150 galaxies in a single view! ASKAP’s WALLABY survey science team images the Hydra cluster region

The first set of science images and cubes for the WALLABY pilot survey have been released onto the CSIRO ASKAP Science Data Archive CASDA, marking significant progress towards the 6th milestone on the ASKAP countdown to full survey science

These data products focus on the Hydra cluster, which is a massive system comprising hundreds of galaxies and is well-suited for detailed studies of galaxy evolution. This is the first release of high resolution 3D spectral line data cubes from an ASKAP pilot survey, representing a significant leap in the volume of publicly accessible data available from ASKAP

Figure 1: Image of the distribution of neutral hydrogen gas in the galaxies of the Hydra cluster region. The image was produced by the WALLABY source finding pipeline and contains almost 150 individual galaxies. Image credit: T. Westmeier / WALLABY team.

Hydra cluster

Galaxy clusters are considered to be the largest gravitationally-bound objects in the Universe. These structures typically contain hundreds to thousands of galaxies being held together by mutual gravitation in an environment that contains an enormous amount of dark matter.

As galaxies move within the cluster, they undergo changes brought on by the hot gas and plasma occupying the regions between galaxies. On the edge of clusters, galaxies dance around and interact with each other before succumbing to the pull of the denser environment.

The Hydra cluster is located more than 50 Mpc (160 million light years) away. At this distance, the entire cluster can fit into a single 5.5° × 5.5° ASKAP field of view. This cluster contains a high number of spiral and irregular type galaxies, which are readily detectable by tuning ASKAP to the WALLABY frequency range (currently 1152 – 1440 MHz).

ASKAP observations & data processing

The new images were produced using 32 hours of observations spanning across two adjacent ASKAP fields (total area of ~60 square degrees) to ensure coverage of the entire cluster, as well as the outskirt areas to sample a wide variety of environmental densities and evolutionary processes.

160 TB of raw data was processed using the Pawsey Super Computing Centre and ASKAPsoft. The latter is a software package specifically developed to handle the large amounts of data streaming out of ASKAP.

The processed image cubes were mosaicked together to create a single full field cube with a sensitivity level that enables the detection of low-mass dwarf galaxies and diffuse gas-rich streams; along with sufficient angular resolution to resolve detailed features of individual galaxies.

These observations represent some of the most comprehensive high-resolution images ever taken with an interferometer of the general Hydra cluster region.

Source finding & scientific analysis

To provide the WALLABY team with an initial data release for their scientific analysis, the WALLABY source finding pipeline, SoFiA 2, was run on a subregion of the full data cube centred on the actual Hydra cluster itself.

The chosen sub-region covers an area of about 3.0° × 5.5° on the sky and extends out to a redshift of 0.05 (lookback time of roughly 700 million years). SoFiA 2 detected the neutral hydrogen emission from almost 150 galaxies (see Figure 1) and demonstrates the vast improvement of WALLABY over the previous HI Parkes All Sky Survey (HIPASS), which was completed in 2002 and detected a mere 8 galaxies across the same region of the sky.

With such large data products, one of the challenges is finding the best way for humans to select, visualise and interpret key features.

In addition to a catalogue of all detected galaxies, SoFiA 2 also provides a range of other useful data products, including a small sub-cube, an integrated spectrum and images of the flux distribution and velocity field of each galaxy. These will be used by the WALLABY team in their scientific analysis of the new data. For example, members of the kinematics working group will be using the velocity field information to extract the rotation curve of resolved galaxies to measure the amount of dark matter present. Another team will be looking at the neutral hydrogen mass distribution of the galaxies to study the influence of the cluster environment on the amount of gas these galaxies can contain.

Apart from these global studies, there are several individual objects of scientific interest in the initial Hydra data release.

One example is close galaxy pairs which are in the process of interacting or merging, resulting in part of their hydrogen reservoir being ejected into long gaseous tails due to the strong gravitational forces associated with the merger. The study of such interacting galaxies is particularly interesting as the resulting ejection of gas could have the potential to reduce or even shut off the process of star formation in the affected galaxies.

Outlook

As WALLABY team members immerse themselves into the richness of the Hydra pilot data and begin writing scientific papers describing the results of their studies; observations of two more WALLABY pilot survey fields, in the direction of the NGC 4636 galaxy group and the Norma galaxy cluster, have already been completed.

The WALLABY pilot survey marks only the beginning of a new era in the study of neutral hydrogen across the sky that will ultimately see the number of galaxies imaged at high angular resolution increase from hundreds to hundreds of thousands. The full WALLABY survey will revolutionise our understanding of galaxy evolution and the pivotal role of the gas in the life and death of galaxies.

WALLABY survey science team

The Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY; https://wallaby-survey.org) is a large international collaboration of currently 144 team members from five different continents. It is led by Lister Staveley-Smith from the International Centre for Radio Astronomy Research (ICRAR) in Perth.