2021A&A...646A..96C

Feedback from active galactic nuclei (AGN) is thought to be key in shaping the life cycle of their host galaxies by regulating star-formation activity. Therefore, to understand the impact of AGN on star formation, it is essential to trace the molecular gas out of which stars form. In this paper we present the first systematic study of the CO properties of AGN hosts at z≃2 for a sample of 27 X-ray selected AGN spanning two orders of magnitude in AGN bolometric luminosity (logL_bol/(erg/s)=44.7-46.9) by using ALMA Band 3 observations of the CO(3-2) transition (∼1'' angular resolution). To search for evidence of AGN feedback on the CO properties of the host galaxies, we compared our AGN with a sample of inactive (i.e., non-AGN) galaxies from the PHIBSS survey with similar redshift, stellar masses, and star-formation rates (SFRs). We used the same CO transition as a consistent proxy for the gas mass for the two samples in order to avoid systematics involved when assuming conversion factors (e.g., excitation corrections and α_CO). By adopting a Bayesian approach to take upper limits into account, we analyzed CO luminosities as a function of stellar masses and SFRs, as well as the ratio L_CO(3–2)'/M_* (a proxy for the gas fraction). The two samples show statistically consistent trends in the L_CO(3–2)'-L_FIR and L_CO(3–2)'-M_* planes. However, there are indications that AGN feature lower CO(3-2) luminosities (0.4-0.7dex) than inactive galaxies at the 2-3σ level when we focus on the subset of parameters where the results are better constrained (i.e., L_FIR≃10^12.2L_☉ and M_*>10¹¹M_☉) and on the distribution of the mean log(L_CO(3–2)'/M_*. Therefore, even by conservatively assuming the same excitation factor r₃₁, we would find lower molecular gas masses in AGN, and assuming higher r₃₁ would exacerbate this difference. We interpret our result as a hint of the potential effect of AGN activity (such as radiation and outflows), which may be able to heat, excite, dissociate, and/or deplete the gas reservoir of the host galaxies. Better SFR measurements and deeper CO observations for AGN as well as larger and more uniformly selected samples of both AGN and inactive galaxies are required to confirm whether there is a true difference between the two populations.