Testing the Universality of the Stellar IMF with Chandra and HST
  The stellar initial mass function (IMF), which is often assumed to be
universal across unresolved stellar populations, has recently been suggested to
be "bottom-heavy" for massive ellipticals. In these galaxies, the prevalence of
gravity-sensitive absorption lines (e.g. Na I and Ca II) in their near-IR
spectra implies an excess of low-mass ($m <= 0.5$ $M_\odot$) stars over that
expected from a canonical IMF observed in low-mass ellipticals. A direct
extrapolation of such a bottom-heavy IMF to high stellar masses ($m >= 8$
$M_\odot$) would lead to a corresponding deficit of neutron stars and black
holes, and therefore of low-mass X-ray binaries (LMXBs), per unit near-IR
luminosity in these galaxies. Peacock et al. (2014) searched for evidence of
this trend and found that the observed number of LMXBs per unit $K$-band
luminosity ($N/L_K$) was nearly constant. We extend this work using new and
archival Chandra X-ray Observatory (Chandra) and Hubble Space Telescope (HST)
observations of seven low-mass ellipticals where $N/L_K$ is expected to be the
largest and compare these data with a variety of IMF models to test which are
consistent with the observed $N/L_K$. We reproduce the result of Peacock et al.
(2014), strengthening the constraint that the slope of the IMF at $m >= 8$
$M_\odot$ must be consistent with a Kroupa-like IMF. We construct an IMF model
that is a linear combination of a Milky Way-like IMF and a broken power-law
IMF, with a steep slope ($\alpha_1=$ $3.84$) for stars < 0.5 $M_\odot$ (as
suggested by near-IR indices), and that flattens out ($\alpha_2=$ $2.14$) for
stars > 0.5 $M_\odot$, and discuss its wider ramifications and limitations.
