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

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posted on 2019-09-13, 09:04 authored by G Torrealba, V Belokurov, SE Koposov, TS Li, MG Walker, JL Sanders, A Geringer-Sameth, DB Zucker, K Kuehn, NW Evans, W Dehnen
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 MV = −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).


This project was developed in part at the 2018 NYC Gaia Sprint, hosted by the Center for Computational Astrophysics of the Flatiron Institute in New York City. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 308024. SEK and MGW are supported by National Science Foundation grant AST-1813881. GT acknowledges support from the Ministry of Science and Technology grant MOST 105-2112-M-001-028-MY3, and a Career Development Award (to YTL) from Academia Sinica. This work presents results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC is provided by national institutions, in particular the institutions participating in the Gaia MultiLateral Agreement (MLA). The Gaia mission website is The Gaia archive website is This paper includes data gathered with Anglo-Australian Telescope at Siding Spring Observatory in Australia. We acknowledge the traditional owners of the land on which the AAT stands, the Gamilaraay people, and pay our respects to elders past and present. This project used data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES) collaboration. Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitch



Monthly Notices of the Royal Astronomical Society, 2019, 488(2), pp. 2743–2766

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/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy


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Monthly Notices of the Royal Astronomical Society


Oxford University Press (OUP), Royal Astronomical Society





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