Submillimeter Interferometry of the Luminous Infrared Galaxy NGC 4418: A Hidden Hot Nucleus with an Inflow and an Outflow

Journal article, 2013

We have observed the nucleus of the nearby luminous infrared galaxy NGC 4418 with subarcsec resolution at 860 and 450 μmfor the first time to characterize its hidden power source. A∼20 pc (0″.1) hot dusty core was found inside a 100 pc scale concentration of molecular gas at the galactic center. The 860 μm continuum core has a deconvolved (peak) brightness temperature of 120–210 K. The CO(3–2) peak brightness temperature there is as high as 90 K at 50 pc resolution. The core has a bolometric luminosity of about 1011 L☉ which accounts for most of the galaxy luminosity. It is Compton thick (NH≳1025 cm−2) and has a high luminosity-to-mass ratio (L/M) ∼ 500 L☉ M☉−1 as well as a high luminosity surface density 108.5±0.5L☉ pc−2. These parameters are consistent with an active galactic nucleus to be the main luminosity source (with an Eddington ratio about 0.3), while they can be also due to a young starburst near its maximum L/M. We also found an optical color (reddening) feature that we attribute to an outflow cone emanating from the nucleus. The hidden hot nucleus thus shows evidence of both an inflow, previously seen with absorption lines, and the new outflow reported here in a different direction. The nucleus must be rapidly evolving with these gas flows.