Stellar Photometry in the Inner Bulge of M31 Using the Hubble Space Telescope Wide Field Camera

Abstract

We present photometry of two fields in the M31 bulge imaged with the Hubble Space Telescope (HST) Wide-Field Camera. The nuclear field (r <40" = 150 pc) giant branch extends to I = 19.5, M_I_ = - 5 (Cousins system), a full 0.9 mag brighter than the giant-branch tips of metal-poor Galactic globular clusters and M31 halo fields. This is also ~1.5 mag brighter than the giant branches of metal-rich Galactic globular clusters, but is no brighter than Mould's (1986) M31 bulge field 1 kpc from the nucleus. The data also suggest that the brightest stars may be preferentially concentrated to the center. The 648 luminous stars detected in 2 x 10^9^ L_sun_ is ~25% that expected from a hypothetical population of evolved asymptotic giant branch (AGB) stars with lifetimes ~10^5^ yr, with the cautionary note that we are near the detection limit. The number of bright stars is also consistent with the progeny of blue stragglers, if one uses a lifetime for the thermal-pulsing AGB of 2 x 10^6^ yr. We strongly caution that incompleteness becomes severe below I = 19.9 mag and that future surveys are likely to find numbers of bright stars too large to accommodate the blue straggler progeny hypothesis. We have imaged an additional field 2' = 500 pc south of the nucleus. The brightest stars in this field are also I = 19.5, but bright stars appear less numerous than in the nuclear field. If the population resembles that of the Galactic bulge, then M_bol_ = - 4.5 is a lower limit to the giant-branch tip luminosity; infrared studies should reveal stars 0.5 mag or more brighter. Either high-metallicity or (more likely) age~10 Gyr may be responsible for the presence of these luminous AGB stars. These observations confirm that previous groundbased infrared studies (e.g., Rich & Mould 1991) very likely detect an extended giant branch and not spurious luminous stars caused by crowding or disk contamination. However, published integrated colors for the M31 bulge/nucleus are extremely red, making it difficult to accommodate a young or intermediate-age population.