A Herschel/PACS mapping of far-infrared emission lines from the nuclear spiral in M31

The nuclear spiral in M31, consisting of ionized and neutral dusty gas clouds, shows optical emission lines characteristic of LINERs. Yet the lack of UV radiation from either an active nucleus or massive young stars makes the ionizing source of this nearest LINER a longstanding puzzle. We propose a PACS spectroscopic mapping of several fine structure lines, including C II, O I and O III,from the nuclear spiral. We will derive for the first time the spatial distribution and kinematics of the circumnuclear neutral gas in M31, with an unparalleled linear resolution of 40 parsec for a massive external galaxy. These FIR lines, together with the optical and midIR emission lines mapped by HST and Spitzer observations, will enable us to assess the importance of various ionization/excitation mechanisms. Specifically, we will test the cosmicray heating scenario, which is favored by energetics considerations for the nuclear spiral. This study will advance our understanding of the physical regulation of galactic circumnuclear environments, which is in turn crucial to understanding the evolution of supermassive black holes and their host galaxies. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]