IC 485: A new candidate disc-maser galaxy at ∼100 Mpc

Milliarcsecond resolution study of the galaxy nucleus and the H₂O megamaser

¹ Dipartimento di Fisica, Universitá degli Studi di Cagliari, S.P. Monserrato-Sestu km 0,700, 09042 Monserrato (CA), Italy
² INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
e-mail: elisabetta.ladu@inaf.it
³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁴ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁵ Center for Astrophysics – Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁶ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA

Received: 24 August 2023
Accepted: 10 October 2023

Abstract

Context. Masers are a unique tool with which to investigate the emitting gas in the innermost regions of active galactic nuclei and to map accretion discs and tori orbiting around supermassive black holes. IC 485, which is classified as a low ionisation nuclear emission-line region (LINER) or Seyfert galaxy, hosts a bright H₂O maser whose nature is still unclear. Indeed, the maser could be a nuclear disc maser, a jet or outflow maser, or even the very first example of a so-called inclined water maser disc.

Aims. We aim to clarify and investigate the nature of the H₂O maser in IC 485 by determining the location and distribution of the maser emission at milliarcsecond resolution and by associating it with the main nuclear components of the galaxy. In a broader context, this work might also provide further information that could be used to better understand the physics and disc–jet geometry in LINER or Seyfert galaxies.

Methods. We observed the nuclear region of IC 485 in continuum and spectral-line mode with the Very Long Baseline Array (VLBA) and with the European VLBI Network (EVN). Here, we report multi-epoch (six epochs) and multi-band (three bands: L, C, and K) observations made in 2018, with linear scales from ∼3 to 0.2 pc.

Results. We detected two 22 GHz H₂O maser components separated in velocity by 472 km s⁻¹, with one centred at the systemic velocity of the nuclear region of IC 485 and the other at a redshifted velocity. We measured for the first time the absolute positions of these components with an accuracy of better than one milliarcsecond. Under the assumption of a maser associated with an edge-on disc in Keplerian rotation, the estimated enclosed mass is M_BH = 1.2 × 10⁷ M_⊙, which is consistent with the expected mass for a SMBH in a LINER or Seyfert galaxy. Continuum compact sources have also been detected in the nuclear region of the galaxy, although at a low level of significance.

Conclusions. The linear distribution of the detected maser components and a comparison with the high-sensitivity single-dish spectrum strongly suggest that the bulk of the maser emission is associated with an edge-on accretion disc. This makes IC 485 a new candidate disc-maser galaxy at a distance of 122 Mpc. In particular, thanks to the upcoming radio facilities (e.g., the Square Kilometer Array and the next-generation Very Large Array), IC 485 will play an important role – alongside other sources at similar distances – in our understanding of active galactic nuclei in an unexplored volume of the Universe.