Black Hole Growth to z = 2 - I: Improved Virial Methods for Measuring M_BH and L/L_Edd

TL;DR

This study improves methods for estimating black hole mass and accretion rates in active galactic nuclei up to redshift 2, by calibrating spectral lines and proposing new bolometric corrections, revealing evolution trends over cosmic time.

Contribution

It introduces a new luminosity-dependent bolometric correction, recalibrates the use of MgII for black hole mass estimates, and evaluates the reliability of CIV and MgII lines for such measurements.

Findings

MgII line width correlates with Hbeta for FWHM<6000 km/s

CIV line width shows no correlation with Hbeta or MgII, limiting its use for mass estimates

L/L_Edd increases with redshift up to z~1, then flattens at near-maximal values

Abstract

We analyze several large samples of AGN 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 SDSS, 2QZ and 2SLAQ surveys at z<2, and a compilation of smaller samples with 0<z<5. We critically evaluate the usage of the MgII(2798A) and CIV(1549A) 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 Hbeta. We present a new, luminosity-dependent bolometric correction for the monochromatic luminosity at 3000A, L_3000, which is lower by a factor of 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 is proportional to L_3000^0.62. We find that FWHM(MgII)\simeqFWHM(Hb) for all sources with FWHM(MgII)<6000 km/s. Beyond this FWHM, the MgII line width seems to saturate. The spectral region of the MgII line can thus be used to reproduce Hb-based estimates of M_BH and L/L_Edd, with negligible systematic differences and a scatter of 0.3 dex. The width of the CIV line, on the other hand, shows no correlation with either that of the Hb or the MgII lines and we could not identify the reason for this discrepancy. The scatter of M_BH(CIV), relative to M_BH(Hb) is of almost 0.5 dex. Moreover, 46% of the sources have FWHM(CIV)<FWHM(Hb), in contrast with the basic premise of the virial method, which predicts FWHM(CIV)/FWHM(Hbeta)~1.9. Thus, the CIV 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~1 flattens towards the expected maximal value of L/L_Edd~1, with lower-M_BH sources showing higher values of L/L_Edd at all redshifts. [Abridged]