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NGC 1316: A Merger in Progress (1Jun95) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 42, June 1995) Glen Mackie and Giuseppina Fabbiano (Harvard-Smithsonian Center for Astrophysics) used the CTIO Schmidt telescope to show that the elliptical galaxy NGC 1316 may be the site of a galaxy-merger in progress. Narrow-band images obtained by the two astronomers identify a region of extended emission that may be the gaseous remnant of a low-mass galaxy cannibalized by NGC 1316. The "fuel" provided by this event, or other ones like it, may help power the nuclear activity in NGC 1316. [Photo not included] The southwest quadrant of NGC 1316. Grayscale depicts residual B band light after model galaxy subtraction. The main feature is the tidal tail L_1. Contours show H alpha + [NII] line emission, with the EELR indicated. The line emission was derived from narrowband exposures centered on 6606 and 6693 Angstroms. NGC 1316 sits at the heart of the Fornax group of galaxies. It has unusual morphology, featuring strong central dust bands, as well as low surface brightness shells and tidal tails at large radii. A gas disk appears to be associated with the dust lane in the nucleus. NGC 1316 also exhibits low-ionization nuclear emission, an unresolved UV bright nucleus, and a steep spectrum radio core with dual-opposing jets. On the larger scale, a double-lobe radio continuum source with a diameter of ~35' extends well beyond the galaxy in the optical. All of these features may point to a common cause---recent mergers of gas-rich galaxies. This possibility was raised by Schweizer (1980 ApJ, 237, 303), who presented extensive optical observations of NGC 1316, and argued that this galaxy may have had several low-mass, gas-rich mergers over the last 2 X 10^9 yrs. Mackie and Fabbiano obtained deep B-band and narrowband Ha + [N II] emission line images, aided by the 30' field provided by the CTIO Schmidt. They discovered an extended (81" X 27" or 9 X 3 kpc) emission line region (EELR) at a projected distance of 35 kpc from the nucleus of NGC 1316 (see the figure). Intriguingly, the EELR is located at the base of a ~90 X 35 kpc tidal tail (L_1 in Schweizer 1980) that has a smooth light distribution and is morphologically consistent with the stellar remnant of a low mass merger (Hernquist and Quinn 1989 ApJ, 342, 1). The extreme length of L_1 and its narrowness argue for a disk progenitor. The EELR's major axis is roughly perpendicular to the predominant direction of L_1. The discovery of the EELR supports the merger scenario, but raises new questions on the nature of gas interaction during such events. The large size and position of EELR argues against it being a giant H II region, and probably connects it with the merger event. However, simulations of low-mass mergers (Weil and Hernquist 1993, ApJ, 405, 142) suggests that the separate stellar and gaseous components of low-mass mergers rapidly segregate, with the gas flowing quickly into the nucleus of the primary galaxy. The smooth light distribution and extent of L_1 suggests that it is substantially evolved, however. L_1 is estimated to be ~5 X 10^8 yrs old. This might imply that any gas intrinsic to its progenitor should already reside in the nucleus of NGC 1316. There is no evidence for a concentration of residual B-band light near the EELR. NGC 1316 is not a strong X-ray source, but inspection of ROSAT X-ray images shows that the EELR is positioned on the edge of an extended region of broadband (0.2-2.4 keV) emission extending away from the main nuclear X-ray emission of NGC 1316. This suggests that the EELR is not only specially positioned in the tidal tail, but also positioned at a hot gas interface region. Based on the above, the most likely ionization source of EELR is shock excitation (predicted by Schweizer (1980) based solely on the existence of dust lanes that occur 1' from the EELR), although optical spectroscopy will be required to verify this hypothesis. The Mackie and Fabbiano observations may imply that we are witnessing a galaxy merger at a unique time in its evolution. The morphology and optical luminosity of the tidal tail imply the progenitor was at least a 3 X 10^9 L_o disk galaxy. The optical properties of NGC 1316 suggest that the recent frequency of infall or merger events is 1 per 5 X 10^8 yrs or higher. This adds weight to the possibility that gas-rich mergers provide fuel for nuclear activity. The discovery of the EELR was due to the wide, ~0.5d field of view of the CTIO Schmidt coupled with the excellent quantum efficiency its CCD detector. It is appropriate that the CTIO Schmidt was used for this discovery, as it was also used almost 20 years ago by Schweizer in his pioneering study of NGC 1316.
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