Photometric Metallicities and Kinematics of 357,881 Low-mass Stars in the Local Galactic Halo with Gaia DR2 and SDSS SEGUE/APOGEE

Although the majority of stars in the Milky Way are low-mass (GKM) main-sequence stars, studies of the kinematics and chemical composition of the Galactic halo often focus on the less common but more luminous giants, because they can be studied to larger distances. In this study, we tap on the much larger population of nearby low-mass stars in the halo, by assembling a catalog of 357,881 nearby (d < 1.5 kpc) halo stars, which combines photometric metallicity estimates with kinematics. Our census mostly consists of metal-poor, low-mass stars with large proper motions (> 40 mas/yr) from Gaia DR2, and selected from a reduced proper motion diagram.Although most candidates are relatively close to the Sun (d_peak ~ 600 pc), we apply a reddening correction using the extinction maps of Green et al. (2019) and Andrea et al. (2018) to recover their intrinsic magnitudes and colors. Among our sample, 4,952 stars have spectroscopic metallicities and radial velocities obtained from either the SDSS/SEGUE or APOGEE-2 databases. Metallicities confirm that our sample consists of metal-poor stars, with -3.5 < [M/H] < -0.5. The distribution of these spectroscopic calibrators in the HR diagram provides reference points that allow us to define a photometric metallicity grid, which we then apply to calculate photometric metallicities for the entire catalog. Since radial velocity data is not yet available for most of our halo stars, we perform a kinematics analysis using alternative methods that rely on proper motions and parallax alone: proper motions in Galactic coordinates (l, b) of stars in six patches on the sky (Galactic poles, Galactic center and anticenter, Solar apex and antapex), can be converted to projections in Galactic (U, V, W) velocity space, and additional patches provide a “tomographic” view of their velocity-space distribution. Our results show that the low-mass halo stars share the same kinematic distribution as reported in recent studies of more luminous Galactic halo stars, and notably show the Gaia-Enceladus Stream and the Helmi Stream. We further explore how the velocity-space distribution of low-mass stars changes with metallicity, and tentatively identify possible chemically distinct sub-populations, which could potentially be revealed in greater detail by future Gaia DR3 data.