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

TL;DR

This study uses submillimeter interferometry to reveal a compact, hot, luminous nucleus in NGC 4418, showing evidence of both inflow and outflow gas dynamics, suggesting a rapidly evolving core possibly powered by an AGN or starburst.

Contribution

First high-resolution submillimeter observations of NGC 4418's nucleus, revealing a hot, luminous core with inflow and outflow features, advancing understanding of its hidden power source.

Findings

Detected a ~20 pc hot dusty core with high brightness temperature.

Identified inflow and outflow gas motions in the nucleus.

Estimated the core's luminosity to be about 10^{11} Lsun.

Abstract

We have observed the nucleus of the nearby luminous infrared galaxy NGC 4418 with subarcsec resolution at 860 and 450 micron for 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 micron 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 10^{11} Lsun, which accounts for most of the galaxy luminosity. It is Compton thick (N_H >~ 10^{25} cm^{-2}) and has a high luminosity-to-mass ratio ~500 Lsun/Msun as well as a high luminosity surface density 10^{8.5+-0.5} Lsun pc^{-2}. These parameters are consistent with an AGN 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.