2017ApJ...835..183C

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 0.5M_☉) 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 8M_☉) 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. 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 and Hubble Space Telescope 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., strengthening the constraint that the slope of the IMF at m 8M_☉ 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 (α₁=3.84) for stars <0.5 M_☉ (as suggested by near-IR indices), and that flattens out (α₂=2.14) for stars >0.5 M_☉, and discuss its wider ramifications and limitations.