Hydrogen loss hints at the impending death of a galaxy
Using the Australian SKA Pathfinder (ASKAP) radio telescope, based at CSIRO’s Murchison Radio-astronomy Observatory in Western Australia, researchers have witnessed what they claim is the beginning of the end for one of the Milky Way’s neighbouring galaxies.
The neighbour in question, the Small Magellanic Cloud (SMC), is a dwarf galaxy based less than 200,000 light years from the Milky Way. Alongside its sibling, the Large Magellanic Cloud (LMC), it is just close enough to Earth to be visible in the night sky with the naked eye — and with other dwarf galaxies located substantially further away, that makes it an ideal subject for study.
Professor Naomi McClure-Griffiths, from the Research School of Astronomy & Astrophysics at the Australian National University (ANU), has been studying the SMC as part of her work on the evolution of galaxies. Along with a team that includes Dr David McConnell from the CSIRO, she has been probing the interactions between the small galaxy and its environment — and as the world’s fastest survey radio telescope, ASKAP has been key to the project’s success.
“The Magellanic Clouds are objects of interest in the Southern Sky, and they’ve always been on the list of interesting things that ASKAP would look at once it was operational,” Dr McConnell said. “Once we got to the point of having a reasonable fraction of the telescope operational, the Small Magellanic Cloud was an obvious choice to make some test observations.”
The last radio telescope to image the SMC was CSIRO’s Australia Telescope Compact Array (ATCA), based in Narrabri, northern NSW, which comprises six 22-m-diameter antennas. But while the 30-year-old array has received various upgrades over the years, it can still only form one beam in the sky at any one time, and so had to undertake 320 separate pointings in order to image the SMC.
“It had to essentially point at lots and lots of positions across the sky,” Dr McConnell said. “Each one of those takes time, and so that means there’s a lot of time involved in making observations — and then there’s also a lot of complexity in stitching all those little tiny images together, to make one big picture.”
By contrast, ASKAP contains 36 antennas, each measuring 12 m in diameter, spread across an area of 6 km. And while only 16 of these antennas were operational at the time of the study, that was enough to image the entire Small Magellanic Cloud in a single panoramic shot taken over three nights, capturing features three times finer than what had been achieved previously. Data from CSIRO’s Parkes radio telescope was also added to pick up fainter details.
“ASKAP has specially designed and quite novel receivers, in a structure called a phased array feed,” Dr McConnell explained. “And that’s a bit analogous to the difference between a one-pixel camera and a 36-pixel camera. We can form simultaneously 36 beams on the sky; we configure them in a closely packed pattern over the object to be studied, and the receiver allows us to form a single image over the whole pattern.”
Significantly, ASKAP’s image of the SMC reveals a powerful outflow of neutral hydrogen gas (HI), the main ingredient of stars, extending at least 2 kiloparsecs from the star-forming bar of the galaxy and making its way towards the nearby Magellanic Stream of gas clouds, which encircles the Milky Way.
“We’re looking at a particular emission that is made by hydrogen atoms, and by analysing that in a spectral sense, by looking at the different strengths of that signal related to the wavelength of the radiation, we can tell how fast the hydrogen we’re looking at is moving towards us or away from us,” Dr McConnell said.
Writing in the journal Nature Astronomy, the researchers claim that the SMC is currently experiencing a particularly large outflow of HI, which they assume originated in its most recent burst of star formation — and that’s a problem, because the SMC doesn’t have a strong enough gravitational field to retain this valuable material.
“The Small Magellanic Cloud is a lot smaller than our own galaxy, and it doesn’t have a huge amount of mass — and so its gravitational field is not as strong,” Dr McConnell explained. “And so any gas that gets pushed around, if it gets a hard enough shove, it will just leave the galaxy.
“Whenever there are supernovae — when stars reach the end of their life and go bang — they explode and push the surrounding gas away. And in the Small Magellanic Cloud, there have been quite a lot of supernovae over the recent past — and when I say recent, I mean millions of years. There have been new stars being born, big heavy stars that get through their life pretty quickly and then go bang as a supernova, and when there’s a lot of them in the one place, that pushes a lot of gas. And the SMC is so small that that gas is pushed so hard that it’s just running away, leaving the galaxy.”
Furthermore, the researchers discovered that this outflow is up to an order of magnitude greater than the SMC’s star-formation rate — so for every Sun-sized star the SMC makes, it loses up to 10 times that amount of hydrogen gas. If the SMC loses all its hydrogen it will also lose its ability to create new stars, and thus its ability to survive.
“This gas that we can see leaving the Small Magellanic Cloud is lost for future star formation,” Dr McConnell said. “Any gas it loses limits the number of new stars it can make. And so ultimately, the size of the Cloud will just diminish, and it won’t have enough gas to make more stars.”
So where exactly is all this hydrogen gas going? The theory is that it is feeding directly into the Magellanic Stream, whose own source of gas has long been speculated. Dr McConnell suggested that the gravitational pull of the Milky Way may also contribute to this outflow and “direct the gas into the stream”.
The Milky Way may also find itself a beneficiary of the SMC’s slow death, as any outflow that doesn’t join the Magellanic Stream may end up spiralling back into our own galaxy. And as the SMC eventually fizzles out — a process that will, admittedly, take billions of years — any remaining material is likely to be similarly taken in by the Milky Way.
So as the Small Magellanic Cloud inches towards its inevitable demise, ASKAP continues to add new capabilities to observe the whole process as best it can. At the time of writing, 28 out of the telescope’s 36 antennas have come online — each one enabling more detailed images than the last.
“We’ll get somewhat sharper images than that one we’ve just made, and the other thing is, we’ll get more detail in the velocity of the gas; in terms of being able to measure the speed of the gas’s motion,” Dr McConnell said. “So both those advances in the telescope will make the images better, and the information more useful.”
Along with further imaging of the Small Magellanic Cloud, the ASKAP team has a long-term plan to observe the Large Magellanic Cloud, the Magellanic Stream and eventually the entire Southern Sky. It therefore appears that when it comes to ASKAP’s imaging capabilities, the sky truly is the limit.
“They’ve got a very big program ahead of them — this is just a little warm-up,” Dr McConnell said.
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