Poster Presentation
Mm/submm Energy Diagnostics in the AGN-Starburst Composite Galaxy NGC 7469 with ALMA
Presenter: Suzuka Nakano (SOKENDAI / NAOJ)
As a promising mechanism of supermassive black hole (SMBH) mass growth, gas accretion onto an SMBH during gas-rich major galaxy merger has been proposed. An SMBH with a high mass accretion rate appears as an Active Galactic Nucleus (AGN). However, since the early phase of AGN is deeply buried in dust and gas, its detection is extremely difficult at the optical wavelength. We need to construct an energy diagnostic method at wavelengths of low dust extinction effects, such as (sub)millimeter. It is known that an AGN is a much stronger X-ray emitting source than a starburst. Because of this, a unique region where physical and chemical properties are determined by X-rays (X-ray dominated region: XDR) is formed around a luminous AGN. It is expected that an XDR shows (sub)millimeter molecular and/or atomic line ratios different from a photodissociation region (PDR) usually developed in a star-forming region. In particular, [CI]/CO line ratio is expected to be a powerful method to distinguish between an XDR and a PDR, because this ratio is predicted to be more than an order of magnitude higher in an XDR than in a PDR, due to efficient dissociation of CO to C in an XDR (e.g., Meijerink et al. 2007). However diagnostic diagrams based on mm/submm line ratios have been explored during the last two decades with no clear success in their discrimination potential, because previous studies are not enough resolution with single dish telescopes.
Here we performed high-spatial-resolution ( ~100 pc) observations of multi-J CO lines and [CI] (1-0) line in the nearby (z=0.016) luminous type-1 Seyfert galaxy NGC 7469 with ALMA. This galaxy hosts a compact circumnuclear gas disk (CND) energetically-dominated by the AGN and a surrounding prominent starburst ring (r = 1.5"). Among detected lines, we found that [CI](1-0) to 13CO(2-1) ratio (both lines would be optically thin) is an order of magnitude higher at the CND (XDR) than at the starburst ring (PDR). This observational trend is consistent with the above-mentioned theoretical predictions. In this presentation, we will show spatial distributions of several line ratios, results of our non-LTE radiative transfer calculations, and discuss how promising our diagnostic is for uncovering dust-obscured energy sources.
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