Kinematics, ages, and metallicities of star clusters in NGC 1316: A 3 Gyr old merger remnant

Abstract

We report on multi-object spectroscopy of 37 candidate star clusters around the giant early-type radio galaxy NGC 1316 (Fornax A), the brightest galaxy in the Fornax cluster. For the star clusters in this sample, we measure a mean radial velocity of 1698 +/- 46 km/s, a rotation velocity of 175 +/- 70 km/s, and a velocity dispersion of 227 +/- 33 km/s within a galactocentric radius of 24 kpc. Using the projected mass estimator and assuming isotropic orbits, the estimated total mass is (6.6 +/- 1.7) E+11 solar masses within a radius of 24 kpc. The mass is uncertain by about a factor of two, depending on the orbital assumptions. The implied M/L_B ratio is in the range 3-6. Four star clusters in our sample are exceptionally luminous (M_V < -12.3). This means that (1) at least this many clusters in NGC 1316 are up to an order of magnitude more luminous than the most luminous star cluster in our Galaxy or M31, and (2) that the S/N ratio of their spectra allows us to measure line strengths with good accuracy. By comparing the colours and equivalent widths of H-alpha and the Ca II triplet absorption lines for those bright star clusters in our sample with those of single-burst population models, we find that they are coeval with an age of 3.0 +/- 0.5 Gyr. Their metallicities are found to be solar to within 0.15 dex. These results show once again that globular clusters with near-solar metallicity do form during galactic mergers, and, moreover, that they can survive disruption processes taking place during the merger (e.g., dynamical friction, tidal disruption), as well as evaporation. In this respect, NGC 1316 provides a hitherto "missing" evolutionary link between young merger remnants of age ~0.5 Gyr such as NGC 3597, NGC 3921 and NGC 7252 on one side, and older giant ellipticals featuring bimodal colour distributions on the other side.