Mid-infrared spectroscopy of the Andromeda galaxy

Abstract

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 9 other regions chosen to cover a range of mid-to-far-infrared colours. In line with previous results, PAH feature ratios (6.2 µm 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 FWHM 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 [NeIII] 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 ′′ north of the nucleus. The PAH feature ratios at this position are atypical with strong emission at 11.2 µm 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.

M 31 circum-nuclear region: A molecular survey with the IRAM interferometer

“…In the near infrared, we observe an excellent agreement with all stellar templates below 5 µm. We do find an excellent agreement of the near infrared measurements with the Hemachandra et al (2015) IRS Spitzer spectra, with a significant silicate 'bump' around 10 µm . It is also very close to the passive galaxy template displayed in Figure 2 of Panuzzo et al (2011).…”

“…Large metallicity templates improve the match with the data in the optical part but not in the UV nor IR (for an age smaller or equal to 10 Gyr). One can note the presence of weak fine-structure emission lines discussed in Hemachandra et al (2015), which confirms the presence some interstellar ionised gas. One can point to relatively large [NeIII] emission line, often correlated to the 24 µm dust emission (e.g.…”

“…We can then plot the spectral energy distribution for each pixel and/or in an integrated area. To compute the spectral energy distribution displayed in Figures 15 and 16, we reproject the maps on the nuclear region observed by Hemachandra et al (2015) with Spitzer/Infrared Spectrograph (IRS). The nucleus was observed with the IRS short-low (SL1, SL2 and SL3) and long-low (LL2) modules which allows observations over the 5.2-20.75 µm band.…”

“…The contribution of the 200 Myr is more important, and it is also necessary to add a large metallicity template to reproduce the UV and optical part of the spectral energy distribution. Here, the stellar background Hemachandra et al (2015) with rectangles. Each rectangle corresponds to a region observed by a different module of Spitzer/IRS.…”

Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

“…• The high sensitivity of Spitzer/IRS has enabled unprecedented PAH spectral mapping of both Galactic and extragalactic sources, providing valuable information on the spatial variations of the PAH characteristics (e.g., size, charge, structure) and their responses to the changing environments [51][52][53][54][55] .…”