Abstract
We report on multi-object spectroscopy in the red spectral region of 37 candidate star clusters in an ∼8×8 arcmin2 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 vhel = 1698±46 km s−1 and a velocity dispersion σ = 227±33 km s−1 within a galactocentric 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±1.7)×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/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 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 ±0.15 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 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.
Highlights
1.1 Extragalactic globular cluster systemsRecent observations with the Hubble Space Telescope (HST) and large-field ground-based CCD cameras have caused rapid advances in our knowledge of the formation and evolution of star cluster systems of galaxies
Well-known aspect of star cluster systems among galaxies is that the number of star clusters per unit galaxy luminosity increases systematically from late-type to early-type galaxies, being,2±3 times higher in elliptical (E) than in spiral galaxies of type Sb and later (Harris & van den Bergh 1981; Harris 1991)
It seems that star clusters in general may form preferentially in high-density regions of starbursts (e.g. Meurer et al 1995), probably from giant molecular clouds whose collapse is being triggered by a 100- to 1000-fold increase in gas pressure due to supernova and shock heating during starbursts (e.g. Elmegreen & Efremov 1997)
Summary
Recent observations with the Hubble Space Telescope (HST) and large-field ground-based CCD cameras have caused rapid advances in our knowledge of the formation and evolution of star cluster systems of galaxies. It is possible that the higher specific frequency of globular clusters in ellipticals relative to that in spirals may be accounted for by secondary populations of globular clusters created during gas-rich mergers In this respect, one interesting feature of the star cluster systems of many giant ellipticals is the presence of bimodal colour distributions, providing clear evidence for the occurrence of asecond event' in the formation of these systems Zepf & Ashman 1993; Whitmore et al 1995; Geisler, Lee & Kim 1996; Kissler-Patig et al 1997; Carlson et al 1999) While such a bimodal colour distribution was predicted from merger models of E galaxy formation by Ashman & Zepf (1992), opinions about the general nature of thesecond event' differ among authors (see detailed review by Ashman & Zepf 1998). The present paper describes new spectroscopic observations aimed at deriving kinematics, ages and metallicities for a suitable sample of star clusters in the merger remnant NGC 1316
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