Mid- to far-infrared properties of star-forming galaxies and active galactic nuclei

Abstract

We study the mid- to far-IR properties of a 24 μm-selected flux-limited sample (S_(24)> 5 mJy) of 154 intermediate redshift (⟨ z ⟩ ~ 0.15), infrared luminous galaxies, drawn from the 5 Milli-Jansky Unbiased Spitzer Extragalactic Survey. By combining existing mid-IR spectroscopy and new Herschel SPIRE submm photometry from the Herschel Multi-tiered Extragalactic Survey, we derived robust total infrared luminosity (L_(IR)) and dust mass (M_(dust)) estimates and infered the relative contribution of the AGN to the infrared energy budget of the sources. We found that the total (8−1000 μm) infrared emission of galaxies with weak 6.2 μm PAH emission (EW_(6.2) ≤ 0.2 μm) is dominated by AGN activity, while for galaxies with EW_(6.2)> 0.2 μm more than 50% of the L_(IR) arises from star formation. We also found that for galaxies detected in the 250–500 μm Herschel bands an AGN has a statistically insignificant effect on the temperature of the cold dust and the far-IR colours of the host galaxy, which are primarily shaped by star formation activity. For star-forming galaxies we reveal an anti-correlation between the L_(IR)-to-rest-frame 8 μm luminosity ratio, IR8 ≡ L_(IR)/L_8  and the strength of PAH features. We found that this anti-correlation is primarily driven by variations in the PAHs emission, and not by variations in the 5−15 μm mid-IR continuum emission. Using the [Ne iii]/[Ne ii] line ratio as a tracer of the hardness of the radiation field, we confirm that galaxies with harder radiation fields tend to exhibit weaker PAH features, and found that they have higher IR8 values and higher dust-mass-weighted luminosities (L_(IR)/M_(dust)), the latter being a proxy for the dust temperature (T_d). We argue that these trends originate either from variations in the environment of the star-forming regions or are caused by variations in the age of the starburst. Finally, we provide scaling relations that will allow estimating L_(IR), based on single-band observations with the mid-infrared instrument, on board the upcoming James Webb Space Telescope.

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