ASCA Observations of "Type 2" LINERs: Evidence for a Stellar Source of Ionization

TL;DR

This study uses ASCA X-ray observations to investigate 'type 2' LINERs, finding no clear evidence of active galactic nuclei and suggesting stellar sources or obscured AGNs as ionization mechanisms.

Contribution

First comprehensive X-ray analysis of 'type 2' LINERs showing extended X-ray emission and challenging the AGN dominance hypothesis.

Findings

Detected X-ray emission in most LINERs except NGC 404.

Soft X-ray emission likely from hot gas due to star formation.

No clear evidence of active galactic nuclei in the sample.

Abstract

We present ASCA observations of LINERs without broad H$\alpha$ emission in their optical spectra. The sample of "type 2" LINERs consists of NGC 404, 4111, 4192, 4457, and 4569. We have detected X-ray emission from all the objects except for NGC 404; among the detected objects are two so-called transition objects (NGC 4192 and NGC 4569), which have been postulated to be composite nuclei having both an HII region and a LINER component. The images of NGC 4111 and NGC 4569 in the soft (0.5-2 keV) and hard (2-7 keV) X-ray bands are extended on scales of several kpc. The X-ray spectra of NGC 4111, NGC 4457 and NGC 4569 are well fitted by a two-component model that consists of soft thermal emission with $kT\sim0.65$ keV and a hard component represented by a power law (photon index $\sim$ 2) or by thermal bremsstrahlung emission ($kT\sim$ several keV). The extended hard X-rays probably come from discrete sources, while the soft emission most likely originates from hot gas produced by active star formation in the host galaxy. We have found no clear evidence for the presence of active galactic nuclei (AGNs) in the sample. If an AGN component is the primary ionization source of the optical emission lines, then it must be heavily obscured with a column density significantly larger than $10^{23}$ cm$^{-2}$. Alternatively, the optical emission could be ionized by a population of exceptionally hot stars.