Circumnuclear Molecular Gas in Megamaser Disk Galaxies NGC 4388 and NGC 1194

TL;DR

This study uses high-resolution near-infrared spectroscopy to analyze the gas dynamics and excitation in the centers of two megamaser disk galaxies, aiming to understand how gas feeds the supermassive black holes.

Contribution

It provides detailed kinematic and excitation diagnostics of nuclear gas in NGC 4388 and NGC 1194 at ~50 pc scales, linking gas motions to the megamaser disks and nuclear activity.

Findings

Detection of a cold stellar core in NGC 4388's center.

Evidence of non-circular gas motions aligned with the megamaser disk.

Ionized gas outflows traced by high-ionization lines and Br-gamma.

Abstract

We explore the warm molecular and ionized gas in the centers of two megamaser disk galaxies using K-band spectroscopy. Our ultimate goal is to determine how gas is funneled onto the accretion disk, here traced by megamaser spots on sub-pc scales. We present NIR IFU data with a resolution of ~50 pc for two galaxies: NGC 4388 with VLT/SINFONI and NGC 1194 with Keck/OSIRIS+AO. The high spatial resolution and rich spectral diagnostics allow us to study both the stellar and gas kinematics as well as gas excitation on scales only an order of magnitude larger than the maser disk. We find a drop in the stellar velocity dispersion in the inner ~100 pc of NGC 4388, a common signature of a dynamically cold central component seen in many active nuclei. We also see evidence for non-circular gas motions in the molecular hydrogen on similar scales, with the gas kinematics on 100-pc scales aligned with the megamaser disk. In contrast, the high ionization lines and Br-gamma trace outflow along the 100 pc-scale jet. In NGC 1194, the continuum from the accreting black hole is very strong, making it difficult to measure robust two-dimensional kinematics, but the spatial distribution and line ratios of the molecular hydrogen and Br-gamma have consistent properties between the two galaxies.