  In this work we explore the impact of the presence of an active galactic
nucleus (AGN) on the mid- and far-infrared (IR) properties of galaxies as well
as the effects of simultaneous AGN and starburst activity in these same
galaxies. To do this we apply a multi-component, multi-band spectral synthesis
technique to a sample of 250 micron selected galaxies of the Herschel
Multi-tiered Extragalactic Survey, with IRS spectra available for all galaxies.
Our results confirm that the inclusion of the IRS spectra plays a crucial role
in the spectral analysis of galaxies with an AGN component improving the
selection of the best-fit hot dust model (torus). We find a correlation between
the obscured star formation rate (SFR) derived from the IR luminosity of the
starburst component, SFR_IR and SFR_PAH, derived from the luminosity of the PAH
features, L_PAH, with SFR_FIR taking higher values than SFR_PAH. The
correlation is different for AGN- and starburst-dominated objects. The ratio of
L_PAH to that of the starburst component, L_PAH/L_SB, is almost constant for
AGN-dominated objects but decreases with increasing L_SB for
starburst-dominated objects. SFR_FIR increases with the accretion luminosity,
L_acc, with the increase less prominent for the very brightest, unobscured
AGN-dominated sources. We find no correlation between the masses of the hot and
cold dust components. We interpret this as a non-constant fraction of gas
driven by the gravitational effects to the AGN while the starburst is ongoing.
We also find no evidence of the AGN affecting the temperature of the cold dust
component, though this conclusion is mostly based on objects with a
non-dominant AGN component. We conclude that our findings do not provide
evidence that the presence of AGN affects the star formation process in the
host galaxy, but rather that the two phenomena occur simultaneously over a wide
range of luminosities.
