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Evolution of Gas and Stars in the Merger Galaxy NGC 1316 (Fornax A)

We present optical and archival X-ray data on the disturbed-morphology radio elliptical galaxy NGC 1316 (Fornax A), which displays numerous low surface brightness shells, loops, and tails. An extended (81″ × 27″, or 9 kpc × 3 kpc) emission-line region (EELR) at a projected distance of 35 kpc from the nucleus has been discovered in an approximately 90 kpc × 35 kpc, 3.0 × 109 LB⊙ tidal tail. The position and extreme size of the EELR suggest that it is related to the merger process. We suggest that the ionization mechanism of the EELR is shock excitation and that the gas is a remnant from the merger progenitor. X-ray emission is detected near two tidal tails. Hot, ∼5 × 106 K, gas is probably the predominant gas component in the tidal-tail interstellar medium. However, based on the current tidal-tail (cold + warm + hot) gas mass, a large fraction of the progenitor gas may already reside in the nucleus of NGC 1316. The numerous and varied tidal-tail system suggests that a disk-disk or disk-E merger could have taken place ≥1 Gyr ago, while a low-mass, gas-rich galaxy would have started to merge ∼0.5 Gyr ago.