Extra-Nuclear Starbursts: Young Luminous Hinge Clumps in Interacting
  Galaxies
  Hinge clumps are luminous knots of star formation near the base of tidal
features in some interacting galaxies. We use archival Hubble Space Telescope
UV/optical/IR images and Chandra X-ray maps along with GALEX UV, Spitzer IR,
and ground-based optical/near-IR images to investigate the star forming
properties in a sample of 12 hinge clumps in five interacting galaxies. The
most extreme of these hinge clumps have star formation rates of 1 - 9
M(sun)/yr, comparable to or larger than the `overlap' region of intense star
formation between the two disks of the colliding galaxy system the Antennae. In
the HST images, we have found remarkably large (~70 pc) and luminous (M(I) ~
12.2 to -16.5) sources at the centers of these hinge clumps, sometimes embedded
in a linear ridge of fainter star clusters. We have found strong X-ray emission
from several of these hinge clumps. In most cases, this emission is
well-resolved with Chandra and has a thermal X-ray spectrum, thus it is likely
due to hot gas associated with the star formation. The ratio of the
extinction-corrected diffuse X-ray luminosity to the mechanical energy rate
(the X-ray production efficiency) for the hinge clumps is similar to that in
the Antennae galaxies, but higher than those for regions in the normal spiral
galaxy NGC 2403. Two of the hinge clumps have point-like X-ray emission much
brighter than expected for hot gas; these sources are likely `ultra-luminous
X-ray sources' (ULXs) due to accretion disks around black holes. The most
extreme of these sources, in Arp 240, has a hard X-ray spectrum and an absorbed
X-ray luminosity of ~2 X 10^41 erg/s; more than expected by single high mass
X-ray binaries (HMXBs), thus it may be either a collection of HMXBs or an
intermediate-mass black hole (>=80 M(sun)). [ abridged ]
