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 [O III] lambda 5007 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 Sersic or Voigt profiles convolved with the seeing. These data, which span a full order of magnitude in IR luminosity (log (L-8 mu m/erg s(-1)) = 44.4-45.4), also provide strong evidence that there is a flattening of the relationship between NLR size and active galactic nucleus luminosity at a seeing-corrected size of similar to 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 NLR around luminous quasars; beyond this size, there is either not enough gas or the gas is over-ionized and does not produce enough [O III] lambda 5007 emission.