A bottom-heavy initial mass function for the accreted blue-halo stars of the Milky Way. (arXiv:2009.05047v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hallakoun_N/0/1/0/all/0/1">Na'ama Hallakoun</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maoz_D/0/1/0/all/0/1">Dan Maoz</a>
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.
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.
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