A Very Large Array Search for Intermediate-Mass Black Holes in Globular
  Clusters in M81
  Nantais et al. used the Hubble Space Telescope to localize probable globular
clusters (GCs) in M81, a spiral galaxy at a distance of 3.63 Mpc. Theory
predicts that GCs can host intermediate-mass black holes (IMBHs) with masses
M_BH \sim 100 - 100,000 M_sun. Finding IMBHs in GCs could validate a formation
channel for seed BHs in the early universe, bolster gravitational-wave
predictions for space missions, and test scaling relations between stellar
systems and the central BHs they host. We used the NRAO Karl G. Jansky Very
Large Array (VLA) to search for the radiative signatures of IMBH accretion from
206 probable GCs in a mosaic of M81. The observing wavelength was 5.5 cm and
the spatial resolution was 1.5 arcsec (26.4 pc). None of the individual GCs are
detected, nor are weighted-mean image stacks of the 206 GCs and the 49 massive
GCs with stellar masses M_star \gtrsim 200,000 M_sun. We apply a semi-empirical
model to predict the mass of an IMBH that, if undergoing accretion in the
long-lived hard X-ray state, is consistent with a given radio luminosity. The
3$\sigma$ radio-luminosity upper limits correspond to mean IMBH masses of
M_BH(all) < 42,000 M_sun for the all-cluster stack and M_BH(massive) < 51,000
M_sun for the massive-cluster stack. We also apply the empirical
fundamental-plane relation to two X-ray-detected clusters, finding that their
individual IMBH masses at 95% confidence are M_BH < 99,000 M_sun and M_BH <
15,000 M_sun. Finally, no analog of HLX-1, a strong IMBH candidate in an
extragalactic star cluster, occurs in any individual GC in M81. This
underscores the uniqueness or rarity of the HLX-1 phenomenon.
