A bottom-heavy initial mass function for the likely-accreted blue-halo stars of the Milky Way

Abstract

We use Gaia DR2 to measure the initial mass function (IMF) of stars within 250 pc and masses in the range of 0.2 < m/M_⊙ < 1.0, separated according to kinematics and metallicity, as determined from Gaia transverse velocity, v_T, and location on the Hertzsprung-Russell diagram (HRD). The predominant thin-disc population (v_T < 40 km s^-1) has an IMF similar to traditional (e.g. Kroupa) stellar IMFs, with star numbers per mass interval dN/dm described by a broken power law, m^-α, and index $\alpha _\textrm {high}=2.03^{+0.14}_{-0.05}$ above m ~ 0.5, shallowing to $\alpha _\textrm {low}=1.34^{+0.11}_{-0.22}$ at m ≲ 0.5. Thick-disc stars (60 km s^-1 <v_T < 150 km s^-1) and stars belonging to the 'high-metallicity' or 'red-sequence' halo (v_T > 100 km s^-1 or v_T > 200 km s^-1, and located above the isochrone on the HRD with a metallicity [M/H] > -0.6) have a somewhat steeper high-mass slope, $\alpha _\textrm {high}=2.35^{+0.97}_{-0.19}$ (and a similar low-mass slope $\alpha _\textrm {low}=1.14^{+0.42}_{-0.50}$). Halo stars from the 'blue sequence', which are characterized by low metallicity ([M/H] < -0.6), however, have a distinct, bottom-heavy IMF, well described by a single power law with $\alpha =1.82^{+0.17}_{-0.14}$ over most of the mass range probed. The IMF of the low-metallicity halo is reminiscent of the Salpeter-like IMF that has been measured in massive early-type galaxies, a stellar population that, like Milky Way halo stars, has a high ratio of α elements to iron, [α/Fe]. Blue-sequence stars are likely the debris from accretion by the Milky Way, ~10 Gyr ago, of the Gaia-Enceladus dwarf galaxy, or similar events. These results hint at a distinct mode of star formation common to two ancient stellar populations - elliptical galaxies and galaxies possibly accreted early-on by ours.