[CII] 158 μm and [NII] 205μm emission in IC342

¹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Leiden Observatory, Leiden University, PO Box 9513, NL 2300 RA Leiden, The Netherlands

Abstract

Atomic fine-structure line emission is a major cooling process in the interstellar medium (ISM). In particular the [C ii] 158 μm line is one of the dominant cooling lines in photon-dominated regions (PDRs). However, it is not confined to PDRs only but can also originate from the ionized gas closely surrounding young massive stars. Here we discuss the question how much of the [C ii] emission in the nucleus of the nearby spiral galaxy IC 342 is contributed by PDRs and by the ionized gas. Particularly the correlation of the [C ii] line with the also observed [N ii] 205 μm emission line coming exclusively from the [H ii] regions is investigated. The emission coming from the ionized gas shows a kinematic component in addition to the general Doppler signature of the molecular gas. We discuss how this requires an adaptation of our understanding of the geometrical structure of the nucleus of IC 342. Applying predictions from numerical models of [H ii] and PDR regions to derive the contribution from the ionized phase to the total [C ii] emission we find that 30–60% of the observed [C ii] intensity stems from the ionized gas if both phases contribute.