NGC 839: Shocks in an M82-like Superwind

TL;DR

This study uses spectroscopic observations of NGC 839 to analyze superwind shocks, revealing shock excitation as a key driver of LINER emission and proposing a formation scenario for E+A galaxies related to M82.

Contribution

It introduces a new grid of low-velocity shock models fitting superwind data, providing diagnostics for shock excitation in galaxy outflows.

Findings

Shock models fit the emission line diagnostics well.

Shock excitation likely causes extended LINER emission.

NGC 839's properties suggest a link to M82-like starburst evolution.

Abstract

We present observations of NGC 839 made with the Wide Field Spectrograph (WiFeS) on the ANU 2.3m telescope. Our data cover a region 25" x 60" at a spatial resolution of ~1.5". The long axis of the field is aligned with the superwind we have discovered in this starburst galaxy. The data cover the range of 3700-7000 {\AA}, with a spectral resolution R~7000 in the red, and R~3000 in the blue. We find that the stellar component of the galaxy is strongly dominated by a fast rotating intermediate-age (~400 Myr) A-Type stellar population, while the gas is concentrated in a bi-conical polar funnel. We have generated flux distributions, emission line ratio diagnostics and velocity maps in both emission and absorption components. We interpret these in the context of a new grid of low-velocity shock models appropriate for galactic-scale outflows. These models are remarkably well fit to the data, providing for the first time model diagnostics for shocks in superwinds and strongly suggesting that shock excitation is largely responsible for the extended LINER emission in the outflowing gas in NGC 839. Our work may have important implications both for extended LINER emission seen in other galaxies, as well as in the interpretation of objects with "composite" spectra. Finally, we present a scenario for the formation of E+A galaxies based upon our observations of NGC 839, and its relation to M82.