Hubble Space Telescope Planetary Camera Images of NGC 1316 (Fornax A)

June 1996 • 1996AJ....111.2212S

Authors • Shaya, E. J. • Dowling, D. M. • Currie, D. G. • Faber, S. M. • Ajhar, E. A. • Lauer, T. R. • Groth, E. J. • Grillmair, C. J. • Lynd, R. • O'Neil, E. J., Jr.

Abstract • We present HST Planetary Camera V and I band images of the central region of the peculiar giant elliptical galaxy NGC 1316. These high resolution images reveal that the central surface brightness rises sharply to 12.1 mag arcsec^-2^ in the I band and 13.5 mag arcsec^-2^ in the V band. The inner profile is well fit by a nonisothermal core model with a core radius of 0.41" +/- 0.02" (34 pc). At an assumed distance of 16.9 Mpc, the deprojected luminosity density reaches ~2.0 x 10^3^ L_sun_ pc^-3^. Outside the inner two or three arcseconds, a constant mass-to-light ratio of ~2.2+/-0.2 is found to fit the observed line width measurements. The line width measurements of the center indicate the existence of either a central dark object of mass 2 x 10^9^ M_sun_, an increase in the stellar mass-to-light ratio by at least a factor of 2 for the inner few arcseconds, or perhaps increasing radial orbit anisotropy towards the center. The mass-to-light ratio run in the center of NGC 1316 resembles that of many other giant ellipticals, some of which are known from other evidence to harbor central massive dark objects (MDOs). The VI color of unreddened regions is found to be uniform at 1.55+/-0.10. The profile does not get significantly bluer near the center as might be expected in a merger, except for perhaps the innermost 0.1". Fits to the unextinguished regions at the center raise concerns that the possible UV- bright point source reported by Fabbiano et al. [ApJ, 434, 67 (1995)] is partially or fully explained by dust clouds wrapped around the central line of sight. The smoothness of the underlying stellar distribution allows analysis of the three-dimensional distribution of the dust. We use two observables; the color excess and the ratio of observed V band flux to that of a symmetric smooth fit. The maximum optical depth is only A_V_ ~ 1.5, and only complexes on the near side of the galaxy are detected. We also examine twenty globular clusters associated with NGC 1316 and report their brightnesses, colors, and limits on tidal radii. The brightest cluster has a luminosity of 9.9 x 10^6^ L_sun_ (M_v_ = -12.7), and the faintest detectable cluster has a luminosity of 2.4 x 10^5^ L_sun_ (M_V_ = -8.6). The globular clusters are just barely resolved, but their core radii are too small to be measured. The tidal radii in this region appear to be <= 35 pc. Although this galaxy seems to have undergone a substantial merger in the recent past, young globular clusters are not detected.

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Carl Grillmair

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