SIMBAD references

Context. Understanding the dominant heating mechanism in the nuclei of galaxies is crucial to understanding star formation in starbursts (SBs), active galactic nuclei (AGN) phenomena, and the relationship between star formation and AGN activity in galaxies. Analysis of the carbon monoxide (¹²CO) rotational ladder versus the infrared continuum emission (hereafter, ¹²CO/IR) in galaxies with different types of activity reveals important differences between them.
Aims. We aim to carry 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 using a multi-transition analysis of the spatial distribution of the ¹²CO emission at different spatial scales. We also want to establish the dominant heating mechanism at work in the inner region of this object at smaller spatial scales (≥200pc).
Methods. We present far-infrared (FIR) and sub-millimeter (sub-mm) ¹²CO line maps and single spectra (from J_up=3 to 20) using the Heterodyne Instrument for the Far Infrared (HIFI), the Photoconductor Array Camera and Spectrometer (PACS), and the Spectral and Photometric Imaging REceiver (SPIRE) onboard Herschel, and the Atacama Pathfinder EXperiment (APEX). We combined the ¹²CO/IR flux ratios and the local thermodynamic equilibrium (LTE) analysis of the ¹²CO images to derive the thermal structure of the interstellar medium (ISM) for spatial scales raging from ≥200pc to 2kpc. In addition, we also present single spectra of low- (¹²CO, ¹³CO and [CI]) and high-density (HCN, HNC, HCO⁺, CS and CH) molecular gas tracers obtained with APEX and HIFI applying LTE and non-LTE (NLTE) analyses. Furthermore, the spectral energy distribution of the continuum emission from the FIR to sub-mm wavelengths is also presented.
Results. From the NLTE analysis of the low- and high-density tracers, we derive gas volume densities (10³-10⁶cm^–3) for NGC 4945 that are similar to those found in other galaxies with different types of activity. From the ¹²CO analysis we find a clear trend in the distribution of the derived temperatures and the ¹²CO/IR ratios. It is remarkable that at intermediate scales (360pc-1kpc, or 19''-57'') we see large temperatures in the direction of the X-ray outflow while at smaller scales (≥200pc-360pc, or ∼9''-19''), 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 NGC4945 located beyond 100pc (≥5'') from the center of the galaxy. This result is further supported by published models, which are able to reproduce the emission observed at high-J (PACS) ¹²CO 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.