Ionization of the diffuse gas in galaxies: Hot low-mass evolved stars at work

TL;DR

This study models the ionization of diffuse gas in NGC 891, showing that hot low-mass evolved stars and OB stars can explain observed emission line ratios and suggesting metallicity is near solar above the galactic plane.

Contribution

It introduces a detailed photoionization model combining OB stars and HOLMES to explain ionization in galaxy halos, emphasizing the role of HOLMES at higher galactic latitudes.

Findings

Electron density decreases with height above the galactic plane.

Models favor solar metallicity in the halo region.

N/O ratio increases with height.

Abstract

We revisit the question of the ionization of the diffuse medium in late type galaxies, by studying NGC 891, the prototype of edge-on spiral galaxies. The most important challenge for the models considered so far was the observed increase of [OIII]/Hbeta, [OII]/Hbeta, and [NII]/Halpha with increasing distance to the galactic plane. We propose a scenario based on the expected population of massive OB stars and hot low-mass evolved stars (HOLMES) in this galaxy to explain this observational fact. In the framework of this scenario we construct a finely meshed grid of photoionization models. For each value of the galactic latitude z we look for the models which simultaneously fit the observed values of the [OIII]/Hbeta, [OII]/Hbeta, and [NII]/Halpha ratios. For each value of z we find a range of solutions which depends on the value of the oxygen abundance. The models which fit the observations indicate a systematic decrease of the electron density with increasing z. They become dominated by the HOLMES with increasing z only when restricting to solar oxygen abundance models, which argues that the metallicity above the galactic plane should be close to solar. They also indicate that N/O increases with increasing z.