Mid-infrared spectroscopy of the Andromeda galaxy

Abstract

We present Spitzer/Infrared Spectrograph (IRS) 5-21 micron 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 9 other regions chosen to cover a range of mid-to-far-infrared colours. In line with previous results, PAH feature ratios (6.2 micron and 7.7 micron features compared to the 11.2 micron 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 micron features and enhanced 11.3 micron feature intensity and FWHM apparently resulted from background-subtraction problems. The nucleus does not show any PAH emission but does show strong silicate emission at 9.7 micron. Furthermore, different spectral features (11.3 micron PAH emission, silicate emission and [NeIII] 15.5 micron 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 microns and 15-20 microns but weak emission at 6--8 microns. The nucleus itself is dominated by stellar light giving rise to a strong blue continuum and silicate emission.