6 mins read 04 Jun 2021

WALLABY Survey Spies Interactions in the Hydra Cluster

Two new papers have been released using ASKAP data to observe the galaxy cluster Hydra as part of the WALLABY survey. This research has found that some of the galaxies in this cluster have been affected by ram pressure stripping.

A galaxy affected by the effects of ram pressure stripping, seen in wisps of gas trailing out behind it. Credit: ESA/Hubble, CC BY 4.0.

To better understand how galaxies evolve, the environments in which they reside must be studied in detail, as these influence how the galaxy transforms over time. Some galaxies live in densely populated regions and interact with other galaxies, accreting gas, colliding and eventually merging, or if they are more isolated, evolving undisturbed. . 

Generally speaking, there are a number of factors that influence this evolution (regardless of it being an isolated or non-isolated system) which include mergers of low or high velocity, the amount of neutral hydrogen in the extended galaxy disk (this is the star formation material), and the tidal interactions that occur between them. 

One of the key parameters of galactic evolution is its star formation rate (SFR) and for this, there needs to be lots of neutral hydrogen gas available as part of the galaxy's structure. Once this neutral hydrogen gas is removed, quenched or ripped away, the SFR falls rapidly, causing galaxies to age. This is presented as reddening in colour as the majority of stars which survive are the cooler red stars, as young, hot blue stars live fast and die young. 

Now, two new papers have been released that have used the ASKAP radio telescope to observe the Hydra Cluster of galaxies, located approximately 190 million light-years away. ASKAP is owned and operated by CSIRO, Australia’s national science agency, as a national research facility located at the Murchison Radio-astronomy Observatory in Western Australia.

The papers form part of WALLABY’s (Widefield ASKAP L-band Legacy All-sky Blind Survey) pilot survey commissioning runs, utilising the ASKAP instrument’s powerful widefield capability, achieved by CSIRO’s phased array feed technology, which allows surveying a much larger portion of the sky faster in radio wavelengths, and to greater resolution. The pilot and pre-surveys conducted so far by the WALLABY project (as well as other surveys) are already producing outstanding results and helping to ensure that the telescope operation and processing pipelines are running at optimal efficiency.

The WALLABY Survey

CSIRO’s ASKAP radio telescope in Western Australia. Credit: CSIRO.

The WALLABY pilot survey is observing the skies to detect the distribution of cold hydrogen gas in galaxy clusters, like that located in Hydra. Whilst the WALLABY pilot surveys only began recently, the project has for a number of years been measuring hydrogen gas distribution around other galaxy clusters, including the Dorado Group of galaxies, the Fornax Galaxy Cluster and the M83 field when ASKAP was commissioning its first 12 dishes. 

WALLABY aims to survey three-quarters of the sky (up to a declination of 30-degrees north) at the 21-centimetre line of neutral hydrogen, detecting ~500,000 galaxies through the project. Neutral hydrogen is abundant throughout the Universe because it was one of the first elements created during Big Bang nucleosynthesis. Because cold hydrogen gas is so distinctive on the radio spectrum (with a frequency of 1.420 GHz), it can be used in astronomy research to look at galaxies and how they evolve and interact with each other. 

Whilst earlier WALLABY surveys only utilised 12 - 16 ASKAP dish antennas over a limited bandwidth, current projects are taking advantage of the full array of 36 dishes at half the bandwidth. This is being targeted at three major galaxy clusters: Hydra Cluster (as reported in these papers), the Norma Cluster and the NGC 4636 group.  In particular, the data from these pilot surveys of the Hydra cluster were obtained over four nights in October and November 2019.

The WALLABY survey is looking to increase knowledge about galaxy formation and evolution, interactions between galaxies, and other processes. This is being done by measuring the properties of cold hydrogen gas of each galaxy by determining its distance, neutral hydrogen mass, total mass, and dark matter content.

The Ram Pressure Stripping of Galaxy ESO 501-G075

A Digitised Sky Survey optical image overlaid with ASKAP HI contours of Hydra showing densities of cold hydrogen gas. ESO 501-G075 is highlighted with an orange dot-dashed circle around it. The red and blue contours indicate the neutral hydrogen gas density. Credit: Reynolds et al. 2021.

The first paper, which was also published in the Monthly Notices of the Royal Astronomical Society, looked specifically at the galaxy ESO 501-G075 in the Hydra cluster. According to this paper, this galaxy appears to have a disturbed hydrogen gas morphology. 

This means that the cold hydrogen gas present in the galaxy is not distributed throughout the galaxy in a way that is expected. The aim of the paper was to investigate the possible environmental causes of this unusual distribution of hydrogen in this galaxy. They conclude that this galaxy has likely been subjected to ram stripping pressure. 

Ram stripping is caused by the movement of galaxies. Within galaxy clusters, there is gas known as the intra-cluster medium. As galaxies move, they push against this medium which causes pressure - similar to the wind a cyclist feels when riding their bike on a calm day. 

If there is enough ram pressure pushing against the galaxy, it can start to force gas, including neutral hydrogen, to be pushed out of the galaxy. This causes the distribution of the hydrogen gas in a galaxy to become unusual as it trails out behind the moving galaxy, appearing as a tail-like structure on the north-east side of the galaxy, with a measured mass of roughly 4.5 x 108 solar masses in the tail alone, accounting for 6% of the total neutral hydrogen mass of the galaxy. The paper from WALLABY it is likely that the galaxy ESO 501-G075 is in the early stages of this ram stripping. 

ESO 501-G075 also resides close to the virial edge of the cluster, exhibiting a line of sight velocity that is close to that of the escape velocity of the cluster, and appears to be dark matter dominated - an exhibiting feature of the entire Hydra Cluster, which has a measured, yet unusual, high dark matter density.

The Hydra Cluster

The Hydra cluster observed by WALLABY showing different slices of velocity offset from the cluster centre. Credit: Wang et al. 2021.

The second paper (preprint) that analyses the WALLABY data looked at the Hydra I cluster of galaxies (which also includes ESO 501-G075) as a whole. This included mapping out the neutral hydrogen in the Hydra cluster over an area of 60-square degrees, and finding 105 galaxy members, whilst resolving 27 of them. 

Contained within this radius is approximately 3.02 x 1014 solar masses, with a velocity dispersion of 602 km./s-1. Within 0.4 radii of the centre of the cluster, there also appears to be 2-3 substructures. 

The researchers found that almost two-thirds of the galaxies within the Hydra cluster showed evidence of early-stage ram stripping - especially those near the cluster centre, and explored the rates at which this process could occur in this cluster. 

They estimated that based on how much neutral hydrogen could still be stripped from these galaxies, that it would take hundreds of millions of years for the mass of hydrogen in these galaxies to significantly change. This information, supported by the new findings from these WALLABY pilot surveys, are important as it helps build up an understanding of the evolution of galaxies. 


We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site where ASKAP is located.

Read an overview of WALLABY