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High-accuracy black hole (BH) masses require excellent spatial resolutionthat is only achievable for galaxies within ~100 Mpc using present-daytechnology. At larger distances, BH masses are often estimated withsingle-epoch scaling relations for active galactic nuclei. This method requiresonly luminosity and the velocity dispersion of the broad line region (BLR) tocalculate a virial product, and an additional virial factor, $f$, to determineBH mass. The accuracy of these single-epoch masses, however, is unknown, andthere are few empirical constraints on the variance of $f$ between objects. Weattempt to calibrate single-epoch BH masses using spectropolarimetricmeasurements of nine megamaser galaxies from which we measure the velocitydistribution of the BLR. We do not find strong evidence for a correlationbetween the virial products used for single-epoch masses and dynamical mass,neither for the megamaser sample alone or when combined with dynamical massesfrom reverberation mapping modeling. Furthermore, we find evidence that thevirial parameter $f$ varies between objects, but we do not find strong evidencefor a correlation with other observable parameters such as luminosity or broadline width. Although we cannot definitively rule out the existence of anycorrelation between dynamical mass and virial product, we find tension betweenallowed $f$ values for masers and those widely used in the literature. Weconclude that the single-epoch method requires further investigation if it isto be used successfully to infer BH masses.

astrophysical journal/the astrophysical journal

Spectropolarimetric measurements of hidden broad lines in nearby  megamaser galaxies: a lack of clear evidence for a correlation between black  hole masses and virial products