The stellar population and luminosity function in M31 bulge and Inner Disk Fields

Abstract

Listen

Translate article

view Abstract Citations (36) References (56) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Stellar Population and Luminosity Function in M31 Bulge and Inner Disk Fields Rich, R. Michael ; Mould, J. R. ; Graham, James R. Abstract We report infrared photometry and stellar identifications for stars in five fields in the M31 bulge located from 2 to 11 arcmin from the nucleus. These fields have been chosen such that the bulge/disk star ratio predicted from Kent's [AJ, 97, 1614(1989)] small bulge model varies from 7:1 to 1:5, allowing a study of near pure disk and near pure bulge stellar populations. We reject the hypothesis of Davies et al. [AJ, 102, 1729 (1991)] that luminous stars found within 500 pc of the nucleus are due to a contaminating disk population. We find that the bulge contains stars in excess of M_bol_ = - 5 mag and that the bulge luminosity function has a distinct shape different from the disk fields. We find many stars redder than (J-K) =2 mag, and suggest that these stars may be the counterparts of the IRAS-selected Galactic bulge Miras studied by Whitelock et al. [MNRAS, 248, 276 (1991)]. The number of bright stars (M_bol_ < -5 mag) falls off more rapidly than the r band surface brightness. By building model fields out of a bulge luminosity function and artificial stars, we are able to show that the change in the luminosity function toward the center cannot be explained simply by the mismeasurement of overcrowded star images. However, these tests also raise the possibility that the asymptotic giant branch (AGB) tip may be ~1 mag fainter than actually measured in our most crowded field, reaching only M_bol_ = -5. We compare observed counts of AGB stars with those predicted from theoretical lifetimes using a technique of general interest for this problem, the Fuel Consumption Theorem [Renzini & Buzzoni, 1986, Spectral Evolution of Galaxies (Reidel, Dordrecht)]. Our methodology is generally applicable to the study of other resolved extragalactic stellar populations. The number of observed stars per magnitude up to a luminosity of M_bol_ = - 5.5 mag is consistent with AGB evolution of the whole population of the innermost bulge field with the standard lifetime on the AGB of 1.3 Myr/mag. Blue straggler progeny [Renzini & Greggio 1990, Bulges of Galaxies (ESO, Garching), p. 47] do not make a major contribution to this luminous population. Two fields 500 pc from the nucleus are similar to the bulge luminosity function of Frogel & Whitford [ApJ, 320, 199 (1987)) if brightened by 0.3 mag (R_0_=8 kpc). The field studied by Rich & Mould [AJ, 101, 1286(1991)] has been reobserved two years later. We find that many stars have varied, some by as much as 1 mag at K. However, the shape of the luminosity function is unchanged. We advance the possibility that the bulge of M31 may be younger than the oldest Galactic globular clusters. We note that M33 has recently been found to have an r^1/4-law spheroid consisting of intermediate-age stars; bulges can form later than the old halo population. Publication: The Astronomical Journal Pub Date: December 1993 DOI: 10.1086/116798 Bibcode: 1993AJ....106.2252R Keywords: Andromeda Galaxy; Astronomical Photometry; Galactic Bulge; Infrared Photometry; Stellar Luminosity; Asymptotic Giant Branch Stars; Identifying; Stellar Magnitude; Astronomy; GALAXIES: INDIVIDUAL: M31; GALAXIES: STELLAR CONTENT; GALAXIES: NUCLEI full text sources ADS | data products SIMBAD (7) NED (1)