FAR-UV PROPERTIES OF THE NUCLEAR REGION OF M31

Comparison of the HST far-UV and visible images of the nucleus of M31 deepens the mystery of the two brightness peaks recently discovered by Lauer et al. [AJ, 106, 1436 (1993)]. At 175 nm the brightest point is the optically fainter peak (P2) that is close to the dynamical center. The very center of P2 has a UV upturn that is much greater than that of the bulge light, while the UV upturn of the optically brighter but off-center P1 is very similar to that of its surroundings. The excess FUV radiation from P2 is closely confined to its center and has a total flux density of 3 μJy, a level that is less likely to come from stellar radiation than from a high-frequency extension of the radio source at the center of M31. A surrounding region of 1.8 pc radius has a somewhat smaller UV upturn than the rest of the bulge, but there is some lingering possibility that this depression might be an artifact of our correction of the spherical aberration. Our improved decomposition of the V image removes the need to postulate a dust lane near the center. We confirm that P1 is very compact, and we derive a luminosity for it similar to that found by Lauer et al. The implications of all this are briefly discussed.