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

Journal Article · 12 June 2019 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/stz1624 · OSTI ID:1544920

Torrealba, G.  ^[1]; Belokurov, V.  ^[2]; Koposov, S. E.  ^[3]; Li, T. S. ^[4]; Walker, M. G. ^[5]; Sanders, J. L.  ^[6]; Geringer-Sameth, A. ^[7]; Zucker, D. B. ^[8]; Kuehn, K. ^[9]; Evans, N. W. ^[6]; Dehnen, W.  ^[10]

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1. Academia Sinica, Taipei (Taiwan). Inst. of Astronomy and Astrophysics
2. Univ. of Cambridge, Cambridge (United Kingdom). Inst. of Astronomy; Flatiron Inst., New York, NY (United States). Center for Computational Astrophysics
3. Univ. of Cambridge, Cambridge (United Kingdom). Inst. of Astronomy; Carnegie Mellon Univ., Pittsburgh, PA (United States). Dept. of Physics, McWilliams Center for Cosmology
4. Fermi National Accelerator Lab., Batavia, IL (United States); Univ. of Chicago, Chicago, IL (United States). Kavli Inst. for Cosmological Physics
5. Carnegie Mellon Univ., Pittsburgh, PA (United States). Dept. of Physics, McWilliams Center for Cosmology
6. Univ. of Cambridge, Cambridge (United Kingdom). Inst. of Astronomy
7. Imperial College London, London (United Kingdom). Blackett Lab., Dept. of Phsyics
8. Macquarie Univ., Sydney (Australia). Dept. of Physics & Astronomy and Macquarie Univ. Research Centre for Astronomy
9. Macquarie Univ., North Ryde (Australia). Australian Astronomical Optics
10. Univ. of Leicester, Leicester (United Kingdom). Dept. of Physics & Astronomy; Sternwarte der Ludwig-Maximilians-Univ., Munchen (Germany)

In this paper, 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 ~ 11° and spans 1.26°, which corresponds to ~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 ~31.9 mag arcsec^-2), ~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 ± 0.5 km s^-1, its velocity dispersion, 5.7 ± 1.1 km s^-1, and mean metallicity, [Fe/H] = -1.4. From these properties we conclude that Antlia 2 inhabits one of the least dense dark matter (DM) haloes 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).

• Research Organization: Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), High Energy Physics (HEP)
• Grant/Contract Number: AC02-07CH11359
• OSTI ID: 1544920
• Alternate ID(s): OSTI ID: 1570216
• Report Number(s): arXiv:1811.04082; FERMILAB-PUB-19-517-AE; oai:inspirehep.net:1703311; TRN: US2001372
• Journal Information: Monthly Notices of the Royal Astronomical Society, Vol. 488, Issue 2; ISSN 0035-8711
• Publisher: Royal Astronomical SocietyCopyright Statement
• Country of Publication: United States
• Language: English