A New Sample of (Wandering) Massive Black Holes in Dwarf Galaxies from
High Resolution Radio Observations
We present a sample of nearby dwarf galaxies with radio-selected accreting
massive black holes (BHs), the majority of which are non-nuclear. We observed
111 galaxies using sensitive, high-resolution observations from the Karl G.
Jansky Very Large Array (VLA) in its most extended A-configuration at X-band
(~8-12 GHz), yielding a typical angular resolution of ~0.25" and rms noise of
~15 uJy. Our targets were selected by cross matching galaxies with stellar
masses M_stellar < 3 x 10^9 M_sun and redshifts z<0.055 in the NASA-Sloan Atlas
with the VLA Faint Images of the Radio Sky at Twenty centimeters (FIRST)
Survey. With our new high-resolution VLA observations, we detect compact radio
sources towards 39 galaxies and carefully evaluate possible origins for the
radio emission including thermal HII regions, supernova remnants, younger radio
supernovae, background interlopers, and AGNs in the target galaxies. We find
that 13 dwarf galaxies almost certainly host active massive BHs despite the
fact that only one object was previously identified as having optical
signatures of an AGN. We also identify a candidate dual radio AGN in a more
massive galaxy system. The majority of the radio-detected BHs are offset from
the center of the host galaxies with some systems showing signs of
interactions/mergers. Our results indicate that massive BHs need not always
live in the nuclei of dwarf galaxies, confirming predictions from simulations.
Moreover, searches attempting to constrain BH seed formation using observations
of dwarf galaxies need to account for such a population of "wandering" BHs.