Estimating sizes of type-2 AGN narrow-line regions from multiple survey spectra – a demonstration

Abstract

ABSTRACT In the Letter, an interesting method is proposed to estimate size of narrow emission-lines regions (NLRs) of a type-2 AGN SDSS J083823.91 + 490241.1 (=SDSS J0838) at a redshift of 0.101, by comparing spectroscopic properties through the SDSS fibre (MJD = 51873) (diameter of 3 arcsec) and through the eBOSS fibre (MJD = 55277) (diameter of 2 arcsec). After subtractions of ppxf method determined host galaxy contributions, the narrow emission lines of SDSS J0838 in the SDSS spectrum and in the eBOSS spectrum can be well measured by Gaussian functions, leading more than 90 per cent of [O iii] emissions to be covered by the eBOSS fibre with diameter of 2 arcsec. Meanwhile, both none broad emission components and none-variabilities of ZTF 3 yr-long g-/r-band light curves can be applied to confirm SDSS J0838 as a type-2 AGN, indicating few orientation effects on the projected NLRs size in SDSS J0838. Therefore, upper limit about 1 arcsec (2250 pc) of the NLRs size can be reasonably accepted in SDSS J0838. Combining with the intrinsic reddening corrected [O iii] line luminosity, the upper limit of NLRs size in SDSS J0838 well lies within the 99.9999 per cent confidence bands of the R-L empirical relation for NLRs in AGN.