2015MNRAS.454..818H

We present Spitzer/Infrared Spectrograph (IRS) 5-21 µm spectroscopic maps towards 12 regions in the Andromeda galaxy (M31). These regions include the nucleus, bulge, an active region in the star-forming ring and nine other regions chosen to cover a range of mid-to-far-infrared colours. In line with previous results, polycyclic aromatic hydrocarbon (PAH) feature ratios (6.2 and 7.7 µm features compared to the 11.2 µm feature) measured from our extracted M31 spectra, except the nucleus, strongly correlate. The equivalent widths of the main PAH features, as a function of metallicity and radiation hardness, are consistent with those observed for other nearby spiral and starburst galaxies. Reprocessed data from the ISOCAM instrument on the Infrared Space Observatory agree with the IRS data; early reports of suppressed 6-8 µm features and enhanced 11.3 µm feature intensity and full width at half-maximum apparently resulted from background-subtraction problems. The nucleus does not show any PAH emission but does show strong silicate emission at 9.7 µm. Furthermore, different spectral features (11.3 µm PAH emission, silicate emission and [Ne iii] 15.5 µm line emission) have distinct spatial distributions in the nuclear region: the silicate emission is strongest towards the stellar nucleus, while the PAH emission peaks 15 arcsec north of the nucleus. The PAH feature ratios at this position are atypical with strong emission at 11.2 and 15-20 µm but weak emission at 6-8 µm. The nucleus itself is dominated by stellar light giving rise to a strong blue continuum and silicate emission.