Double, double, toil, and trouble

The tails, bubbles, and knots of the local compact obscured nucleus galaxy NGC 4418

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: wethers@chalmers.se
² National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
³ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
⁴ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
⁵ Institut de radioastronomie millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁶ Wisconsin IceCube Particle Astrophysics Center, Madison, WI 53703, USA
⁷ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
⁸ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22903, USA
⁹ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL University, Sorbonne University, Paris, France
¹⁰ Institute of Astrophysics, Foundation for Research and Technology Hellas (FORTH), Heraklion 70013, Greece
¹¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹² Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁴ Astronomy Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
¹⁵ National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁶ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
¹⁷ Joint ALMA Observatory, Alonso de Córdova, 3107 Vitacura, Santiago 763-0355, Chile
¹⁸ Department of Astronomy, The University of Tokyo, 7-3-1, Hongo, Bunkyo, Tokyo 113-0033, Japan
¹⁹ Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago 22, Chile
²⁰ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
²¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²² Department of Physics and Astronomy, University College London, Gower Street, WC1E 6BT London, UK

Received: 16 June 2023
Accepted: 1 December 2023

Abstract

Context. Compact obscured nuclei (CONs) are an extremely obscured (N_H2 > 10²⁵ cm⁻²) class of galaxy nuclei thought to exist in 20–40 per cent of nearby (ultra-)luminous infrared galaxies While they have been proposed to represent a key phase of the active galactic nucleus (AGN) feedback cycle, the nature of these CONs – what powers them, their dynamics, and their impact on the host galaxy – remains unknown.

Aims. This work analyses the galaxy-scale optical properties of the local CON NGC 4418 (z = 0.00727). The key aims of the study are to understand the impact of nuclear outflows on the host galaxy and infer the power source of its CON. Through the mapping of the galaxy spectra and kinematics, we seek to identify new structures in NGC 4418 to ultimately reveal more about the CON’s history, its impact on the host, and, more generally, the role CONs play in galaxy evolution.

Methods. We present new, targeted integral field unit observations of the galaxy with the Multi-Unit Spectroscopic Explorer (MUSE). For the first time, we mapped the ionised and neutral gas components of the galaxy, along with their dynamical structure, to reveal several previously unknown features of the galaxy.

Results. We confirm the presence of a previously postulated, blueshifted outflow along the minor axis of NGC 4418. We find this outflow to be decelerating and, for the first time, show it to extend in both directions from the nucleus. We report the discovery of two further outflow structures: a redshifted southern outflow connected to a tail of ionised gas surrounding the galaxy and a blueshifted bubble to the north. In addition to these features, we find the [O III] emission reveals the presence of knots across the galaxy, which are consistent with regions of the galaxy that have been photoionised by an AGN.

Conclusions. We identify several new features in NGC 4418, including a bubble structure, a reddened outflow, and [O III] knot structures throughout the galaxy. We additionally confirm the presence of a bilateral blueshifted outflow along the minor axis. Based on the properties of these features, we conclude that the CON in NGC 4418 is most likely powered by AGN activity.