Mid-infrared spectroscopy of the Andromeda galaxy
  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.
