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Description
Understanding the dominant heating mechanism in the nuclei of galaxies is crucial to understand star formation in starbursts (SB), active galactic nuclei (AGN) phenomena and the relationship between the star formation and AGN activity in galaxies. The analysis of the carbon monoxide (^12^CO) rotational ladder versus the infrared continuum emission (hereafter, ^12^CO/IR) in galaxies with different type of activity have shown important differences between them. 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. We also want to establish the dominant heating mechanism at work in the inner region of this object at smaller spatial scales (<~200pc). We present far-infrared (FIR) and sub-millimeter (sub-mm) ^12^CO 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 ^12^CO/IR flux ratios and the local thermodynamic equilibrium (LTE) analysis of the ^12^CO 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 (^12^CO, ^13^CO and [CI]) and high density (HCN, HNC, HCO^+^, CS and CH) molecular gas tracers obtained with APEX and HIFI applying LTE and non-LTE analyses. Furthermore, the Spectral Energy Distribution (SED) of the continuum emission from the far-IR to sub-mm wavelengths is also presented. From the non-LTE analysis of the low and high density tracers we derive in NGC 4945 gas volume densities (10^3^-10^6^cm^-3^) similar to those found in other galaxies with different type of activity. 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 (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 ^12^CO multi-transition analysis suggests that mechanical heating, like shocks or turbulence, dominates the heating of the ISM in the nucleus of NGC4945 located beyond 100 pc (&500) from the center of the galaxy. This result is further supported by the Kazandjian et al. (2015, Cat. J/AJ/133/504) 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.
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