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Exploring the Galaxy’s halo and very metal-weak thick disc with <i>SkyMapper</i> and <i>Gaia</i> DR2

G. CordoniDipartimento di Fisica e Astronomia ‘Galileo Galilei’ – Università di Padova, Vicolo dell’Osservatorio 3, Padova, IT-35122, ItalyG. S. Da CostaARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), AustraliaDavid YongARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), AustraliaDougal MackeyARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), AustraliaA. F. MarinoDipartimento di Fisica e Astronomia ‘Galileo Galilei’ – Università di Padova, Vicolo dell’Osservatorio 3, Padova, IT-35122, ItalyStephanie MontyResearch School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, AustraliaThomas NordlanderARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), AustraliaJohn E. NorrisResearch School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, AustraliaM. AsplundMax Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, D-85748 Garching, GermanyM. S. BessellResearch School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, AustraliaAndrew R. CaseyFaculty of Information Technology, Monash University, Wellington Rd, Clayton, VIC 3800, AustraliaAnna FrebelDepartment of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USAK. LindAlbaNovaSimon J. MurphyResearch School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, AustraliaB. SchmidtResearch School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, AustraliaXudong GaoMax-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, GermanyT. Xylakis-DornbuschLandessternwarte, Heidelberg University, Königstuhl 12, D-69117, Heidelberg, GermanyA. M. AmarsiTheoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120 Uppsala, SwedenA. P. MiloneDipartimento di Fisica e Astronomia ‘Galileo Galilei’ – Università di Padova, Vicolo dell’Osservatorio 3, Padova, IT-35122, Italy

2020en

ABI

Annotatsiya

ABSTRACT In this work, we combine spectroscopic information from the SkyMapper survey for Extremely Metal-Poor stars and astrometry from Gaia DR2 to investigate the kinematics of a sample of 475 stars with a metallicity range of $-6.5 \le \rm [Fe/H] \le -2.05$ dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the Gaia Sausage and Gaia Sequoia accretion events, respectively. The most metal poor of these candidates have metallicities of $\rm [Fe/H]=-3.31\, \mathrm{ and }\, -3.74$, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that ∼21 per cent of the sample have orbits that remain confined to within 3 kpc of the Galactic plane, that is, |Zmax| ≤ 3 kpc. Of particular interest is a subsample (∼11 per cent of the total) of low |Zmax| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = –4.30 and the subsample is best interpreted as the very low-metallicity tail of the metal-weak thick disc population. The low |Zmax|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Zmax|, high eccentricity pro- and retrograde stars are plausibly associated with the Gaia Sausage system. We find that a small fraction of our sample (∼4 per cent of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted.

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DOI: 10.1093/mnras/staa3417

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