Centro de Astrobiología (INTA)
Mechanical heating at parsec scales in the nearby AGN-SB composite galaxy NGC 4945
Understanding the dominant heating mechanism in the nuclei of galaxies is crucial to understand star formation in starburst (SB), the active galactic nuclei (AGN) phenomena and the relationship between the star formation and AGN activity in galaxies. We study the nearby composite SB--AGN galaxy, NGC 4945, by a multi-transition analysis of the spatial distribution of the 12^CO emission to establish which is the dominant heating mechanism. We present far--infrared (FIR) and sub-millimeter (sub-mm) 12^CO line (from J_up= 4 to 19) maps and single spectra 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, along with APEX data. We combined the Spectral Line Energy Distribution (SLED) and the LTE analysis of the 12^CO images to derive the thermal structure of the Interstellar Medium (ISM) for spatial scales raging from 150 pc to 2 kpc. Our main results obtained from the 12^CO analysis show that a clear trend is found in the distribution of the derived temperatures and the SLED/IR ratios. It is remarkable that at scales of 150-300 pc, the highest temperature, derived from the high-J lines, is not found toward the nucleus, but toward the galaxy plane. At intermediate scales (350 pc-1 kpc) we also see large temperatures in the direction of the X-ray outflow. The thermal structure derived from the 12^CO multi-transition analysis suggests that shocks dominate 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 published 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. The results obtained in this work seem to confirm that the presence of the AGN in NGC 4945 has little impact on the thermal properties of its nuclear starburst. IR observations at higher spatial resolution are required to characterize both the dust and molecular line emissions. Spectroscopic and photometric observations like those could be achieved by the instruments onboard SOFIA (e.g., HAWC+, CASIMIR and/or GREAT) are needed in order to characterize the physical conditions of temperature and density as well as the structure of the emission itself in the very inner regions of NGC 4945.