The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2 (1811.04082v3)

Abstract: 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).

Discovery of Antlia 2: A Comprehensive Analysis of a Diffuse Galactic Dwarf Satellite

The analysis presented in this paper addresses the identification and characterization of a large, low surface brightness satellite galaxy of the Milky Way, named Antlia 2, discovered through data obtained from Gaia Data Release 2 (DR2). This research focuses on the unique properties of Antlia 2, emphasizing its size, luminosity, dark matter content, and potential formation mechanisms. The paper combines observational data with dynamical models to elucidate the origins and present state of this dwarf galaxy.

Observational Discovery

Antlia 2 was identified at a distance of approximately 130 kpc, located behind the Milky Way's disk, making it challenging to detect. The spatial extent of the galaxy is similar to the Large Magellanic Cloud but with an exceedingly faint luminosity of $M_V = -9$, representing surface brightness levels as low as $31.9$ mag/arcsec$^2$. This finding delineates it as 100 times more diffuse than ultra diffuse galaxies traditionally known. The methods used for identification included photometry, astrometry, and variability data available in Gaia DR2, complemented by deep imaging from the DECam survey and spectroscopic observations from AAOmega on the AAT.

Key Measurements and Properties

The spectroscopic analysis revealed Antlia 2's systemic velocity to be $290.9 \pm 0.5$ km/s, alongside a velocity dispersion of $5.7 \pm 1.1$ km/s and an average metallicity [Fe/H] = $-1.4$. These properties suggest that Antlia 2 exists in a very low-density dark matter halo compared to other galaxies. A noteworthy aspect of this paper is the dynamical modeling pointing to a combination of a cored dark matter profile and significant tidal interactions as potential explanations for Antlia 2's observed characteristics.

Interpretation of Results and Implications

The implications of this discovery are manifold. The disproportionately large size and faint luminosity of Antlia 2 challenge existing models of dwarf galaxy formation and evolution. It may necessitate reconsideration of feedback processes in galaxy formation or alternative forms of dark matter, such as ultra-light bosons. Moreover, it highlights the possibility of a population of similarly diffuse galaxies that remain unseen, potentially extending the ultra-faint galaxy regime with a new class of "stealth" galaxies.

Future Directions

This paper opens avenues for further exploration into the nature of these ultra-diffuse galaxies. Detailed follow-up observations are essential to confirm Antlia 2’s kinematic and chemical properties, assess its potential tidal debris, and refine models of its dynamical history. Such exploration could unveil new mechanisms of dwarf galaxy evolution, enhance understanding of the dark matter halo's structural diversity, and contribute to advancing the field of galaxy formation theories.