We report the discovery of a Milky Way satellite in the constellation of Antlia .
The Antlia 2 dwarf galaxy is located behind the Galactic disc at a latitude of b \sim 11 ^ { \circ } and spans 1.26 degrees , which corresponds to \sim 2.9 kpc at its distance of 130 kpc .
While similar in spatial extent to the Large Magellanic Cloud , Antlia 2 is orders of magnitude fainter at M _ { V } = -9 mag , making it by far the lowest surface brightness system known ( at \sim 31.9 mag/arcsec ^ { 2 } ) , \sim 100 times more diffuse than the so-called ultra diffuse galaxies .
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 signal .
We have also obtained follow-up spectroscopy using AAOmega on the AAT , identifying 159 member stars , and we used them to measure the dwarf ’ s systemic velocity , 290.9 \pm 0.5 km/s , its velocity dispersion , 5.7 \pm 1.1 km/s , and mean metallicity , [ Fe/H ] = -1.4 .
From these properties we conclude that Antlia 2 inhabits one of the least dense Dark Matter ( DM ) halos probed to date .
Dynamical modelling and tidal-disruption simulations suggest that a combination of a cored DM profile and strong tidal stripping may explain the observed properties of this satellite .
The origin of this core may be consistent with aggressive feedback , or may even require alternatives to cold dark matter ( such as ultra-light bosons ) .