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