Black hole mass is a fundamental property of active galactic nuclei (AGNs).In the distant universe, black hole mass is commonly estimated using the MgII,Hbeta, or Halpha emission line widths and the optical/UV continuum or lineluminosities, as proxies for the characteristic velocity and size of thebroad-line region. Although they all have a common calibration in the localuniverse, a number of different recipes are currently used in the literature.It is important to verify the relative accuracy and consistency of the recipes,as systematic changes could mimic evolutionary trends when comparing varioussamples. At z=0.36, all three lines can be observed at optical wavelengths,providing a unique opportunity to compare different empirical recipes. We usespectra from the Keck Telescope and the Sloan Digital Sky Survey to compareblack hole mass estimators for a sample of nineteen AGNs at this redshift. Wecompare popular recipes available from the literature, finding that massestimates can differ up to 0.38+-0.05 dex in the mean (or 0.13+-0.05 dex, ifthe same virial coefficient is adopted). Finally, we provide a set of 30internally self consistent recipes for determining black hole mass from avariety of observables. The intrinsic scatter between cross-calibrated recipesis in the range 0.1-0.3 dex. This should be considered as a lower limit to theuncertainty of the black hole mass estimators.
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