A bottom-heavy initial mass function for the accreted blue-halo stars of the Milky Way
Na'ama Hallakoun, Dan Maoz
We use Gaia DR2 to measure the initial mass function (IMF) of stars within
250 pc and masses in the range 0.2 < m/Msun < 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) has an IMF similar to traditional (e.g.
Kroupa 2001) stellar IMFs, with star numbers per mass interval dN/dm described
by a broken power law, m^alpha, and index alpha_high=-1.99 +0.05/-0.11 above
m~0.5, shallowing to alpha_low=-1.26 +0.12/-0.13 at m~<0.5. Thick-disc stars
(60 km/s < v_T < 150 km/s) and stars belonging to the "high-metallicity" or
"red-sequence" halo (v_T > 100 km/s or v_T > 200 km/s, and located above the
isochrone on the HRD with metallicity [M.H] > -0.6) have a somewhat steeper
high-mass slope, alpha_high=-2.31 +0.30/-0.81 (and a similar low-mass slope
alpha_low=-1.05 +0.21/-0.65). Halo stars from the "blue sequence", which are
characterised by low-metallicity ([M/H] < -0.6) , however, have a distinct,
bottom-heavy IMF, well-described by a single power law with alpha=-2.17
+0.10/-0.17 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 alpha elements to iron, [alpha/Fe]. Blue-sequence stars are
likely the debris from accretion by the Milky Way, ~10 Gyrs ago, of a
moderate-mass galaxy or galaxies. These results hint at a distinct mode of star
formation common to two ancient stellar populations---elliptical galaxies and
galaxies accreted early-on by ours.