Optical properties of the central region of NGC 1316 - A small bright core in a giant D galaxy

This paper presents new photometric, spectroscopic, and astrometric observations of the central region of NGC 1316 obtained with the 4 m telescope on Cerro Tololo. High-resolution photographic photometry is combined with existing photoelectric data to give brightness profiles that extend from the center to 450" (71 kpc) west. From these profiles, the core appears to be merely the peak of the highly concentrated spheroid; the intrinsic core radius is 0".6 ± 0".2 (95 ± 32 pc) and the central surface brightness V = 13.3 ± 0.3 mag arcsec-2 after proper correction for effects of seeing. A seeing-convolved King model of concentration index C =2.68 ± 0.05 fits the data well over the inner 10 kpc but falls off too steeply beyond, indicating that the galaxy has an even higher concentration index. Fits with an added point source suggest weakly that there may be an unresolved reddened nucleus of V ≳ l5.2(MV ≳ -17.4) buried within the core. However, there is no evidence for any measurable, abnormal mass concentration at the center. The core spectrum shows strong absorption lines typical of an old stellar population, a few weak emission lines from ionized gas, and no sign of any nonthermal source (B ≳ 16.1 mag). Hence the presumed nucleus within the core is now fairly inactive or strongly reddened, or both. The line-of-sight velocity dispersion of 255 ± 15 km s-1 is low for a galaxy of MV = -23.8, implies a core mass of 2.3 (±0.8) × 109 Msun, and gives an exceptionally low M/LB = l.8 ± O.2(M/LB)sun that remains to be explained. A discussion of available observations suggests that the small bright core of NGC 1316 is not abnormal for a giant D galaxy. The opinion that cD cores are generally large and faint lacks solid observational support and seems to be based on the questionable theoretical concept of homologous mergers. The best available theoretical and observational evidence suggests that, on the contrary, merging is nonhomologous. Therefore, the strong concentration of NGC 1316 does not contradict previously found evidence that the galaxy accreted some companions recently. The existing observations indicate that cD cores vary by factors of at least 80 in size (rc =0.1-8 kpc) and 500 in surface brightness. Table 8 summarizes the main parameters of the core, whose center position is α = 3h20m47s209 ± 0s.015, δ = -37°23'08".22 ± 0".23 (equinox 1950.0).