The Size of the Narrow-Line Emitting Region in the Seyfert 1 Galaxy NGC 5548 from Emission-Line Variability

TL;DR

This study reveals that the narrow [O III] emission lines in NGC 5548 vary over time, indicating a smaller and denser emission region than previously thought, which impacts long-term flux calibration practices.

Contribution

It provides the first measurement of the size and density of the narrow-line region in NGC 5548 using emission-line variability data.

Findings

Narrow [O III] flux varies with time

Narrow-line region radius is 1-3 parsecs

Region density is approximately 10^5 cm^{-3}

Abstract

The narrow [O III] 4959, 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 ~ 10^5 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 Hbeta emission-line light curves for the period 1988 to 2008.