Infrared Spectroscopy of the Diffuse Ionized Halo of NGC 891

TL;DR

This study uses infrared spectroscopy to analyze the ionization and PAH features in the halo of NGC 891, revealing the need for additional ionization sources and detecting extraplanar PAH emissions with specific scale-heights.

Contribution

It provides the first spectroscopic detection of extraplanar PAHs in a normal galaxy and demonstrates that halo ionization cannot be explained by photo-ionization alone, suggesting secondary ionization sources.

Findings

[Ne III]/[Ne II] ratio is enhanced in the halo, indicating harder ionizing radiation.

Photo-ionization models cannot reproduce the observed ionization trend, implying additional ionization sources.

Detected extraplanar PAH features with estimated scale-heights of 330-530 pc.

Abstract

We present infrared spectroscopy from the Spitzer Space Telescope at one disk position and two positions at a height of 1 kpc from the disk in the edge-on spiral NGC 891, with the primary goal of studying halo ionization. Our main result is that the [Ne III]/[Ne II] ratio, which provides a measure of the hardness of the ionizing spectrum free from the major problems plaguing optical line ratios, is enhanced in the extraplanar pointings relative to the disk pointing. Using a 2D Monte Carlo-based photo-ionization code which accounts for the effects of radiation field hardening, we find that this trend cannot be reproduced by any plausible photo-ionization model, and that a secondary source of ionization must therefore operate in gaseous halos. We also present the first spectroscopic detections of extraplanar PAH features in an external normal galaxy. If they are in an exponential layer, very rough emission scale-heights of 330-530 pc are implied for the various features. Extinction may be non-negligible in the midplane and reduce these scale-heights significantly. There is little significant variation in the relative emission from the various features between disk and extraplanar environment. Only the 17.4 micron feature is significantly enhanced in the extraplanar gas compared to the other features, possibly indicating a preference for larger PAHs in the halo.