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

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 ∼ 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 Hβ emission-line light curves for the period 1988 to 2008.

Anatomy of the AGN in NGC 5548

“…While the NLR is sufficiently large to be spatially resolved in some AGNs, it is unresolved in NGC 5548. However, Peterson et al (2013) measure a size for the [O III]-emitting region of 1−3 pc based on emission-line variability, consistent with the photoionization prediction of Kraemer et al (1998). Similarly, Detmers et al (2009) estimate a radius of 1−15 pc for the X-ray NLR.…”

“…Variability has been presented by Detmers et al (2009), using O VII f (forbidden line) in the X-rays, and both Peterson et al (2013) and Denney et al (2014), using [O III] in the optical (the papers refer to NGC 5548 in the former two cases and Mrk 590 in the latter).…”

Spectroscopic Indication of a Centi-Parsec Supermassive Black Hole Binary in the Galactic Center of Ngc 5548

“…This leads to F 5100 =3.4×10 −15 erg s −1 cm −2 Å −1 for a V-band magnitude of 14.99 . We compared our host galaxy corrections with those given in Peterson et al (2013) and find that the differences are generally smaller than 10%.…”

“…Figures 3(a) and (b) plot light curves 7 We note that Peterson et al (2013) of the 5100 Å continuum and Hβ fluxes, respectively. Appendix A shows all the Hβ profiles.…”

“…Since we only focus on the effect of host galaxy contamination on the broad Hβ profile, we neglect other components such as narrow Hβ, [O III], Balmer continuum, and Fe II blends. We set the host galaxy flux at 5100 Å to the value determined from the AGN Watch campaigns, F gal =3.75× 10 −15 erg s −1 cm −2 Å −1 (Peterson et al 2013). Note that this choice does not affect our test because the results only depend on the flux ratio between the central AGN and the host galaxy starlight F AGN /F gal at 5100 Å.…”