The Study Of Narrow Emission Lines In Active Galactic Nuclei

Narrow emission lines from Active Galactic Nuclei (AGNs) are produced bygas photoionized by AGN continuum on scales of pc to 10² pc scales. They carryimportant information about the ionizing continuum, which is "hiddened" in alarge fraction of AGNs, and the physical conditions and kinematics of gas on thosescales. Because of its relative large size, Narrow Line Region (NLR) is the onlystructure that can be directed imaged with current optical telescopes. Imagingof very extended NLR with ground optical telescope has been proved to be verysuccessful, however, the real breakthrough comes from Hubble Space Telescope(HST) imaging and image spectroscopy of NLR in variety type of AGN, whichreveals very details of NLR structure. Comparing to image studies, spectroscopy isa traditional tool in the NLR study. With the abundant lines, the AGN spectrumprovides very rich information on the physics of NLR, the ionizing continuum,and gas kinematics, which may provide improtant constraints on the enviromentof the AGN. In this thesis, we carried out, a study of the NLR structure in nearbylow luminosity AGNs using the data from HST, and narrow Feâ…¡emission linesand outflows in a peculiar quasar.Low level nuclear activity occurs in the a substantial fraction of nearby brightgalaxies. Majority of them can be classified spectroscopically as either Seyfertgalaxies or LINERs. Though the phenom has been studied for nearly thirty years,its nature is still controversial. We retrieved HST WFPC2 images of 23 lowluminosity AGN in the sample of Palomer spectroscopic survey carried by Ho etel. (1995), and study in detail the structure of NLR. We find in all cases, NLRs arewell resolved, and their morphology differs from object to object. Chimps, linearstructure, spiral arms or ring are dected, while there is no significant differencebetween typeâ… and typeâ…¡, and between Seyferts and LINERs. We find thatNLR size and narrow line luminosity are strongly correlated for both LINERsand low luminosity Seyferts, and that the size of Hα+[Nâ…¡] emission line regionscales with Hαluminosity as R_NLR∝L_Hα^0.44±0.06,consistent with the extention of NLR size-luminosity relation definded for luminuous Seyfert galaxies and PGquasars(Berinert et al.2002; Schmitt et al. 2003) to two orders of magnitudelower in luminosity and to low level acticities of Seyfert galaxies, and they maybepowered by the central active galactic nucleus.We report the identification of SDSS J102839.11+450009.4 as a peculiarlyinteresting quasar. It shows very prominent narrow Feâ…¡emission lines(NELs)in Feâ…¡, remarkably different from the broad Feâ…¡multiplets commonly observedin quasars and Seyfert 1 galaxies. The narrow Feâ…¡spectrum is dominated bytransitions to the low lying even parity ground levels in UV, and forbidden tran-sitions in optical; and thus is very likely formed in a warm collisionally ionizedlow density medium. Comparison with photoionization model suggests that flu-orescence of Feâ…¡by a soft continuum is probabily is also important to explainthe Feâ…¡line ratios and Feâ…¡line to Mgâ…¡and Sâ…¡4074. These narrow Feâ…¡lines have similar width as [Oâ…¢] and other narrow lines, but are blueshifted rel-ative to [Oâ…¢] by～170 km s^-1, In addition to the unusual narrow Feâ…¡emissionlines, the SDSS spectrum also reveals narrow absorption lines (NALs) in Hβ, Hγ,Hδ, HeⅠλλ3889,3189,2946,2830 and MgⅡλλ2796,2803. Very few quasars show Heâ… NALs, and even fewer exhibit Balmer absorption lines. The Balmer and Heâ… ab-sorption lines are blueshifted with respect to the systematic velocity by around670 km s^-1, while two Mgâ…¡absorption systems are identified, blueshifted by 1200km s^-1 and 670 km s^-1, respectively. The Heâ… requires a column density of hydro-gen in an Hâ…¡region of greater than 10²⁰ cm^-2. The unique spectral propertiesmake this quasar a promising probe to the formation of Feâ…¡emission lines andoutflows, and the possible relation between the two.