The Size of the Narrow-line-emitting Region in the Seyfert 1 Galaxy NGC 5548 from Emission-line Variability
Kochanek, C. S.; Vestergaard, M.; Peterson, B. M.; Pogge, R. W.; Ciroi, S.; De Rosa, G.; Bentz, M. C.; Dalla Bontà, E.; Grier, C. J.; Denney, K. D.; Kilerci-Eser, E.
United States, Denmark, France, Italy
Abstract
The narrow [O III] λλ4959, 5007 emission-line fluxes in the spectrum of the well-studied Seyfert 1 galaxy NGC 5548 are shown to vary with time. From this we show that the narrow-line-emitting region has a radius of only 1-3 pc and is denser (n e ~ 105 cm-3) than previously supposed. The [O III] line width is consistent with virial motions at this radius given previous determinations of the black hole mass. Since the [O III] emission-line flux is usually assumed to be constant and is therefore used to calibrate spectroscopic monitoring data, the variability has ramifications for the long-term secular variations of continuum and emission-line fluxes, though it has no effect on shorter-term reverberation studies. We present corrected optical continuum and broad Hβ emission-line light curves for the period 1988-2008.