Publication Date:
2019-07-13
Description:
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 〈 or approx. = 0.5 Stellar Mass) 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 〉 or approx. = 8 Stellar Mass) 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/LK) 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/LK 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/LK. We reproduce the result of Peacock et al., strengthening the constraint that the slope of the IMF at m 〉 or approx. = 8 Stellar Mass 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 (alpha1 = 3.84) for stars 〈 0.5 Stellar Mass (as suggested by near-IR indices), and that flattens out (alpha2 = 2.14) for stars 〉 0.5 Stellar Mass, and discuss its wider ramifications and limitations.
Keywords:
Astrophysics
Type:
GSFC-E-DAA-TN45844
,
The Astrophysical Journal (ISSN 2041-8205) (e-ISSN 2041-8213); 835; 2; 183
Format:
text
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