Discovery of the Remnant of S Andromedae (SN 1885) in M31

We have discovered the long sought after remnant of the celebrated 1885 supernova, commonly known as S Andromedae, which occurred in the nuclear bulge of M31. Optical CCD images of M31's nuclear bulge taken with a narrow-passband filter centered on the Fe I resonance line at 3860 A show an unresolved spot of obscuration located within 1" of SN 1885's reported position. The absorption feature does not show on images taken using a broader filter centered farther to the blue at 3476 A, nor does it correspond to any previously reported dust cloud in M31's nuclear region. Our detection of SN 1885's remnant in Fe I absorption suggests that (1) the remnant contains appreciable iron-rich ejecta, consistent with a Type I classification; (2) the ejecta are still largely cold and freely expanding; (3) S And's Fe 1 absorption-line profiles must be quite broad, ~ 125 A; and (4) S And probably occurred on the side of M31's bulge nearer to us. Future absorption-line observations of S And have the potential of becoming among the most informative observations ever taken of a Type I supernova.