Relation Between The Variations In Broad MgⅡ λ2798 Emission-line And 3000(?) Continuum

As a practical and powerful tool to study the structure and kinematics of the BLRs,reverberation mapping(RM)experiments have been successful for ≥ 60 AGNs to date.According to these RM observations,the size of the BLR as measured for a particular emission line is closely related to the luminosity of the AGN in the approximate form R ∝ L1/2.R-L relation has already led to the development of the so-called single-epoch virial black hole(BH)mass estimators.This empirical method has been widely applied in studies of estimating AGN BH masses based on broad emission-lines(BELs)and thus helps us broadly understand numerous physical processes associated with the central BH,as well as the assembly history of the SMBH population across cosmic time.RM studies,as they are known,have been traditionally performed mostly on low-luminosity AGN at low redshift(z<0.3)using the Hβ λ4863 emission-line to measure active BH mass.For AGNs at redshifts beyond 1,rest-frame ultraviolet(UV)BELs are required,such as Mg ii λ2798,a crucial line of RM interest that can be observed in the optical at 0.3<z<2.However,RM results of MgⅡ line are quite scarce.This is initially interpreted that the MgⅡ emission-line responds more slowly to continuum changes than H(3 and that the MgⅡ-emitting region is larger than that of Hβ.This thesis here mainly investigates the first interpretation,that is to predict the detailed response of the MgⅡ emission-line to variations in the ionizing 3000 A contin-uum.We use the quasar data(0.65<z<1.5)from the compilation of the SDSS-DR7Q and BOSS-DR12Q,each with at least 2 spectroscopic epochs in SDSS-Ⅰ/Ⅱ/Ⅲ surveys.Additionally,we adopt certain signal-to-noise ratio(S/N)selection criteria to obtain the variations of MgⅡ and continuum.In the meantime,we notice that additional flux deficit is confirmed in SDSS-Ⅲ BOSS relative to SDSS-Ⅰ/Ⅱ due to the difference in flux calibration from SDSS-Ⅰ/Ⅱ to BOSS.To obtain an accurate and reliable measure-ment of the intrinsic variations of MgⅡ and continuum for each quasar,we develop an independent and empirical correction to the line flux,which is called narrow-line flux-recalibration.Through narrow-line flux-recalibration,we find a fairly close correlation(Spear-man ρ = 0.593)between the variations in broad MgⅡ and the continuum,which is consistent with the idea that MgⅡ is varying in response to the continuum emission variations.Adopting the modified weighted least squares regression method,we statis-tically constrain the responsivity α of the broad MgⅡ for the sources in different lumi-nosity bins after eliminating intrinsic biases introduced by the rescaling process itself.The results indicate that the MgⅡ variations are well scaled with the continuum vari-ations with a continuum-level dependent emission-line responsivity α = α(Lcont(t)).Furthermore,we confirm an average α = 0.464(median 0.428),suggesting that high signal-to-noise flux measurements are likely required to robustly detect the intrinsic variability and reliable lag of MgⅡ.