A photometric and spectroscopic survey for young stars in the halo of M31

We have obtained UBV CCD photometry of a 0.5 deg² field toward the halo of M31 (the Andromeda Galaxy), using the 2.5 m Isaac Newton Telescope on La Palma. These observations have allowed us to identify nine blue stellar objects, with (U - B) < -0.4 and (B - V) < 0.0, in the magnitude range B = 21.5-22.5, typical of main-sequence early B-type stars at the distance of Andromeda. Hence these objects may be normal Population I stars at large distances (> 3 kpc) from the plane of M31. Follow-up low-resolution spectra of these objects obtained with the William Herschel Telescope, also on La Palma, do not have the requisite signal-to-noise ratios for a definitive conclusion regarding their nature, although one source appears to be a degenerate star in the halo of our Galaxy and has thus been eliminated from the candidate list. Positive identification of Balmer absorption features in the stellar spectra at the known radial velocity shift of the Andromeda system would provide very strong evidence for the general existence of normal B-type stars in the halos of spiral galaxies.