SUPER

IV. CO(J = 3–2) properties of active galactic nucleus hosts at cosmic noon revealed by ALMA^⋆

¹ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
e-mail: c.circosta@ucl.ac.uk
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
⁴ School of Mathematics, Statistics and Physics, Newcastle University NE1 7RU, UK
⁵ European Southern Observatory, Alonso de Cordova 3107, Casilla 19, Santiago 19001, Chile
⁶ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ INAF IASF-Milano, Via Alfonso Corti 12, 20133 Milano, Italy
⁸ Dipartimento di Fisica e Astronomia dell’Università degli Studi di Bologna, via P. Gobetti 93/2, 40129 Bologna, Italy
⁹ INAF/OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via P. Gobetti 93/3, 40129 Bologna, Italy
¹⁰ MPE, Giessenbach-Str. 1, 85748 Garching bei München, Germany
¹¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
¹³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy
¹⁶ Centro de Astrobiología (CAB, CSIC–INTA), Departamento de Astrofísica, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁷ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone (Roma), Italy
¹⁸ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹⁹ Graduate school of Science and Engineering, Saitama Univ. 255 Shimo-Okubo, Sakura-ku, Saitama City, Saitama 338-8570, Japan

Received: 26 August 2020
Accepted: 25 November 2020

Abstract

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 (log L_bol / erg s⁻¹ = 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 (0.4–0.7 dex) 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 ≈ 10^12.2 L_⊙ and M_* > 10¹¹ M_⊙) 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.