科研之友
微信
新浪微博
Facebook
分享链接
摘要
We analyse several large samples of active galactic nuclei (AGNs) in order to establish the best tools required to study the evolution of black hole mass (M-BH) and normalized accretion rate (L/L-Edd). The data include spectra from the Sloan Digital Sky Survey, 2dF QSO Redshift survey and 2dF SDSS LRG And QSO survey public surveys at z < 2, and a compilation of smaller samples with 0 < z < 5. We critically evaluate the usage of the Mg II lambda 2798 and C IV lambda 1549 lines, and adjacent continuum bands, as estimators of M-BH and L/L-Edd, by focusing on sources where one of these lines is observed together with H beta. We present a new, luminosity-dependent bolometric correction for the monochromatic luminosity at 3000 angstrom, L-3000, which is lower by a factor of similar to 1.75 than those used in previous studies. We also re-calibrate the use of L-3000 as an indicator for the size of the broad emission-line region (R-BLR) and find that R-BLR. L-3000(0.62), in agreement with previous results. We find that FWHM(MgII) similar or equal to FWHM(H beta) for all sources with FWHM(MgII) less than or similar to 6000 km s(-1). Beyond this full width at half-maximum (FWHM), the MgII line width seems to saturate. The spectral region of the MgII line can thus be used to reproduce H beta-based estimates of M-BH and L/L-Edd, with negligible systematic differences and a scatter of similar to 0.3 dex. The width of the C IV line, on the other hand, shows no correlation with either that of the H beta or the Mg II lines and we could not identify the reason for this discrepancy. The scatter of M-BH(C IV), relative to M-BH(H beta), is of almost 0.5 dex. Moreover, 46 per cent of the sources have FWHM(C IV) less than or similar to FWHM(H beta), in contrast with the basic premise of the virial method, which predicts FWHM(C IV)/FWHM(H beta) less than or similar to root 3.7, based on reverberation mapping experiments. This fundamental discrepancy cannot be corrected based on the continuum slope or any C IV-related observable. Thus, the C IV line cannot be used to obtain precise estimates of M-BH. We conclude by presenting the observed evolution of M-BH and L/L-Edd with cosmic epoch. The steep rise of L/L-Edd with redshift up to z similar or equal to 1 flattens towards the expected maximal value of L/L-Edd similar or equal to 1, with lower M-BH sources showing higher values of L/L-Edd at all redshifts. These trends will be further analysed in a forthcoming paper.
- 出版日期2012-12
