The HST view of the innermost narrow line region

TL;DR

This study uses HST observations to analyze the innermost narrow line region in low-luminosity AGN, revealing differences between LINERs and Seyferts related to the density and obscuration of ionized gas regions.

Contribution

It provides new insights into the structure of the innermost narrow line region in LINERs and Seyferts, highlighting the role of the circumnuclear torus and spatial resolution effects.

Findings

LINERs have a dense, compact ionized gas region at about 3 pc.

HST resolves a denser ILR component in LINERs not seen in Seyferts.

The presence of the torus affects the visibility of the ILR in Seyferts.

Abstract

We analyze the properties of the innermost narrow line region in a sample of low-luminosity AGN. We select 33 LINERs (bona fide AGN) and Seyfert galaxies from the optical spectroscopic Palomar survey observed by HST/STIS. We find that in LINERs the [NII] and [OI] lines are broader than the [SII] line and that the [NII]/[SII] flux ratio increases when moving from ground-based to HST spectra. This effect is more pronounced considering the wings of the lines. Our interpretation is that, as a result of superior HST spatial resolution, we isolate a compact region of dense ionized gas in LINERs, located at a typical distance of about 3 pc and with a gas density of about 10$^4$-10$^5$ cm$^{-3}$, which we identify with the outer portion of the intermediate line region (ILR). Instead, we do not observe these kinds of effects in Seyferts; this may be the result of a stronger dilution from the NLR emission, since the HST slit maps a larger region in these sources. Alternatively, we argue that the innermost, higher density component of the ILR is only present in Seyferts, while it is truncated at larger radii because of the presence of the circumnuclear torus. The ILR is only visible in its entirety in LINERs because the obscuring torus is not present in these sources.