Near-infrared mapping of M31

Martinez-Roger, C.; Phillips, J. P.; Sanchez Magro, C.
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Astronomy and Astrophysics (ISSN 0004-6361), vol. 161, no. 2, June 1986, p. 237-243.

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Moderate resolution near-infrared mapping of the M31 bulge component is presented, and the structure, and radial color index and surface brightness variations are discussed. The near-infrared surface brightness, mu(r), is matched well by Hubble and de Vaucouleurs functions for major axis distances of r greater than 30 arcsec, although they fail to explain the observed behavior at smaller radii. The near-infrared radial gradient in mu is steeper than at visual wavelengths, however, and this is also reflected in color index variations. The metallicity adduced using single star burst models appears to be reasonably large at the core (M/H of about 0.2), and decreases slowly outwards; a result which confirms and supplements various optical estimates. The present results also confirm a change of apparent isophotal ellipticity compared with optical wavelengths, with ellipticity taking a typical value epsilon of about 0.3. It is postulated that this may be due to observation of a redder stellar component of the bulge having a flatter oblate spheroidal distribution.