The ionization mechanism of NGC 185: how to fake a Seyfert galaxy?

TL;DR

This study investigates how NGC 185 exhibits Seyfert-like emission lines without hosting an active nucleus, suggesting stellar processes like planetary nebulae and supernova remnants can mimic active galactic nuclei signatures.

Contribution

The paper demonstrates that stellar phenomena such as planetary nebulae and supernova remnants can produce Seyfert-like emission line ratios in dwarf galaxies, challenging the assumption of active nuclei.

Findings

Planetary nebulae fluxes can explain the spectrum but not the line ratios.

A mixture of supernova remnants and planetary nebulae can mimic Seyfert line ratios.

Chemical evolution models support the presence of supernova remnants in NGC 185.

Abstract

NGC 185 is a dwarf spheroidal satellite of the Andromeda galaxy. From mid-1990s onwards it was revealed that dwarf spheroidals often display a varied and in some cases complex star formation history. In an optical survey of bright nearby galaxies, NGC 185 was classified as a Seyfert galaxy based on its emission line ratios. However, although the emission lines in this object formally place it in the category of Seyferts, it is probable that this galaxy does not contain a genuine active nucleus. NGC 185 was not detected in radio surveys either in 6 or 20 cm, or X-ray observations, which means that the Seyfert-like line ratios may be produced by stellar processes. In this work, we try to identify the possible ionization mechanisms for this galaxy. We discussed the possibility of the line emissions being produced by planetary nebulae (PNe), using deep spectroscopy observations obtained with GMOS-N, at Gemini. Although the fluxes of the PNe are high enough to explain the integrated spectrum, the line ratios are very far from the values for the Seyfert classification. We then proposed that a mixture of supernova remnants and PNe could be the source of the ionization, and we show that a composition of these two objects do mimic Seyfert-like line ratios. We used chemical evolution models to predict the supernova rates and to support the idea that these supernova remnants should be present in the galaxy.