AMUSE-VIRGO. III: mid-infrared photometry of early-type galaxies and limits on obscured nuclear emission

TL;DR

This study uses mid-infrared imaging to investigate low-level nuclear activity in Virgo early-type galaxies, finding that metallicity gradients likely explain MIR excesses rather than active nuclei, indicating low accretion rates.

Contribution

It provides the first comprehensive MIR survey of Virgo early-type galaxies, distinguishing between dust, metallicity effects, and nuclear activity as sources of MIR emission.

Findings

53 out of 95 galaxies detected in MIR

MIR emission is more centrally concentrated than optical light

Most MIR excess explained by metallicity gradients

Abstract

We complete our census of low-level nuclear activity in Virgo Cluster early-type galaxies by searching for obscured emission using Spitzer Space Telescope mid-infrared (MIR) imaging at 24mu. Of a total sample of 95 early-type galaxies, 53 objects are detected, including 16 showing kiloparsec-scale dust in optical images. One dimensional and two dimensional surface photometry of the 37 detections without extended dust features reveals that the MIR light is more centrally concentrated than the optical light as traced by Hubble Space Telescope F850LP-band images. No such modeling was performed for the sources with dust detected in the optical images. We explore several possible sources of the MIR excess emission, including obscured nuclear emission. We find that radial metallicity gradients in the stellar population appear to be a natural and most likely explanation for the observed behavior in a majority of the sources. Alternatively, if the concentrated MIR emission were due to nuclear activity, it would imply a MIR-to-X luminosity ratio ~5-10 for the low luminosity AGN detected in X-rays by our survey. This ratio is an order of magnitude larger than that of typical low-luminosity AGN and would imply an unusual spectral energy distribution. We conclude that the black holes found by our survey in quiescent early-type galaxies in Virgo have low bolometric Eddington ratios arising from low accretion rates and/or highly radiatively inefficient accretion.