Black hole masses for samples of active galactic nuclei ( AGN ) are currently estimated from single-epoch optical spectra .
In particular , the size of the broad-line emitting region needed to compute the black hole mass is derived from the optical or ultraviolet continuum luminosity .
Here we consider the relationship between the broad-line region size , R , and the near-infrared ( near-IR ) AGN continuum luminosity , L , as the near-IR continuum suffers less dust extinction than at shorter wavelengths and the prospects for separating the AGN continuum from host-galaxy starlight are better in the near-IR than in the optical .
For a relationship of the form R \propto L ^ { \alpha } , we obtain for a sample of 14 reverberation-mapped AGN a best-fit slope of \alpha = 0.5 \pm 0.1 , which is consistent with the slope of the relationship in the optical band and with the value of 0.5 naïvely expected from photoionisation theory .
Black hole masses can then be estimated from the near-IR virial product , which is calculated using the strong and unblended Paschen broad emission lines ( Pa \alpha or Pa \beta ) .