Widespread Shocks in the Nucleus of NGC 404 Revealed by Near-infrared Integral Field Spectroscopy

TL;DR

This study uses high-resolution near-infrared integral field spectroscopy to reveal that shocks, likely from a supernova remnant, dominate the LINER emission in the nucleus of NGC 404, challenging the black hole accretion hypothesis.

Contribution

It provides the first detailed spatially-resolved evidence that shocks, rather than accretion, excite LINER emission in NGC 404 using near-infrared spectroscopy.

Findings

Shocks are the dominant excitation mechanism in NGC 404's nucleus.

Molecular gas is in a rotating disk with thermal excitation.

High [FeII]/Paβ ratios indicate strong shock activity.

Abstract

We present high spatial resolution, integral field spectrograph (IFS) observations of the nearby LINER (low ionization nuclear emission line region) galaxy NGC 404 at 1.25 $\mu$m (J band) and 2.2 $\mu$m (K band) near-infrared wavelengths. Although NGC 404 is thought to host an intermediate mass black hole at its center, it has been unclear whether accretion onto the black hole or another mechanism such as shock excitation drives its LINER emission at optical/near-infrared wavelengths. We use the OSIRIS IFS at Keck Observatory behind laser guide star adaptive optics to map the strength and kinematics of [FeII], H$_2$, and hydrogen recombination lines at spatial resolutions of 1 pc across the central 30 pc of the galaxy. The H$_2$ gas is in a central rotating disk and ratios of multiple H$_2$ lines indicate that the molecular gas is thermally excited, with some contribution from UV fluorescence. The [FeII] emission is more extended and diffuse than the molecular gas and has a different kinematic structure that reaches higher velocities/dispersions. We also map the strength of the CO stellar absorption feature and constrain the dominant age of the nuclear stellar population to $\sim$1 Gyr. Finally, we find regions across the nucleus of NGC 404 with [FeII]/Pa$\beta$ line ratios up to 6.5, $\sim$2.5 times higher than the ratio measured from spatially-integrated spectra. From these high line ratios, we conclude that shocks are the dominate physical mechanism exciting NGC 404's LINER emission and argue that a possible source of this shock excitation is a supernova remnant.