Bottom-heavy initial mass function in a nearby compact l{star} galaxy.

Abstract

We present orbit-based dynamical models and stellar population analysis of Sloan Digital Sky Survey J151741.75-004217.6, a low-redshift (z = 0.116) early-type galaxy (ETG) which, for its moderate luminosity, has an exceptionally high velocity dispersion. We aim to determine the central black hole (BH) mass (M_•), the i-band stellar mass-to-light ratio (Υ_{*, i}) and the slope of the initial mass function (IMF). Combining constraints from Hubble Space Telescope imaging and long-slit kinematic data with those from fitting the spectrum with stellar populations models of varying IMFs, we show that this galaxy has a large fraction of low-mass stars, significantly higher than implied even by a Salpeter IMF. We exclude a Chabrier/Kroupa as well as a unimodal (i.e. single-segment) IMF, while a bimodal (low-mass tapered) shape is consistent with the dynamical constraints. Thereby, our study demonstrates that a very bottom-heavy IMF can exist even in an L* ETG. We place an upper limit of 10^10.5 M_⊙ on M_•, which still leaves open the possibility of an extremely massive BH.