SALT Long-slit Spectroscopy of Luminous Obscured Quasars: An Upper Limit on the Size of the Narrow-Line Region?

TL;DR

This study uses SALT long-slit spectroscopy to measure the size of narrow-line regions in luminous obscured quasars, revealing a potential size limit and a strong link between NLR size and mid-IR luminosity, indicating significant AGN influence.

Contribution

It provides new measurements of NLR sizes in luminous quasars and establishes a steeper relationship between NLR size and mid-IR luminosity than previously known.

Findings

NLR size scales approximately as L_8micron^(1/2).

Evidence suggests a maximum NLR size of 10-20 kpc.

NLR size correlates with L_[OIII] as L_[OIII] ~ L_8micron^(1.4).

Abstract

We present spatially resolved long-slit spectroscopy from the Southern African Large Telescope (SALT) to examine the spatial extent of the narrow-line regions (NLRs) of a sample of 8 luminous obscured quasars at 0.10 < z < 0.43. Our results are consistent with an observed shallow slope in the relationship between NLR size and L_[OIII], which has been interpreted to indicate that NLR size is limited by the density and ionization state of the NLR gas rather than the availability of ionizing photons. We also explore how the NLR size scales with a more direct measure of instantaneous AGN power using mid-IR photometry from WISE, which probes warm to hot dust near the central black hole and so, unlike [OIII], does not depend on the properties of the NLR. Using our results as well as samples from the literature, we obtain a power-law relationship between NLR size and L_8micron that is significantly steeper than that observed for NLR size and L_[OIII]. We find that the size of the NLR goes approximately as L^(1/2)_8micron, as expected from the simple scenario of constant-density clouds illuminated by a central ionizing source. We further see tentative evidence for a flattening of the relationship between NLR size and L_8micron at the high luminosity end, and propose that we are seeing a limiting NLR size of 10 - 20 kpc, beyond which the availability of gas to ionize becomes too low. We find that L_[OIII] ~ L_8micron^(1.4), consistent with a picture in which the L_[OIII] is dependent on the volume of the NLR. These results indicate that high-luminosity quasars have a strong effect in ionizing the available gas in a galaxy.