Bottom-heavy initial mass function in a nearby compact L*-galaxy

Abstract

We present orbit-based dynamical models and stellar population analysis of galaxy SDSS 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 mass (M_bh), the i-band stellar mass-to-light ratio, and the low-mass slope of the initial mass function (IMF). Combining constraints from HST imaging and longslit kinematic data with those from fitting the SDSS spectrum with stellar populations models of varying IMF, 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_sun on M_bh, which still leaves open the possibility of an extremely massive BH.