The globular cluster system of NGC 1316 IV. Nature of the star cluster
  complex SH2
  The light of the merger remnant NGC 1316 is dominated by old and
intermediate-age stars. The only sign of current star formation in this big
galaxy is the HII region SH2, an isolated star cluster complex with a ring-like
morphology and an estimated age of 0.1 Gyr at a galactocentric distance of
about 35 kpc. A nearby intermediate-age globular cluster, surrounded by weak
line emission and a few more young star clusters, is kinematically associated.
The origin of this complex is enigmatic. The nebular emission lines permit a
metallicity determination which can discriminate between a dwarf galaxy or
other possible precursors. We used the Integrated Field Unit of the VIMOS
instrument at the Very Large Telescope of the European Southern Observatory to
study the morphology, kinematics, and metallicity employing line maps, velocity
maps, and line diagnostics of a few characteristic spectra. The line ratios of
different spectra vary, indicating highly structured HII regions, but define a
locus of uniform metallicity. The strong-line diagnostic diagrams and empirical
calibrations point to a nearly solar or even super-solar oxygen abundance. The
velocity dispersion of the gas is highest in the region offset from the bright
clusters. Star formation may be active on a low level. There is evidence for a
large-scale disk-like structure in the region of SH2, which would make the
similar radial velocity of the nearby globular cluster easier to understand.
The high metallicity does not fit to a dwarf galaxy as progenitor. We favour
the scenario of a free-floating gaseous complex having its origin in the merger
2 Gyr ago. Over a long period the densities increased secularly until finally
the threshold for star formation was reached. SH2 illustrates how massive star
clusters can form outside starbursts and without a considerable field
population.
