THE SIZE OF THE NARROW-LINE-EMITTING REGION IN THE SEYFERT 1 GALAXY NGC 5548 FROM EMISSION-LINE VARIABILITY

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.