The hidden giant: discovery of an enormous Galactic dwarf satellite in
  Gaia DR2
  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
$\sim2.9$ 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 $\sim31.9$
mag/arcsec$^2$), $\sim100$ times more diffuse than the so-called ultra diffuse
galaxies. The satellite was identified using a combination of astrometry,
photometry and variability data from \textit{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, $290.9\pm0.5$km/s, its velocity dispersion, $5.7\pm1.1$
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).
