Gaia Spots a ‘Ghost’ Galaxy

Antlia 2 is slightly larger than the Large Magellanic Cloud and is almost 1/3 of the Milky Way itself. (Image: V. Belokurov and A. Smith (Cambridge, UK and CCA, New York, US) based on the images by Marcus and Gail Davies and Robert Gendler)
Antlia 2 is slightly larger than the Large Magellanic Cloud and is almost 1/3 of the Milky Way itself. (Image: V. Belokurov and A. Smith (Cambridge, UK and CCA, New York, US) based on the images by Marcus and Gail Davies and Robert Gendler)

An international team of astronomers, including from the University of Cambridge, discovered the massive object when trawling through data from the European Space Agency’s Gaia satellite. The object, named Antlia 2 (or Ant 2), has avoided detection until now thanks to its extremely low density as well as a perfectly-chosen hiding place, behind the shroud of the Milky Way’s disc. The researchers have published their results online.

Ant 2 is known as a dwarf galaxy. As structures emerged in the early Universe, dwarfs were the first galaxies to form, and so most of their stars are old, low-mass and metal-poor. But compared to the other known dwarf satellites of our Galaxy, 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 Ant 2 even more unusual is how little light it gives out. Compared to the LMC, another satellite of the Milky Way, Ant 2 is 10,000 times fainter. In other words, it is either far too large for its luminosity or far too dim for its size. Gabriel Torrealba, the paper’s lead author, said:

The ESA’s Gaia mission has produced the richest star catalog to date, including high-precision measurements of nearly 1.7 billion stars and revealing previously unseen details of our home Galaxy. Earlier this year, Gaia’s second data release made new details of stars in the Milky Way available to scientists worldwide.

What Antlia 2 would look like if you could see it from Earth. Credit: G. Torrealba (Academia Sinica, Taiwan), V. Belokurov (Cambridge, UK and CCA, New York, US) based on the image by ESO/S. Brunier

What Antlia 2 would look like if you could see it from Earth. (Image: G. Torrealba (Academia Sinica, Taiwan), V. Belokurov (Cambridge, UK and CCA, New York, US) based on the image by ESO/S. Brunier)

The researchers behind the current study — from Taiwan, the UK, the U.S., Australia, and Germany — searched the new Gaia data for Milky Way satellites by using RR Lyrae stars. These stars are old and metal-poor, typical of those found in a dwarf galaxy.

RR Lyrae change their brightness with a period of half a day and can be located thanks to these well-defined pulses. Co-author Vasily Belokurov, from Cambridge’s Institute of Astronomy, said:

The team contacted colleagues at the Anglo-Australian Telescope (AAT) in Australia, but when they checked the coordinates for Ant 2, they realized they had a limited window of opportunity to get follow-up data. They were able to measure the spectra of more than 100 red giant stars just before the Earth’s motion around the Sun rendered Ant 2 unobservable for months.

Antlia 2 is a giant, but low mass, dwarf galaxy. As Antlia 2 orbits around the Milky Way, it is likely that stars are torn from the dwarf galaxy and deposited throughout the outskirts of the Milky Way. The orange stars show the results of a computer model of Antlia 2. The background shows the Gaia satellite’s view of the entire night sky. The Large Magellanic Cloud (LMC) is visible below the Milky Way disc — although similar to Antlia 2 in size, the LMC is 10,000 times brighter. Credit: J. Sanders (Cambridge, UK) based on the image by Gaia Data Processing and Analysis Consortium (DPAC); A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal Read more at: https://phys.org/news/2018-11-gaia-ghost-galaxy-door.html#jCp

Antlia 2 is a giant, but low mass, dwarf galaxy. As Antlia 2 orbits around the Milky Way, it is likely that stars are torn from the dwarf galaxy and deposited throughout the outskirts of the Milky Way. The orange stars show the results of a computer model of Antlia 2. The background shows the Gaia satellite’s view of the entire night sky. The Large Magellanic Cloud (LMC) is visible below the Milky Way disc — although similar to Antlia 2 in size, the LMC is 10,000 times brighter. (Image: J. Sanders (Cambridge, UK) based on the image by Gaia Data Processing and Analysis Consortium (DPAC); A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal)

The spectra enabled the team to confirm that the ghostly object they spotted was real: All the stars were moving together. Ant 2 never comes too close to the Milky Way, always staying at least 40 kiloparsecs (about 130,000 light-years) away. The researchers were also able to obtain the galaxy’s mass, which was much lower than expected for an object of its size.

Co-author Sergey Koposov, from Carnegie Mellon University, said:

If it is impossible to puff the dwarf up by removing matter from it, then Ant 2 had to have been born huge. The team has yet to figure out the exact process that made Ant 2 so extended. While 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. Co-author Jason Sanders, also from Cambridge, said:

Alternatively, Ant 2’s low density could mean that a modification to the dark matter properties is needed. The currently favored theory predicts dark matter 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. Co-author Matthew Walker, also from Carnegie Mellon University, said:

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. Solving the Ant 2 puzzle may help researchers understand how the first structures in the early Universe emerged.

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.

Provided by: University of Cambridge [Note: Materials may be edited for content and length.]

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