The complex, dusty narrow-line region of NGC 4388: Gas-jet interactions, outflows, and extinction revealed by near-IR spectroscopy

TL;DR

This study uses near-infrared spectroscopy to analyze the complex gas dynamics, dust extinction, and ionization mechanisms in the narrow-line region of NGC 4388, revealing the influence of jets, shocks, and obscuration.

Contribution

First spatially-extended emission line mapping of NGC 4388's NLR, highlighting the roles of shocks and radiation in gas excitation and obscuration.

Findings

Heavily obscured nuclear region with E(B-V) ~1.9 mag.

High-ionization emission extends at least 200 pc from the nucleus.

Shocks from radio jets significantly contribute to gas excitation.

Abstract

We present Gemini/GNIRS spectroscopy of the Seyfert 2 galaxy NGC 4388, with simultaneous coverage from 0.85 - 2.5 $\mu$m. Several spatially-extended emission lines are detected for the first time, both in the obscured and unobscured portion of the optical narrow line region (NLR), allowing us to assess the combined effects of the central continuum source, outflowing gas and shocks generated by the radio jet on the central 280 pc gas. The HI and [FeII] lines allow us to map the extinction affecting the NLR. We found that the nuclear region is heavily obscured, with E(B-V) ~1.9 mag. To the NE of the nucleus and up to ~150 pc, the extinction remains large, ~1 mag or larger, consistent with the system of dust lanes seen in optical imaging. We derived position-velocity diagrams for the most prominent lines as well as for the stellar component. Only the molecular gas and the stellar component display a well-organized pattern consistent with disk rotation. Other emission lines are kinematically perturbed or show little evidence of rotation. Extended high-ionization emission of sulfur, silicon and calcium is observed to distances of at least 200 pc both NE and SW of the nucleus. We compared flux ratios between these lines with photoionization models and conclude that radiation from the central source alone cannot explain the observed high-ionization spectrum. Shocks between the radio-jet and the ambient gas are very likely an additional source of excitation. We conclude that NGC 4388 is a prime laboratory to study the interplay between all these mechanisms.