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 candidate star clusters in an ∼8 × aarcmin² field centred on the giant early-type radio galaxy NGC 1316 (Fornax A), the brightest galaxy in the Fornax cluster. Out of this sample, 24 targets are found to be genuine star clusters associated with NGC 1316, and 13 targets are Galactic foreground stars. For the star cluster sample, we measure a mean heliocentric velocity υ_hel = 1698 ± 46 km s^-1 and a velocity dispersion σ = 227 ± 33 km s^-1 within a galacto-centric radius of 24 kpc. Partly responsible for the velocity dispersion is a significant rotation in the star cluster system, with a mean velocity of ∼175 ± 70 km s^-1 along a position angle of ∼6° ± 18°. Using the projected mass estimator and assuming isotropic orbits, the estimated total mass is (6.6 ± 1.V) × 10¹¹ M_⊙ within a radius of 24 kpc. The mass is uncertain by 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 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 that (2) the S/N ratio of their spectra allows us to measure line strengths with good accuracy. By comparing the measured colours and equivalent widths of Hα and the Ca II triplet (λλ 8498, 8542, 8662 Å) absorption lines for those bright star clusters in our sample with those of single-burst population models (the Bruzual & Charlot 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 ±0.15dex. 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 ago 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 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, 3921 and 7252 on one side and older giant ellipticals featuring bimodal colour distributions on the other side.