Dynamical Relics of the Ancient Galactic Halo

Abstract

We search for dynamical substructures in the LAMOST DR3 very metal-poor (VMP) star catalog. After cross-matching with Gaia DR2, there are ∼ 3300 VMP stars with available high-quality astrometric information that have halo-like kinematics. We apply a method based on self-organizing maps, StarGO, to find groups clustered in the 4D space of orbital energy and angular momentum. We identify 57 dynamically tagged groups, which we label DTG-1 to DTG-57. Most of them belong to existing massive substructures in the nearby halo, such as the Gaia Sausage or Sequoia. The stream identified by Helmi et al. is recovered, but the two disjoint portions of the substructure appear to have distinct dynamical properties. The very retrograde substructure Rg5 found previously by Myeong et al. is also retrieved. We report six new DTGs with highly retrograde orbits, two with very prograde orbits, and 12 with polar orbits. By mapping other datasets (APOGEE halo stars, and catalogs of r-process-enhanced and CEMP stars) onto the trained neuron map, we can associate stars with detailed chemical abundances to the DTGs, and look for associations with chemically peculiar stars. The highly eccentric Gaia Sausage groups contain representatives both of debris from the satellite itself (which is α-poor) and the Splashed Disk, sent up into eccentric halo orbits from the encounter (and is α-rich). The new prograde substructures also appear to be associated with the Splashed Disk. The DTGs belonging to the Gaia Sausage host two relatively metal-rich r-II stars and six CEMP stars in different sub-classes, consistent with the idea that the Gaia Sausage progenitor is a massive dwarf galaxy. Rg5 is dynamically associated with two highly r-process-enhanced stars with [Fe/H] ∼ −3. This finding indicates that its progenitor might be an ultra-faint dwarf galaxy that has experienced r-process enrichment from neutron star mergers.

TheR-Process Alliance: Fourth Data Release from the Search forR-process-enhanced Stars in the Galactic Halo

“…Quantified chemical identifications provide clues as to the formation history of the Milky Way, since stars with similar metallicities and levels of r-process enrichment have also been found to be dynamically linked in small associations (see, e.g., Roederer et al 2018a;Yuan et al 2020). Current and future RPA efforts will help to refine the mapping of r-processenhanced stars into their parent dynamical groups, so that we may learn more about the natal environment in which the r-process occurred in each of these now-disrupted systems.…”

“…These dynamical groups could have once been members of satellite galaxies that hosted a prolific r-process event prior to their disruption into the Galactic halo. Additional explorations of this hypothesis are presently underway (e.g., Yuan et al 2020;D. Gudin et al 2020, in preparation).…”

The S2 Stream: the shreds of a primitive dwarf galaxy.*

“…We can also use this set of high quality members to identify other S2 stars from the literature with chemical data. In particular, we searched through the 12 S2 members with high resolution spectroscopy claimed by Roederer et al (2010) and the 6 S2 members with APOGEE spectroscopy found by Yuan et al (2020).…”

“…We retain 10 of these stars as safe S2 members -7 from Roederer et al (2010) and 3 from Yuan et al (2020). They are shown as the filled triangles and circles in Fig.…”

“…The arrival of the unprecedented high-quality astrometry from the Gaia space observatory (Gaia Collaboration et al 2016) has completely changed the game of searching for the halo sub-structure. The last three years have witnessed an avalanche of discoveries of disrupted Galactic satellites in the Solar vicinity (Helmi et al 2017;Myeong et al 2017a;Belokurov et al 2018;Myeong et al 2018c,a,b;Koppelman et al 2018;Myeong et al 2019;Borsato et al 2020;O'Hare et al 2020;Yuan et al 2020) and further out from the Sun (Ibata et al 2018;Malhan et al 2018;Koposov et al 2019;Ibata et al 2019b,a;Grillmair 2019). The more distant streams are powerful probes of the Galactic gravitational potential (Koposov et al 2010;Gibbons et al 2014;Bowden et al 2015;Bovy et al 2016;Bonaca & Hogg 2018;Erkal et al 2019;Malhan & Ibata 2019).…”