Gabriel Torrealba of ASIAA finds the most diffuse galaxy in the universe

(ASIAA, the Academia Sinica Institute of Astronomy and Astrophysics, released the news on November 14th., 2018)

An international team led by Dr. Gabriel Torrealba of ASIAA today announced the discovery of an enormous galaxy lurking in the outskirts of the Milky Way. The collaboration of scientists from Taiwan, the UK, the US, Australia and Germany has unearthed the amazing find by trawling through data from the ESA’s Gaia satellite. The object, named Antlia 2 (or Ant 2) after the constellation in which it's located, has avoided detection until today thanks to its astonishingly low density as well as a perfectly-chosen hiding place, behind the shroud of the Milky Way’s disk.

Ant 2 is what is known as a dwarf galaxy. As structures emerged in the early Universe, dwarfs were the first galaxies to form. That is why most of their stars are old, low-mass and metal-poor (deficient in heavy elements). But compared to the other known dwarf satellites of our Galaxy, the physical extent of Ant 2 is immense: it is as big as the Large Magellanic Cloud (LMC), and a third the size of the Milky Way itself. What makes it even more unusual is how little light it gives out. Compared to the LMC for example, it is 10,000 times fainter. In other words, Ant 2 is either far too large for its luminosity or far too dim for its size.

Gaia is ESA’s current Cornerstone mission, a robotic space observatory in operation since 2014. Gaia has the angular resolution of the Hubble Space Telescope, but it does not collect images of the sky; instead it measures the positions and brightnesses of every star that crosses its field of view. Gaia Data Release 2 in April this year made the motions and distances of 1 billion stars available to the world for the first time. Torrealba and colleagues were a part of a larger team of scientists gathered at New York’s Gaia Sprint, an event dedicated solely to the exploration of the freshly-released Gaia data.

"The Sprint is an occasion when all sorts of ideas are thrown around, even the craziest ones,” continues Torrealba. “Ours was simple: to try to find new satellites of the Milky Way using RR Lyrae stars. And there it was, a little group of RR Lyrae sitting just above the Galactic disk.”

RR Lyra is a type of an old and metal-poor star, exactly the kind one expects to find in a dwarf galaxy. RR Lyrae change their brightness with a period of half a day. It is with this well-defined, unmistakable pulsation that they betray their location and the location of the satellite galaxy they inhabit.

"When Gabriel showed his result at the Sprint, there was little surprise in the room. After all, RR Lyrae had been found in every known dwarf satellite,” says Vasily Belokurov, co-author of the discovery. “Yet, when we looked closer at the location on the sky it turned out we found something new, as no previously identified object came up in any of the databases we searched through."

The team rushed to contact their colleagues at the Anglo-Australian Telescope (AAT) in Siding Spring Observatory, Australia. "But when we checked the coordinates for Ant 2, we realised we had only a very limited window of opportunity to get follow-up data," notes Daniel Zucker, one of the Australian collaborators. For several more precious days, the mysterious object could still be observed from Earth, before the Earth's motion around the Sun rendered Ant 2 off-limits for months. If they were lucky with the weather, there was a chance to take spectra of other stars around the handful of RR Lyrae they found; with AAT spectra in hand, they would be able to confirm the nature of the object. And in luck they were: even though Ant 2 could just be seen, low above the horizon, the team got spectra of more than a hundred red giant stars in the phantom galaxy.

“On a first look at the spectra, I thought we must be getting too many Milky Way stars mixed in our sample, because the signal seemed suspiciously strong," comments team member Ting Li. "But it turned out almost all of the spectra actually belonged to Ant 2. Thanks to the Gaia data, we got a 99% pure selection of dwarf galaxy stars."

The AAT spectra were an important piece of the puzzle. First of all, the team were able to confirm that the ghostly object was real: all the stars were moving together. Combining the redshift of Ant 2 with its proper motion, they determined the orbit of the galaxy and learned that it never comes too close to the Milky Way, always staying at least 40 kpc away. From the spread in the individual stellar redshifts, they also obtained the mass of the galaxy. The mass, however, was clearly far short of what was expected. The combination of small mass and enormous size meant an anomalously low density.

“The simplest explanation of why Ant 2 appears to have so little mass today is that it is being taken apart by the Galactic tides,” opines co-author Sergey Koposov. “What remains unexplained, however, is the object’s giant size. Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow."

If it is impossible to puff the dwarf up by removing matter from it, then it had to have been born huge. The team has yet to figure out the exact process that made Ant 2 so extended. While it appears that objects of this size and luminosity have not been predicted by current models of galaxy formation, recently it has been speculated that some dwarfs could be inflated by vigorous star formation. Stellar winds and supernova explosions would push away the unused gas, weakening the gravity that binds the galaxy and allowing the dark matter to drift outward as well.

"Even if star formation could re-shape the Dark Matter distribution in Ant 2 as it was put together, it must have acted with unprecedented efficiency", points out Jason Sanders, a team member from Cambridge.

Alternatively, Ant 2's low density could mean that a modification to the Dark Matter properties is needed. The currently favored Cold Dark Matter is predicted to pack tightly in the centers of galaxies. Given how fluffy the new dwarf appears to be, a dark matter particle which is less keen to cluster may be required.

“Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball,” points out Matthew Walker, also on the paper. “We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of nearly invisible dwarfs similar to this one.”

Indeed, the gap between Ant 2 and the rest of the Galactic dwarfs is so wide that this may well be an indication that some important physics is missing in the models of dwarf galaxy formation. This is an exciting puzzle and solving it will help shed light on the details of the emergence of the first structures in the early Universe. Finding more objects like Ant 2 will show just how common such ghostly galaxies are, and the team is busy looking for other similar galaxies in the Gaia data.

(This article is based on the Press Release prepared by the collaboration team.)

Reference:

The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2 , G. Torrealba et al., 2018, submitted to Monthly Notices of the Royal Astronomical Society [https://academic.oup.com/mnras, preprint: https://arxiv.org/abs/1811.04082].