The globular cluster system of NGC 1316 IV. Nature of the star cluster complex SH2

Abstract

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.