Detection of Enhanced Central Mass-to-Light Ratios in Low-Mass Early-Type Galaxies: Evidence for Black Holes?

TL;DR

This study measures central mass-to-light ratios in low-mass early-type galaxies, finding enhancements likely due to black holes or IMF variations, with some galaxies hosting over-massive black holes, supporting galaxy stripping scenarios.

Contribution

It provides the first dynamical measurements of central M/L ratios in low-mass early-type galaxies and links these to black hole presence and galaxy evolution.

Findings

Central M/L ratios are higher than at larger radii in 80% of galaxies.

Black hole masses are estimated, with some galaxies hosting over-massive black holes.

M/L enhancements are consistent with those in ultracompact dwarf galaxies.

Abstract

We present dynamical measurements of the central mass-to-light ratio ($M/L$) of a sample of 27 low-mass early-type ATLAS$^{3D}$ galaxies. We consider all ATLAS$^{3D}$ galaxies with 9.7$<$log(M$_\star/$M$_\odot)$$<$10.5 in our analysis, selecting out galaxies with available high-resolution Hubble Space Telescope (HST) data, and eliminating galaxies with significant central color gradients or obvious dust features. We use the HST images to derive mass models for these galaxies and combine these with the central velocity dispersion values from ATLAS$^{3D}$ data to obtain a central dynamical $M/L$ estimate. These central dynamical $M/L$s are higher than dynamical $M/L$s derived at larger radii and stellar population estimates of the galaxy centers in $\sim$80\% of galaxies, with a median enhancement of $\sim$14\% and a statistical significance of 3.3$\sigma$. We show that the enhancement in the central $M/L$ is best described either by the presence of black holes in these galaxies or by radial IMF variations. Assuming a black hole model, we derive black hole masses for the sample of galaxies. In two galaxies, NGC 4458 and NGC 4660, the data suggests significantly over-massive BHs, while in most others only upper limits are obtained. We also show that the level of $M/L$ enhancements we see in these early-type galaxy nuclei are consistent with the larger enhancements seen in ultracompact dwarf galaxies (UCDs), supporting the scenario where massive UCDs are created by stripping galaxies of these masses.