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

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, vT, and location on the Hertzsprung-Russell diagram (HRD). The predominant thin-disc population (vT < 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 αhigh=2.03+0.14-0.05 above m ∼0.5, shallowing to αlow=1.34+0.11-0.22 at m ≤ 0.5. Thick-disc stars (60 km s-1 <vT < 150 km s-1) and stars belonging to the 'high-metallicity' or 'red-sequence' halo (vT > 100 km s-1 or vT > 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, α high=2.35+0.97-0.19 (and a similar low-mass slope α 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 α = 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.