A point source survey of M31 with the Spitzer Space Telescope
  We explore the stellar population of M31 in a Spitzer Space Telescope survey utilizing IRAC and MIPS observations. Red supergiants are the brightest objects seen in the infrared; they are a prominent evolutionary phase. Due to their circumstellar envelopes, many of these radiate the bulk of their luminosity at IRAC wavelengths and do not stand out in the near infrared or optically. Going fainter, we see large numbers of luminous asymptotic giant branch (AGB) stars, many of which are known long period variables. Relative to M33, the AGB carbon star population of M31 appears sparse, but this needs to be spectroscopically confirmed. 
Jeremy Mould, Pauline Barmby, Karl Gordon, S.P. Willner, M.L.N. Ashby,   R.D. Gehrz, Roberta Humphries, and Charles E Woodward