The Central Regions of M31 in the 3-5 μm Wavelength Region

Abstract

Images obtained with NIRI on the Gemini North telescope are used to investigate the photometric properties of the central regions of M31 in the 3-5 μm wavelength range. The light distribution in the central arcsecond differs from what is seen in the near-infrared in the sense that the difference in peak brightness between P1 and P2 is larger in M' than in K'; no obvious signature of P3 is detected in M'. These results can be explained if there is a source of emission that contributes ~20% of the peak M' light of P1, has an effective temperature of no more than a few hundred K, and is located between P1 and P2. Based on the red K - M' color of this source, it is suggested that the emission originates in a circumstellar dust shell surrounding a single bright asymptotic giant branch (AGB) star. Tests of this hypothesis are described. A bright source that is ~8'' from the center of the galaxy is also detected in M'. This object has red colors and an absolute brightness in M' that is similar to the most highly evolved AGB stars in the solar neighborhood; hence, it is likely to be a very evolved AGB star embedded in a circumstellar envelope. The K-band brightness of this star is close to the peak expected for AGB evolution, and an age of only a few hundred million years is estimated, which is comparable to that of the P3 star cluster. Finally, using high angular resolution near-infrared adaptive optics images as a guide, a sample of unblended AGB stars outside of the central few arcseconds is defined in L'. The (L', K - L') color-magnitude diagram of these sources shows a dominant AGB population with a peak L' brightness and a range of K - L' colors that are similar to those of the most luminous M giants in the Galactic bulge.