Astronomers have discovered next to Milky Way galaxy-"Ghost"

Mindy Sparks
November 17, 2018

Researchers from Universidad de Zaragoza, King's College London and the Institute of Astronomy in the United Kingdom have been studying a stellar stream left behind by a dwarf spheroidal galaxy that was devoured by the Milky Way aeons ago. In a paper presented at the online library of preprints arXiv.orgscientists describe 2 Antlia (Ant 2) as a small galaxy,"Ghost", unique among about 60 companions of the milky Way.

Astronomers have calculated dark matter due to its gravitational effects on the known matter and believe that it is the gravitational glue that holds galaxies together. This faint galaxy was found hiding in the region behind the Milky Way's disc - probably the reason why it could not be spotted by astronomers up until now.

The Large Magellanic Cloud, the Milky Way Galaxy and Antlia 2 (from left to right).

"The satellite was identified using a combination of astrometry, photometry and variability data from Gaia Data Release 2, and its nature confirmed with deep archival DECam imaging, which revealed a conspicuous BHB [blue horizontal branch] signal in agreement with distance obtained from Gaia RR Lyrae", the study says. Still, while the galaxy does have the expected RR Lyrae, it's enormous size and low mass remain puzzling.

Working quickly, the team measured the spectra of more than 100 red giant stars-just before Earth's orbit around the Sun rendered Ant 2 unobservable for months.

The observations made were enough to confirm that Ant 2 is real and that it never comes closer to the Milky Way than 130,000 light-years.


However, the team is yet to figure out the exact process that made Ant 2 so extended.

In March, a separate team of astronomers found the galaxy NGC 1052-DF2 that is missing most of its dark matter that challenges currently-accepted theories of galaxy formation and seeks scientists to re-look at the nature of planet formation as well as the studies on the dark matter. However, this does not explain its impressive size. "Normally, as galaxies lose mass to the Milky Way's tides, they shrink, not grow", Koposov said.

Dr Matthew Walker, from Carnegie Mellon University, added: "We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of almost invisible dwarfs similar to this one". The team continues to search through the Gaia data for other "ghost" galaxies like Ant 2 to hopefully answer such questions about galaxies like this.

Alternatively, Ant 2's low density could mean that a modification to the dark matter properties is needed.

But O'Hare and his colleagues calculated the affect of the S1 stream in our part of the galaxy and predicted possible signatures of the dark matter, which could help inform and support efforts to locate and study the elusive substance. "We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of almost invisible dwarfs similar to this one". As structures emerged in the early Universe, dwarfs were the first galaxies to form, and so most of their stars are now old, relatively low in terms of their mass, and poor in metals.

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