Dark Clouds of Hydrogen Found in the Eridanus Supergroup

In a remote part of Western Australia, construction is about to commence on a new radio telescope array that has been 30 years in the planning. As one part of the Square Kilometre Array (SKA), it will consist of more than 130,000 low-frequency antennas shaped like Christmas trees. When complete, the instrument will be able to look back billions of years in time to the formation of the first stars and galaxies.

The SKA won’t be fully operational until later in the decade, but astronomers are not sitting idly by and waiting for the one-of-a-kind research facility to be built. They are busy developing the technologies that will be needed for the SKA to explore the unknown frontiers of science and astronomy. The telescope will also be rolled out in phases, with each phase commencing new science observations, and likely yielding new science results.

One precursor to SKA that is providing crucial design, assembly and deployment guidance is the Australian SKA Pathfinder (ASKAP), developed and operated by Australia’s national science agency, CSIRO. But ASKAP is more than just a technology demonstrator for the more sensitive SKA. It is currently conducting its own ground-breaking, world-first research as part of its commissioning operations.

One of the high priority surveys currently running on ASKAP is the Widefield ASKAP L-band Legacy All-sky Blind survey, or, more simply, WALLABY. The aim of WALLABY is to use the radio telescope to observe three-quarters of the sky in the 21-cm line of neutral hydrogen at high resolution. Doing so gives astronomers a picture of the distribution of matter in galaxies, including the way the stars move and where star formation is occurring.

The 21-cm hydrogen line, or HI line, is a spectral line that appears in the radio spectrum when a change occurs in the energy state of neutral hydrogen atoms. Although it takes about 10-million for a given hydrogen atom to change state in this way, the sheer quantity of hydrogen in our Universe means that it is commonly observed and studied by astronomers using radio telescopes.

Unlike visible light, the radio-wavelength radiation that causes the HI line penetrates through dust clouds. And by using radio telescopes, astronomers can observe gas from across the galaxy, even if other telescopes cannot. Although the visible light from stars may be obscured by dust, the 21-cm hydrogen line has given astronomers fairly compelling evidence that the Milky Way has a clear spiral structure to it.

But astronomers are interested in more than just the shape of our own galaxy. They also want to know how it formed, how other galaxies form, and what part their surrounding environment plays in galaxy evolution. And so, they’ve been looking at the Eridanus supergroup using data from WALLABY.  

In the constellation Eridanus lies a loose grouping of galaxies about 70-million light-years away. Nearby to the Eridanus group are two other galaxy sub-groups, one centred around NGC 1407, and the other around NGC 1332. Together, all three form the Eridanus supergroup.

Astronomers have found evidence that this supergroup is in the early stages of formation by observing the merger characteristics exhibited by the subgroups, and believe that one day they may end up as an even larger aggregate known as a cluster. The scientists who are studying galaxy evolution are particularly interested in Eridanus because it is in the subcluster merging phase.

A close look at the distorted distributions of hydrogen gas around individual galaxies in the group confirms that there are ongoing gravitational, or tidal, interactions amongst the subgroups.

The Eridanus supergroup is also a bit different to other galaxy groups in that many of the galaxies have less hydrogen gas than expected. Almost half of the galaxies in the supergroup are considered to be HI deficient, a greater proportion than observed in other galaxy groups located in our general vicinity.

These are some of the findings of a recent study using WALLABY data by a team of Australian and international astronomers, and they confirm that galaxies in the Eridanus supergroup are just a bit different to your average galaxy. But the team has also found two enormous clouds of hydrogen gas that don’t seem to be associated with any visible galaxies.

Fascinating results like these are proof of why it’s so important to look at the sky in different wavelengths. If we just relied on optical surveys of the same area it would look a lot like empty space. But at radio wavelengths, it shines brightly for a variety of different objects, events and phenomena. 

The dark clouds themselves were interesting enough to be the subject of a second study. The question on astronomer’s minds is whether they are clouds of gas ripped from galaxies in the Eridanus group during past gravitational interactions, or they are, in fact, associated with very dim galaxies.

In support of the first hypothesis, that they have a tidal origin, is that examples of HI tidal remnants have been found in other galaxy group interactions in previous studies. But these clouds are further away from potential optical counterparts than is commonly observed.

In support of the second hypothesis, that they are ancient dark galaxies, is that cosmological simulations have produced some very close virtual analogues to the clouds. But there is one significant difference. The analogues are rotating, whereas the clouds are not. At least, not clearly rotating, according to this recent survey data.

Interestingly, the HI sizes and masses of the two clouds are also consistent with other low mass galaxies in the local Universe, including our neighbours, the Large and Small Magellanic Clouds. Though, a stark point of difference is that the Magellanic Clouds were named for their cloudy appearance in the visible bands, not because they are clouds of HI gas (which of course, is not visible to our eyes or optical telescopes).

The end result is that astronomers cannot be certain how the clouds originated. But they are confident that the results from WALLABY suggest that the upcoming large area HI surveys will have a significant impact on our understanding of dim galaxies and the physical processes that shape them.

Pre-pilot and pilot ASKAP observations (such as those conducted in the WALLABY and EMU surveys) have proved to be an improvement on past observations made with some of the CSIRO’s older telescopes, such as the Australian Telescope Compact Array (ATCA) based in Narrabri in rural NSW. 

While ATCA, the Parkes Radio Telescope, and the Mopra Radio Telescope are all certain to play an important role in radio astronomy for years to come, we can expect lots more ground-breaking science results to come from our investment in the SKA project in the future. 

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