A study led by a University of Pretoria (UP) master’s student using South Africa’s MeerKAT telescope has led to the discovery of a group of 20 galaxies. This large galaxy group is likely the most neutral hydrogen gas-rich group ever discovered, and it is the first time this group has been identified, despite residing in a very well-studied area of the sky.

Optical image of the galaxy group with three-colour optical images of each member galaxy. The red outline indicates the extent of the neutral hydrogen gas around each galaxy. The central image, also showing the many thousands of background galaxies, is one degree on each side, large enough to fit four full moons. (Shilpa Ranchod/MIGHTEE/HSC project)

The research was led by Shilpa Ranchod, an MSc student in the University of Pretoria’s (UP) Department of Physics, supervised by Professor Roger Deane, Founder of the UP Radio Astronomy Research Group, and now Extraordinary Professor. “The distribution of neutral hydrogen gas in these galaxies has revealed interesting, disturbed morphologies suggesting that these galaxies are group members and are being influenced by their cosmic neighbours in the group,” Ranchod said. “For example, we found an interacting pair of galaxies that will potentially merge to form a new galaxy with a completely transformed appearance.”

A three-colour optical image of two of the group galaxies. The diffuse red structure is the neutral hydrogen gas, which envelops both galaxies, and indicates they are in the process of merging. (MIGHTEE/HSC Project)

Ranchod added that the MeerKAT observations show a galaxy group in its early stages of formation, which is extremely rare. “Through this, we are able to understand how galaxy groups are assembled and evolve. This group inhabits an area of sky that has been studied by many other telescopes, but the group structure has been revealed for the first time due to MeerKAT’s excellent sensitivity.”

Most star-forming galaxies are embedded within a cloud of cold neutral hydrogen gas, which acts as the raw fuel from which stars can eventually form. This gas is extremely faint, and can only be detected in radio wavelengths. It is diffuse, and extends beyond the visible part of the galaxy. By observing this hydrogen gas, astronomers are able to understand the evolutionary processes that take place in galaxies.

The majority of galaxies in the Universe reside in groups. However, it is rare to detect a group with such a large number of group members with so much neutral hydrogen. This suggests that the group is still in the process of assembly, as it has not undergone evolutionary processes that would remove this gas from the galaxies.

This galaxy group was discovered by the MeerKAT International Gigahertz Tiered Extragalactic Exploration (MIGHTEE) survey. It is one of the large survey projects in progress using South Africa’s MeerKAT telescope, and involves a team of South African and international astronomers.

The MeerKAT radio telescope in the Northern Cape, South Africa’s precursor to the Square Kilometre Array (SKA), aims to answer fundamental questions about the formation and evolution of galaxies. Its exceptional sensitivity provides astronomers with further insight into the drivers of galaxy evolution.

South Africa’s MeerKAT telescope. (South African Radio Astronomy Observatory)

“MeerKAT continues to impress us with new discoveries, thanks to some brilliant South African engineers who have delivered a world-leading instrument,” said Professor Deane. “To see our bright young students take hold of the scientific opportunities this presents and carry out internationally acclaimed research is both a rewarding and essential step as we plan ahead toward the Square Kilometre Array era."

Ranchod’s galaxy group was found in a survey that produces hundreds of terabytes of data, which are processed via the cloud computing facility hosted by the inter-university Institute of Data-Intensive Astronomy (IDIA), a partnership between the Universities of Pretoria, Cape Town, and the Western Cape.

Professor Chris Theron, Head of UP’s Department of Physics, noted that, "By equipping smart and inquisitive students with the hardware and software tools required to carry out big data research, we open enormous discovery potential. In the Department of Physics, we endeavour to do exactly that." Prof Theron represents UP on the inter-university Institute for Data-Intensive Astronomy management team.

This discovery has been published in the Monthly Notices of the Royal Astronomical Society. Click here to read the publicly available pre-print.