hidden structures in the Zone of Avoidance — a blind MeerKAT HI Survey of the Vela Supercluster

Authors: Sambatriniaina H. A. Rajohnson et al 

First author institution: Department of Astronomy, University of Cape Town, South Africa
Status: Accepted for publication in MNRAS [open access]

Cosmologists use the positions of galaxies to map out Large Scale Structure in the Universe – the largest structures in the Universe, formed by filaments of clustering galaxies or empty voids. However, our view of the Universe is partially obstructed by our own galaxy, the Milky Way. The Zone of Avoidance (ZOA, which lies in the galactic plane) refers to the region of space in the sky that is obscured by the Milky Way’s dust, and as such our understanding of structures in this part of the sky is limited. However astronomers have noticed, for quite a time, there may be a strong gravitational tug on nearby galaxies from a source that appears to lie in this zone; historically it has even been referred to as the Great Attractor in the literature.

In a study conducted in 2017, astronomers found the first evidence that there may be a large cluster (or supercluster) of galaxies behind the ZOA using spectroscopic measurements of the light in this region, in the constellation of Vela. The presence of a supercluster was also supported by other observations, and it has been named the Vela Supercluster. It is hypothesised that Vela could be responsible for the strong tugging of galaxies in this direction. Its presence is expected to contribute at least 50 km/s to the motion of our Local Group (which contains the Milky Way, Andromeda, Triangulum and various other small galaxies which orbit these larger ones).

The authors of today’s paper have used the MeerKAT radio telescope, in South Africa, to follow up observations in the ZOA. They refer to the observations they detect as Vela-HI, which follows up from previously collected data in Vela-SMGPS – the Vela-SARAO MeerKAT Galactic Plane Survey (SARAO is the South African Radio Astronomy Observatory). This telescope maps the intensity of matter in the Universe by tracing faint radio emission from neutral hydrogen (HI). A higher intensity of this radiation indicates a higher density of mass or galaxies. In total, the authors detected 719 sources (galaxies) in the survey. A total of 432 of these were below the galactic plane, and 287 were above it. Due to heavy dust obscuration it is harder to detect anything right in the centre of the plane. Figure 1 shows the data that was collected by Vela-HI in a slice of varying galactic latitude, and also in a slice of varying galactic longitude (these are really quite analogous to latitude and longitude on the Earth). 

Comparing the data with previous surveys 

The authors checked the self-consistency of their collected data prior to searching for the presence or Large Scale Structures. The collected data comes from many ‘pointings’ of the telescope in the sky and was grouped together in `mosaics’ on a 2D map. Many of these mosaics have overlapping portions of sky, and in fact 163 of the detected sources were found to be in adjacent overlapping mosaics. This allowed the authors to compare the detected information from each source in separate mosaics and use this to check the information was consistent between the mosaics. For example, they compared the self-consistency and accuracy of the positional information of each source, and used any small differences in this to determine the positional uncertainty in their measurements. 

Furthermore, they compared the measured data with the previously mentioned Vela-SMGPS (for 9 sources with overlapping position in the sky) and also a survey called HIZOA for 87 sources (the Parkes HI ZOA survey – Parkes is a radio telescope in Australia) which also made measurements in the same region of the sky using HI. Comparing various variables with these surveys (the redshift, the width – or range of frequencies – of the measured HI spectral features and the flux – light detected – from each source), they found good agreement between them. Using databases such as NED, they were also able to search for information that may exist from other surveys; they found measurements in the near-infrared (NIR) and optical (visible wavelengths) for 211 of the galaxies. Comparing these measurements, they found good agreement in the redshift measurements from these and their own HI measurements. Figure 2 shows a 2D map of the detections from Vela-HI in the sky, and information from surveys that were found to have optical or NIR information for the sources.

Uncovering Hidden Structures

The authors found their data from Vela-HI and Vela-SMGPS very clearly traces out a continuous structure known as the Hydra Wall (which is much closer to us than the Vela supercluster but is in close enough proximity in the sky to be seen in their data). While this structure is not a new discovery, their data traces out the continuous Hydra Wall for the first time.  Deeper into the ZOA but still inbetween ourselves and Vela, branching structures and a void with a size of approximately 10 Mpc can be seen. In the core of Vela, there are various dense clusters of galaxies that appear to form a wall crossing the ZOA. At deeper redshifts a broad overdensity in the ZOA is hinted at by the data. These structures can be viewed in Figure 3 which contains annotations to illustrate them.

Having done this the authors analyzed the measurements to infer the presence of Large Scale Structures in the core of Vela. In order to ensure the structures that seem to appear (clustering or absence of clustering) are really significant detections of structure, they compared the observations with results from simulations that did not contain any structure. By applying their analysis procedure to a simulation of the survey (with realistic mimicking of the sensitivity parameters of the real survey) but which actually contained no structure (by having a somewhat random distribution of galaxies) they could check their detections were actually significant. The results showed this to be the case, although the authors caution any lack of structure (indicating for example a void) needs to be interpreted carefully as it could still be due to heavy dust obscuration.

Overall, the observations have allowed for a deeper view into the structures that lie in the ZOA and complement existing data. This information allows astronomers to continue the aim of mapping structure in the Universe in greater and greater detail, and perhaps eventually better quantify the impact of a hidden ‘Great Attractor’ on the motions of nearby galaxies. 

Edited by Skylar Grayson

Featured image credit: The Milky Way core rises above the Natural Monument of “Los Barruecos”. By Astrocld, CCA by 4, via Wikimedia Commons.