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P. Goudfrooij (ESA/STScI), M. Kissler--Patig, G. Meylan (ESO), D. Minniti (PUC)
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, using the ESO 3.5-m New Technology Telescope.
For the star cluster sample, we measure a mean heliocentric velocity v\rm hel = 1698 ±46 km s-1, a rotation velocity v\rm rot = 175 ±70 km s-1 and a velocity dispersion \sigma = 227 ±33 km s-1. Using the projected mass estimator and assuming isotropic orbits, the estimated total mass is (6.6 ±1.7) \times 1011 M\odot within a radius of 24 kpc. The mass is uncertain by about a factor of two, depending on the orbital assumptions. The implied \cal{M}/LB ratio is in the range 3\,--\,6.
Four star clusters in our sample are exceptionally luminous (MV < -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 M\,31, and (2) that 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 H\alpha and the Ca\,{\sc ii} triplet (\lambda\lambda\,8498,\,8542,\,8662 Å) absorption lines for those bright star clusters in our sample with those of single-burst population models (of Bruzual & Charlot), 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. We discuss the properties of the main body of NGC~1316 and conclude 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 galaxy 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.
The author(s) of this abstract have provided an email address for comments about the abstract: goudfroo@stsci.edu