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We report the discovery of a Milky Way satellite in the constellation ofAntlia. The Antlia 2 dwarf galaxy is located behind the Galactic disc at alatitude 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 tothe 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 diffusegalaxies. The satellite was identified using a combination of astrometry,photometry and variability data from \textit{Gaia} Data Release 2, and itsnature confirmed with deep archival DECam imaging, which revealed a conspicuousBHB signal. We have also obtained follow-up spectroscopy using AAOmega on theAAT, identifying 159 member stars, and we used them to measure the dwarf'ssystemic 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 concludethat Antlia 2 inhabits one of the least dense Dark Matter (DM) halos probed todate. Dynamical modelling and tidal-disruption simulations suggest that acombination of a cored DM profile and strong tidal stripping may explain theobserved properties of this satellite. The origin of this core may beconsistent with aggressive feedback, or may even require alternatives to colddark matter (such as ultra-light bosons).

The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2