Blasts and shocks in the disc of NGC 4258

TL;DR

This study uses integral field spectroscopy to analyze the gas dynamics in NGC 4258, revealing high-velocity shocks and bipolar outflows likely caused by nuclear jet activity or hypershell structures.

Contribution

First detailed spectroscopic analysis of neutral and ionized gas kinematics in NGC 4258's central region, supporting models of jet-induced shocks or bipolar hypershells.

Findings

Detection of high-velocity neutral gas (120-370 km/s)

Identification of bipolar ionized gas expansion up to 300 km/s

Evidence of shocked gas regions aligned with X-ray emission

Abstract

We present integral field spectroscopic observations of the central region of the active galaxy NGC 4258 obtained with the fibre IFU system INTEGRAL. We have been able to detect cold neutral gas by means of the interstellar NaD doublet absorption and to trace its distribution and kinematics with respect to the underlying disc. The neutral gas is blue-shifted with projected velocities in the 120--370 km/s range. We have also detected peculiar kinematics in part of the ionized gas in this region by means of a careful kinematic decomposition. The bipolar spatial distribution of the broader component is roughly coincident with the morphology of the X-ray diffuse emission. The kinematics of this gas can be explained in terms of expansion at very high (projected) velocities of up to 300 km/s. The observations also reveal the existence of a strip of neutral gas, parallel to the major kinematic axis, that is nearly coincident with a region of very high [SII]/H$\alpha$ ratio tracing the shocked gas. Our observations are consistent with the jet model presented by \cite{wilsonetal01} in which a cocoon originating from the nuclear jet is shocking the gas in the galaxy disc. Alternatively, our observations are also consistent with the bipolar hypershell model of \cite{Sofue80} and \cite{SofueandVogler01}. On balance, we prefer the latter model as the most likely explanation for the puzzling features of this peculiar object.