CO in the ALMA Radio-source Catalogue (ARC): The molecular gas content of radio galaxies as a function of redshift^⋆

¹ Institute of Astronomy, Astrophysics, Space Applications, & Remote Sensing, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
e-mail: anelise.audibert@iac.es
² Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁴ Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis Zografos 15784, Greece
⁵ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, 44001 Teruel, Spain
⁶ Cardiff Hub for Astrophysics Research & Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
⁷ Istituto Nazionale di Astrofisica: Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁸ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio, 3, 35122 Padova, Italy
⁹ Observatoire de Paris, LERMA, Université PSL, Sorbonne Université, CNRS, Paris, France
¹⁰ Collège de France, 11 Pl. Marcelin Berthelot, 75231 Paris, France

Received: 29 March 2022
Accepted: 12 August 2022

Abstract

To evaluate the role of radio activity in galaxy evolution, we designed a large archival CO survey of radio galaxies (RGs) to determine their molecular gas masses at different epochs. We used a sample of 120 RGs representative of the NVSS 1.4 GHz survey, when flux limited at 0.4 Jy. Of those, 66 galaxies belonged to the ALMA Radio-source Catalogue (ARC) of calibrators and had spectral window tunings around CO (1–0), (2–1), (3–2), or (4–3). We reduced their ALMA data, determined their H₂ mass contents, and combined the results with similar results for the remaining 54 galaxies from the literature. We found that, while at all epochs the majority of RGs have undetectable reservoirs, there is a rapid increase in the H₂ mass content of the CO-detected RGs with z. At 1 < z < 2.5, one-fourth of the RGs have at least as much molecular gas as simulations would indicate for a typical halo mass of that epoch. These galaxies plausibly have “normal” or even starbursty hosts. Overall, reservoirs of 10⁷ ≲ M_H2 ≲ 10¹⁰ M_⊙ are seen at z < 0.3, and 10¹⁰ ≲ M_H2 ≲ 10¹² M_⊙ at z> 1. Taking into account the completeness correction of the sample, we created the corresponding H₂ mass functions at 0.005 < z < 0.3 and 1 < z < 2.5. The local mass function reveals that the number density of low-z RGs with detectable molecular gas reservoirs is only a little lower (a factor of ∼4) than that of pure (or little star-forming) type 1 and 2 AGN in simulations. At 1 < z < 2.5, there is a significant decrease in the number density of high-z RGs due to the rarity of bright radio galaxies. An estimate for the missing faint RGs would, nonetheless, bring populations close again. Finally, we find that the volume density of molecular gas locked up in the brightest 1/5000–1/7000 RGs is similar in the examined z bins. This result likely indicates that the inflow rate on one hand and the star-formation depletion rate plus the jet-driven expulsion rate on the other hand counteract each other in the most luminous RGs of each epoch.