The multi-phase ISM in the nearby composite AGN-SB galaxy NGC 4945: large (parsecs) scale mechanical heating

TL;DR

This study uses multi-transition CO observations to analyze the thermal structure of NGC 4945, revealing that mechanical heating mechanisms like shocks and turbulence dominate the galaxy's interstellar medium, especially beyond 100 pc from the nucleus.

Contribution

It provides a detailed multi-scale analysis of the thermal structure of NGC 4945's ISM, highlighting the dominant role of mechanical heating mechanisms.

Findings

Mechanical heating dominates the galaxy's nucleus beyond 100 pc.

High temperatures are observed in the galaxy plane at small scales.

Shocks and turbulence are linked to the barred potential and outflows.

Abstract

We aim at carrying out a comprehensive study of the nearby composite AGN-SB galaxy, NGC 4945, using spectroscopic and photometric data from the Herschel satellite. In particular, we want to characterize the thermal structure in this galaxy by a multi-transitions analysis of the spatial distribution of the 12^CO emission at different spatial scales using HIFI, PACS, SPIRE as well as APEX data. We combined the 12^CO/IR flux ratios and the LTE analysis of the 12^CO images to derive the thermal structure of the Interstellar Medium (ISM) for spatial scales raging from <200 pc to 2 kpc. From the 12^CO analysis we found clear trend in the distribution of the derived temperatures and the 12^CO/IR ratios. It is remarkable that at intermediate scales (360 pc-1 kpc) we see large temperatures in the direction of the X-ray outflow while at smaller scales (<200 pc-360 pc), the highest temperature, derived from the high-J lines, is not found toward the nucleus, but toward the galaxy plane. The thermal structure derived from the 12CO multi-transition analysis suggests that mechanical heating, like shocks or turbulence, dominates the heating of the ISM in the nucleus of NGC 4945 located beyond 100 pc from the center of the galaxy. This result is further supported by the Kazandjian et al. 2015 models, which are able to reproduce the emission observed at high-J (PACS) 12^CO transitions when mechanical heating mechanisms are included. Shocks and/or turbulence are likely produced by the barred potential and the outflow, observed in X-rays.