Potential Black Hole Seeding of the Spiral Galaxy NGC 4424 via an
  Infalling Star Cluster
  Galaxies can grow through their mutual gravitational attraction and
subsequent union. While orbiting a regular high-surface-brightness galaxy, the
body of a low-mass galaxy can be stripped away. However, the stellar heart of
the infalling galaxy, if represented by a tightly-bound nuclear star cluster,
is more resilient. From archival Hubble Space Telescope images, we have
discovered a red, tidally-stretched star cluster positioned ~5 arcseconds (~400
pc in projection) from, and pointing toward the center of, the post-merger
spiral galaxy NGC 4424. The star cluster, which we refer to as `Nikhuli', has a
near-infrared luminosity of (6.88+/-1.85)x10^6 L_{solar,F160W} and likely
represents the nucleus of a captured/wedded galaxy. Moreover, from our Chandra
X-ray Observatory image, Nikhuli is seen to contain a high-energy X-ray point
source, with L_{0.5-8 keV} = 6.31^{+7.50}_{-3.77}x10^{38} erg/s (90%
confidence). We argue that this is more likely to be an active massive black
hole than an X-ray binary. Lacking an outward-pointing comet-like appearance,
the stellar structure of Nikhuli favors infall rather than the ejection from a
gravitational-wave recoil event. A minor merger with a low-mass early-type
galaxy may have sown a massive black hole, aided an X-shaped pseudobulge, and
be sewing a small bulge. The stellar mass and the velocity dispersion of NGC
4424 predict a central black hole of (0.6-1.0)x10^5 M_solar, similar to the
expected intermediate-mass black hole in Nikhuli, and suggestive of a black
hole supply mechanism for bulgeless late-type galaxies. We may potentially be
witnessing black hole seeding by capture and sinking, with a nuclear star
cluster the delivery vehicle.
