A Stacked Search for Intermediate-Mass Black Holes in 337 Extragalactic
  Star Clusters
  Forbes et al. recently used the Hubble Space Telescope to localize hundreds
of candidate star clusters in NGC 1023, an early-type galaxy at a distance of
11.1 Mpc. Old stars dominate the light of 92% of the clusters and
intermediate-age stars dominate the light of the remaining 8%. Theory predicts
that clusters with such ages can host intermediate-mass black holes (IMBHs)
with masses M_BH \lesssim 10^5 M_sun. To investigate this prediction, we used
264 s of 5.5 GHz data from the Karl G. Jansky Very Large Array (VLA) to search
for the radiative signatures of IMBH accretion from 337 candidate clusters in
an image spanning 492 arcsec (26 kpc) with a resolution of 0.40 arcsec (22 pc).
None of the individual clusters are detected, nor are weighted-mean image
stacks of the 311 old clusters, the 26 intermediate-age clusters, and the 20
clusters with stellar masses M_star \gtrsim 7.5 x 10^5 M_sun. The clusters thus
lack radio analogs of HLX-1, a strong IMBH candidate in a cluster in the
early-type galaxy ESO 243-49. This suggests that HLX-1 is accreting gas related
to its cluster's light-dominating young stars. Alternatively, the HLX-1
phenomenon could be so rare that no radio analog is expected in NGC 1023. Also,
using a formalism heretofore applied to star clusters in the Milky Way, the
radio-luminosity upper limit for the massive-cluster stack corresponds to a
mean 3$\sigma$ IMBH mass of M_BH(massive) < 2.3 x 10^5 M_sun, suggesting mean
black-hole mass fractions of M_BH(massive)/M_star < 0.05-0.29.
