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

We report on multi-object spectroscopy in the red spectral region of 37 can didate star clusters in an similar to8 x 8 arcmin(2) field centred on the g iant early-type radio galaxy NGC 1316 (Fornax A), the brightest galaxy in t he Fornax cluster. Out of this sample, 24 targets are found to be genuine s tar clusters associated with NGC 1316, and 13 targets are Galactic foregrou nd stars, For the star cluster sample, we measure a mean heliocentric veloc ity upsilon (hel) = 1698 +/- 46 km s(-1) and a velocity dispersion sigma = 227 +/- 33 km s(-1) within a galactocentric radius of 24 kpc. Partly respon sible for the velocity dispersion is a significant rotation in the star clu ster system, with a mean velocity of similar to 175 +/- 70 km s(-1) along a position angle of similar to6 degrees +/- 18 degrees. Using the projected mass estimator and assuming isotropic orbits, the estimated total mass is ( 6.6 +/- 1.7) x 10(11) M. within a radius of 24 kpc, The mass is uncertain b y about a factor of 2, depending on the orbital assumptions. The implied M/ L-B ratio is in the range 3-6, Four star clusters in our sample are excepti onally luminous (M-V < -12.3). This means that (1) at least this many clust ers in NGC 1316 are up to an order of magnitude more luminous than the most luminous star cluster in our Galaxy or M31, and that (2) the SM ratio of t heir spectra allows us to measure line strengths with good accuracy. By com paring the measured colours and equivalent widths of H<alpha> and the CaII triplet (lambda lambda 8498, 8542, 8662 Angstrom) absorption lines for thos e bright star clusters in our sample with those of single-burst population models (the Bruzual & Chariot models), we find that they are coeval with an age of 3.0 +/- 0.5 Gyr. Their metallicities are found to be solar to withi n +/-0.15 dex. We discuss the properties of the main body of NGC 1316, and conclude that they are consistent with having hosted a major merger 3 Gyr a go as well. The presence of intermediate-age globular clusters in NGC 1316 shows once again that globular clusters with near-solar metallicity do form during galactic mergers, and, moreover, that they can survive disruption p rocesses taking place during the merger (e.g., dynamical friction, tidal di sruption), as well as evaporation. In this respect, NGC 1316 provides a hit herto 'missing' evolutionary link between young merger remnants of age simi lar to0.5 Gyr such as NGC 3597, 3921 and 7252 on one side and older giant e llipticals featuring bimodal colour distributions on the other side.