CON-quest

Searching for the most obscured galaxy nuclei

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: niklas.falstad@chalmers.se
² Observatoire de Paris, LERMA, College de France, CNRS, PSL Univ., Sorbonne University, UPMC, Paris, France
³ Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁵ Department of Astronomy, 530 McCormick Road, University of Virginia, Charlottesville, VA 22904, USA
⁶ Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
⁷ European Southern Observatory, Alonso de Córdova 3107, Vitacura, 763 0355 Santiago, Chile
⁸ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, 763 0355 Santiago, Chile
⁹ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan ROC
¹⁰ Jodrell Bank Centre for Astrophysics, Department of Physics & Astronomy, School of Natural Sciences, The University of Manchester, Manchester M13 9PL, UK
¹¹ Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling, 475 North Charter Street, Madison, WI 53706, USA
¹² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁴ Cosmic Dawn Center (DAWN), DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹⁵ Observatorio de Madrid, OAN-IGN, Alfonso XII, 3, 28014 Madrid, Spain
¹⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁷ Astron. Dept., King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
¹⁸ National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁹ Institute of Astronomy, The University of Tokyo, 2-21-1, Osawa, Mitaka, Tokyo 181-0015, Japan
²⁰ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
²¹ George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USA
²² Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
²³ Chinese Academy of Sciences South America Center for Astronomy (CASSACA), National Astronomical Observatories, CAS, Beijing 100101, PR China
²⁴ Institute of Astrophysics, Foundation for Research and Technology–Hellas (FORTH), Heraklion 70013, Greece
²⁵ Instituto de Astrofísica de Andalucia (CSIC), Glorieta de al Astronomia s/n, 18008 Granada, Spain

Received: 30 August 2020
Accepted: 25 February 2021

Abstract

Context. Some luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) host extremely compact (r < 100 pc) and dusty nuclei. The high extinction associated with large column densities of gas and dust toward these objects render them hard to detect at many wavelengths. The intense infrared radiation arising from warm dust in these sources can provide a significant fraction of the bolometric luminosity of the galaxy and is prone to excite vibrational levels of molecules such as HCN. This results in emission from the rotational transitions of vibrationally excited HCN (HCN-vib); the brightest emission is found in compact obscured nuclei (CONs; Σ_{HCN − vib} > 1 L_⊙ pc⁻² in the J = 3−2 transition). However, there have been no systematic searches for CONs, and it is unknown how common they are.

Aims. We aim to establish how common CONs are in the local Universe (z < 0.08), and whether their prevalence depends on the luminosity or other properties of the host galaxy.

Methods. We conducted an Atacama Large Millimeter/submillimeter Array survey of the rotational J = 3−2 transition of HCN-vib in a volume-limited sample of 46 far-infrared luminous galaxies.

Results. Compact obscured nuclei are identified in 38₋₁₃^+18% of the ULIRGs, 21₋₆^+12% of the LIRGs, and 0₋₀^+9% of the lower luminosity galaxies. We find no dependence on the inclination of the host galaxy, but strong evidence of lower IRAS 25 μm to 60 μm flux density ratios (f₂₅/f₆₀) in CONs (with the exception of one galaxy, NGC 4418) compared to the rest of the sample. Furthermore, we find that CONs have stronger silicate features (s_9.7 μm), but similar polycyclic aromatic hydrocarbon equivalent widths (EQW_6.2 μm) compared to other galaxies. Along with signatures of molecular inflows seen in the far-infrared in most CONs, submillimeter observations also reveal compact, often collimated, outflows.

Conclusions. In the local Universe, CONs are primarily found in (U)LIRGs, in which they are remarkably common. As such systems are often highly disturbed, inclinations are difficult to estimate, and high-resolution continuum observations of the individual nuclei are required to determine if the CON phenomenon is related to the inclinations of the nuclear disks. Further studies of the in- and outflow properties of CONs should also be conducted to investigate how these are connected to each other and to the CON phenomenon. The lower f₂₅/f₆₀ ratios in CONs as well as the results for the mid-infrared diagnostics investigated (EQW_6.2 μm and s_9.7 μm) are consistent with the notion that large dust columns gradually shift the radiation from the hot nucleus to longer wavelengths, making the mid- and far-infrared “photospheres” significantly cooler than the interior regions. Finally, to assess the importance of CONs in the context of galaxy evolution, it is necessary to extend this study to higher redshifts where (U)LIRGs are more common.