Gemini Long-slit Observations of Luminous Obscured Quasars: Further Evidence for an Upper Limit on the Size of the Narrow-Line Region

Abstract

We examine the spatial extent of the narrow-line regions (NLRs) of a sample of 30 luminous obscured quasars at \(0.4 < z < 0.7\) observed with spatially resolved Gemini-N GMOS long-slit spectroscopy. Using the [OIII]\(\lambda5007\) emission feature, we estimate the size of the NLR using a cosmology-independent measurement: the radius where the surface brightness falls to 10\(^{-15}\) erg s\(^{-1}\) cm\(^{-2}\) arcsec\(^{-2}\). We then explore the effects of atmospheric seeing on NLR size measurements and conclude that direct measurements of the NLR size from observed profiles are too large by 0.1 - 0.2 dex on average, as compared to measurements made to best-fit S\'{e}rsic or Voigt profiles convolved with the seeing. These data, which span a full order of magnitude in IR luminosity (\(\log{(L_{8 \mu \mathrm{m}} / \mathrm{erg\, s}^{-1})} = 44.4 - 45.4\)) also provide strong evidence that there is a flattening of the relationship between NLR size and AGN luminosity at a seeing-corrected size of \(\sim 7\) kpc. The objects in this sample have high luminosities which place them in a previously under-explored portion of the size-luminosity relationship. These results support the existence of a maximal size of the narrow-line region around luminous quasars; beyond this size either there is not enough gas, or the gas is over-ionized and does not produce enough [OIII]\(\lambda5007\) emission.