Measuring monster MBHs: maybe mighty, maybe merely massive

Accurate black hole mass (MBH) measurements in high-z galaxies are difficult yet crucial to constrain the growth of supermassive BHs, and to discriminate between competing BH seed models. Recent studies claimed the detection of massive BHs in very distant AGN, implying extreme growth conditions. However, these estimates are usually obtained by extrapolating indirect methods that are calibrated for moderately accreting, low-luminosity AGN in the local universe. To assess the reliability of the single epoch method (SE) in the distant universe, we compute the MBH for a sample of hyper-luminous distant quasars and a sample of highly accreting AGN using the X-ray scaling method. We first verify that this X-ray method yields reliable MBH values for distant highly accreting objects. Then, we carry out a systematic comparison with the SE method and find that these two indirect methods yield consistent MBH over a broad range of luminosities, intrinsic absorption, and accretion rates. The only discrepancies are associated with AGN that are substantially absorbed (underestimated by the SE method), and AGN accreting well above the Eddington limit (overestimated by the SE method). The latter result casts some doubts on the claim of overmassive BHs in highly accreting AGN in the early universe. Our study also reveals that one of the frequently used AGN catalogs consistently underestimates the MBH values by a factor of 2.5. Although this factor is of the order of the uncertainty generally associated with the SE method, we demonstrate that the use of underestimated values may result in potentially misleading conclusions. Specifically, for this AGN sample we confirm strong positive correlations for Gamma vs. lambda_Edd and for the X-ray bolometric correction vs. lambda_Edd, as well as for Gamma vs. the soft excess strength, at odds with the conclusions inferred using underestimated MBH values.