Pedro Beirão (SSC/Caltech) : The Heating and Cooling of the Interstellar Medium in NGC 1097

The most important heating process in the neutral ISM is expected to be the photoelectric effect working on polycyclic aromatic hydrocarbons (PAHs) and small dust grains. The heated gas cools primarily via fine-structure lines such as [CII]158μm and [OI]63μm. In some regions, heating may not be dominated by UV radiation, but by shocks driven by cloud collisions or by X-ray sources such as active galactic nuclei (AGNs). It is therefore important to understand the ISM heating and cooling balance under a variety of conditions. The KINGFISH project (Key Insights on Nearby Galaxies: a Far- Infrared Survey with Herschel) is an open-time Herschel key program which aims to measure the heating and cooling of the gaseous and dust components of the ISM in a sample of 61 nearby galaxies with the PACS and SPIRE instruments. One of the galaxies in the sample is NGC 1097, a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar and an active nucleus. It is an ideal object to examine how the gas heating efficiency varies due to the properties of the gas and dust in different environments. I am going to present a detailed study of the spatial variation of the far infrared (FIR) [CII]158μm and [OI]63μm lines and mid-infrared H2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS, and Spitzer-IRS infrared spectral maps. Diagnostics such as the ([CII]+[OI])/PAH ratio are used to study the variations of gas heating efficiency and by comparing the observed line fluxes to photodissociation region and SED models we trace the dominant heating processes in the ring and the spiral arms of NGC 1097.