SUBMILLIMETER INTERFEROMETRY OF THE LUMINOUS INFRARED GALAXY NGC 4418: A HIDDEN HOT NUCLEUS WITH AN INFLOW AND AN OUTFLOW

Abstract

We have observed the nucleus of the nearby luminous infrared galaxy NGC 4418\nwith subarcsec resolution at 860 and 450 micron for the first time to\ncharacterize its hidden power source. A ~20 pc (0.1") hot dusty core was found\ninside a 100 pc scale concentration of molecular gas at the galactic center.\nThe 860 micron continuum core has a deconvolved (peak) brightness temperature\nof 120-210 K. The CO(3-2) peak brightness temperature there is as high as 90 K\nat 50 pc resolution. The core has a bolometric luminosity of about 10^{11}\nLsun, which accounts for most of the galaxy luminosity. It is Compton thick\n(N_H >~ 10^{25} cm^{-2}) and has a high luminosity-to-mass ratio ~500 Lsun/Msun\nas well as a high luminosity surface density 10^{8.5+-0.5} Lsun pc^{-2}. These\nparameters are consistent with an AGN to be the main luminosity source (with an\nEddington ratio about 0.3) while they can be also due to a young starburst near\nits maximum L/M. We also found an optical color (reddening) feature that we\nattribute to an outflow cone emanating from the nucleus. The hidden hot nucleus\nthus shows evidence of both an inflow, previously seen with absorption lines,\nand the new outflow reported here in a different direction. The nucleus must be\nrapidly evolving with these gas flows.\n