The effect of active galactic nuclei on the cold interstellar medium in
  distant star-forming galaxies
  In the framework of a systematic ALMA study of IR-selected main-sequence and
starburst galaxies at z~1-1.7 at typical ~1" resolution, we report on the
effects of mid-IR- and X-ray-detected active galactic nuclei (AGN) on the
reservoirs and excitation of molecular gas in a sample of 55 objects. We find
detectable nuclear activity in ~30% of the sample. The presence of dusty tori
influences the IR SED of galaxies, as highlighted by the strong correlation
among the AGN contribution to the total IR luminosity budget (fAGN =
LIR,AGN/LIR), its hard X-ray emission, and the Rayleigh-Jeans to mid-IR
(S1.2mm/S24um) observed color, with consequences on the empirical SFR
estimates. Nevertheless, we find only marginal effects of AGN on the CO
(J=2,4,5,7) or neutral carbon ([CI](1-0), [CI](2-1)) line luminosities and on
the derived molecular gas excitation as gauged by line ratios and the full
SLEDs. The [CI] and CO emission up to J=5,7 thus primarily traces the
properties of the host in typical IR luminous galaxies. However, we highlight
the existence of a large variety of line luminosities and ratios despite the
homogeneous selection. In particular, we find a sparse group of AGN-dominated
sources with the highest LIR,AGN/LIR,SFR ratios, >3, that are more luminous in
CO(5-4) than what is predicted by the L'CO(5-4)-LIR,SFR relation, which might
be the result of the nuclear activity. For the general population, our findings
translate into AGN having minimal effects on quantities such as gas and dust
fractions and SFEs. If anything, we find hints of a marginal tendency of AGN
hosts to be compact at far-IR wavelengths and to display 1.8x larger dust
optical depths. In general, this is consistent with a marginal impact of the
nuclear activity on the gas reservoirs and star formation in average
star-forming AGN hosts with LIR>5e11 Lsun, typically underrepresented in
surveys of quasars and SMGs.
