Astronomers have unexpectedly spotted a ‘ghost’ low surface brightness dwarf galaxy on the outer edges of our own Milky Way.
After scanning the latest batch of data from the European Space Agency’s (ESA) Gaia spacecraft, an international team of astronomers found the most diffuse and lowest surface brightness galaxy ever detected. Now dubbed Antlia 2, for the constellation in which it lies, it’s officially a new satellite galaxy of the Milky Way.
“It’s not completely clear how this galaxy came to be so ghostly,” Gabriel Torrealba, the team lead and an astrophysicist at Taiwan’s Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), told me.
About a third the diameter of the Milky Way, Torrealba notes that Antlia 2 is about the same size as the nearby Large Magellanic Cloud (LMC) but shines some 10,000 times fainter.
Such low surface brightness galaxies have very low star-forming rates and produce very few if any supernovae, as noted here earlier.
Part of the reason Antilia 2 had not been spotted to date was simply that it lies in an inherently difficult part of the galactic plane to observe. It’s a region full of dust and an overabundance of bright stars near the galactic center.
But the team was able to use about a hundred old and metal-poor pulsating, so-called ‘RR Lyrae’ stars to probe inside and ultimately identify Antlia 2 in this galactic zone of avoidance.
“The zone of avoidance is basically the part of the sky obscured by the Milky Way’s disk as seen from the Earth,” said Torrealba. “The disk of the Milky Way has a lot of gas and stars, making it extremely crowded and complex.”
Because of this complexity, says Torrealba, any kind of study there becomes very difficult to do. Gaia is able to dig into the Zone of Avoidance, he says, because it provides high-quality proper motions of stars behind the central disk of our Milky Way galaxy. That is, it is able to track stars as they move across the celestial sphere.
Located behind the galactic disk, the team reports in the journal Monthly Notices of the Royal Astronomical Society that Antlia 2 was found using data from Gaia’s latest Data Release 2 (DR-2) as well as follow-up ground-based observations with the Anglo-Australian Telescope in Australia. And although it is clearly a satellite, it never comes closer than about 130,000 light years from the Milky Way.
Torrealba says that Antlia 2 is likeliest one of the oldest dwarf galaxies in the universe, but he and colleagues are still puzzled as to how it became so diffuse.
“One possibility is that Antlia 2 was much more massive in the past, and as it fell into the Milky Way, it lost its mass to become more diffuse,” said Torrealba.
One problem with this idea says Torrealba is that rather than grow, galaxies tend to shrink at the same time they lose stars.
“There seems to be no limit (so far) on how diffuse and low surface bright a galaxy can get,” Stacy McGaugh, an astronomer at Case Western Reserve University in Cleveland, who wasn’t involved in the study, told me.
One of the biggest questions is just how many more such dwarf galaxies may be lurking around our own galaxy. The authors note that the Milky Way may still have between one and three undetected dwarf satellite galaxies with each with 100,000 stellar masses or more.