The Stellar Content of M31's Bulge

Abstract

In this paper we analyze the stellar populations present in M31 using nine sets of adjacent HST-NICMOS Camera 1 and 2 fields with galactocentric distances ranging from 2' to 20'. These infrared observations provide some of the highest spatial resolution measurements of M31 to date; our data place tight constraints on the maximum luminosities of stars in the bulge of M31. The tip of the red giant branch is clearly visible at Mbol ~ -3.8, and the tip of the asymptotic giant branch (AGB) extends to Mbol ~ -5. This AGB peak luminosity is significantly fainter than previously claimed; through direct comparisons and simulations we show that previous measurements were affected by image blending. We do observe field-to-field variations in the luminosity functions, but simulations show that these differences can be produced by blending in the higher surface brightness fields. We conclude that the red giant branch of the bulge of M31 is not measurably different from that of the Milky Way's bulge. We also find an unusually high number of bright blueish stars (7.3/arcmin^2) which appear to be Galactic foreground stars.

28 pages, 20 figures (posted here with significantly reduced resolution), accepted to the AJ

References in corpus (9)

• Evidence for Stellar Substructure in the Halo and Outer Disk of M31
• HST/WFPC2 proper motions in two bulge fields: kinematics and stellar population of the Galactic bulge
• The Stellar Populations of Pixels and Frames
• HST-NICMOS Color Transformations and Photometric Calibrations
• The Brightest AGB Stars in the Inner Bulge of M31
• Long Period Variables in Globular Clusters and the Galactic Bulge: Their Dependence on Metallicity
• HST-NICMOS Observations of M31's Metal Rich Globular Clusters and Their Surrounding Fields: II. Results
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