The effect of active galactic nuclei on the cold interstellar medium in distant star-forming galaxies

Abstract

In the framework of a systematic study with the ALMA interferometer 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 widespread detectable nuclear activity in ∼30% of the sample. The presence of dusty tori influences the IR spectral energy distribution 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 (S1.2 mm/S24 μm) observed color, with evident consequences on the ensuing empirical star formation rate estimates. Nevertheless, we find only marginal effects of the presence and strength of AGN on the carbon monoxide CO (J = 2, 4, 5, 7) or neutral carbon ([C I](3P1 − 3P0), [C I](3P2 − 3P1)) line luminosities and on the derived molecular gas excitation as gauged by line ratios and the full spectral line energy distributions. The [C I] and CO emission up to J = 5, 7 thus primarily traces the properties of the host in typical IR luminous galaxies. However, our analysis highlights 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 star formation efficiencies. If anything, we find hints of a marginal tendency of AGN hosts to be compact at far-IR wavelengths and to display 1.8 times 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 > 5 × 1011 L⊙, typically underrepresented in surveys of quasars and submillimeter galaxies.

Photodissociation and X-Ray Dominated Regions

“…Finally, by contrast with early PDR models suggesting that [C I] is emitted from a thin layer around molecular clouds, both observational studies (Plume et al 1994, Ikeda et al 1999 and more recently Popping et al 2017, Nesvadba et al 2019, Valentino et al 2020 as well as theory and simulations (Papadopoulos et al 2004, Offner et al 2014, Gaches et al 2019, Heintz & Watson 2020 have shown a strong correlation between [C I] emission and that from CO isotopologues, likely resulting from turbulence within clouds. The turbulence produces a complex geometry with many internal surfaces and FUV pathways to dissociate the CO and excite [C I].…”

“…The whole CO ladder of AGN host sources can be another valuable tool to infer the presence of an XDR component contributing to the gas heating (Bradford et al 2009, Hailey-Dunsheath et al 2012, Pozzi et al 2017, Mingozzi et al 2018, Valentino et al 2021 as XDRs are characterized by large column densities of warm molecular gas, where high-J CO lines (J 8) can be efficiently excited. For this reason the CO Spectral Line Energy Distribution (SLED) resulting from XDR models peaks at increasingly higher J for increasing X-ray flux (Vallini et al 2019, and see Figure 8), even though CO emission can be suppressed by X-ray induced CO dissociation (Section 4.4) at intermediate column densities (Kawamuro et al 2020).…”

“…Nevertheless, the unprecedented sensitivity and resolution of ALMA enable observations approaching sub-kpc scales, thus providing a first glimpse on the overall ISM conditions of distant sources. The [C II] 158 µm line is by far the most widely targeted PDR tracer in normal (SFR= 1 − 100 M yr −1 ) sources at high-z, while [O I] 63 and 145 µm (e.g., Rybak et al 2020b, Lee et al 2021), [C I] 609 and 370 µm (e.g., Strandet et al 2017, Valentino et al 2020, CO (e.g., D'Odorico et al 2018, Pavesi et al 2019, Apostolovski et al 2019, and H2O, HF, and OH + lines (e.g., Casey et al 2019, Lehnert et al 2020, Riechers et al 2021, Pensabene et al 2021) are mostly detected in rare/massive and highly star forming galaxies and quasars.…”

The Past and Future of Mid-Infrared Studies of AGN

“…The recent literature is still somewhat divided on the topic of the impact of AGN on the molecular gas reservoir of their hosts. For example, Valentino et al [158] examine the ALMA CO properties of both mid-IR and X-ray AGN and find no significant effect in either amount of molecular gas or its excitation. On the other hand, Circosta et al [159] find that, controlling for other parameters such as stellar mass and star-formation rate, AGN hosts have lower CO 3-2 luminosities suggestive of either gas removal or excitation to higher rotation states.…”

The Giant Metrewave Radio Telescope Cold-HI AT $z\approx1$ Survey

“…Kereš et al 2005;Dekel et al 2009), (2) AGN or stellar feedback, especially in massive galaxies (e.g. Weiner et al 2009;Steidel et al 2010;Kakkad et al 2020;Valentino et al 2021), and (3) heating of the gas reservoir in the CGM (e.g. Schawinski et al 2014).…”