The extended molecular gas of the Circinus galaxy and NGC 1097 as seen by APEX

¹ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, 0315 Oslo, Norway
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

^⋆ Corresponding author: akhil.lasrado@astro.uio.no

Received: 6 March 2025
Accepted: 9 August 2025

Abstract

The outer region of the interstellar medium (ISM) is often witness to dynamically important events in a galaxy’s evolutionary history such as outflows, inflows, tidal interactions, and mergers, as well as dynamical structures affecting its current evolution such as large-scale bars and spiral arms. Studying the imprints of these processes in the diffuse, extended molecular gas is best achieved by a single dish telescope which can cover a large field of view with good sensitivity to large-scale structures. In this work we present results from Atacama Pathfinder EXperiment (APEX) line emission maps of two nearby galaxies: the Circinus galaxy in the CO(3–2) transition, and NGC 1097 in CO(2–1), covering their full optical extents. We detect molecular gas at the largest extents seen for these galaxies yet, at up to 5′ (r ≈ 6 kpc) for the Circinus galaxy, and 4_.^′5 (r ≈ 18 kpc) for NGC 1097, and compute total CO luminosities of L^′_CO(3−2) = (1.5 ± 0.4) × 10⁸ K km s⁻¹ pc² and L^′_CO(2−1) = (7.0 ± 1.7) × 10⁸ K km s⁻¹ pc², corresponding to molecular gas masses (2.1 ± 1.0)×10⁹ M_⊙ and (4.7 ± 1.9)×10⁹ M_⊙, respectively. We further analyze the large-scale gas kinematics through position-velocity diagrams and 3D tilted ring modeling using the ^3DBAROLO code. We detect notable features in both galaxies beyond their well-studied bright central regions: in the Circinus galaxy we detect molecular gas embedded in a bar-like structure, whose kinematic signature is also evident in the major axis position-velocity diagram, and in NGC 1097, we observe tidal molecular gas structures involved in the interaction of NGC 1097 with the companion galaxy NGC 1097A. The clear detection of such structures in the molecular gas shows promise in conducting large-scale molecular gas studies toward nearby galaxies with APEX and, in the future, the Atacama Large Aperture Submillimeter Telescope (AtLAST).