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Lacy, Mark; Rawlings, Steve and Serjeant, Stephen
(1998).
DOI: https://doi.org/10.1046/j.1365-8711.1998.01880.x
URL: http://dx.doi.org/10.1046/j.1365-8711.1998.01880.x
Abstract
We use new high-resolution near-infrared spectroscopy and our previously published optical spectroscopy of the gravitationally lensed Seyfert 2 galaxy F10214+4724 to study both the links between the starburst and AGN in this object and the properties of the line-emitting clouds in the inner narrow-line region. Close inspection of the rest frame UV spectrum reveals interstellar or stellar absorption features consistent with a compact, moderately reddened starburst providing about half the UV light, and explaining the dilution of the UV continuum polarization relative to the broad emission lines. Spectroscopy of the Hα/[N ii] line blend has enabled us to assess the relative contributions of the emission from the narrow-line region of the Seyfert 2, and a moderately reddened emission-line region which we argue is associated with the starburst activity. Estimates of the star formation rate from the unpolarized UV continuum flux and the Hα flux are consistent to within their associated uncertainties. We find we can plausibly explain the unusual emission-line properties of F10214+4724 in terms of conventional models for nearby Seyfert 2 galaxies if lensing is preferentially magnifying the side of the inner narrow-line region between the AGN and the observer, and the other side is both less magnified and partially obscured by the torus. The hydrogen densities of clouds in this region are high enough to make the Balmer lines optically thick and to suppress forbidden emission lines with low critical densities. From the emission-line spectrum we have deduced the column density of both ionized and neutral gas in the narrow-line clouds, and the density of the ionized gas. Using these we have been able to estimate the mass of the inner narrow-line clouds to be ∼ 1 M, and show that the gas:dust ratio NH/E(B−V) in these clouds must be ∼ 1.3 × 1027 m−2 mag−1, significantly higher than the average value in the Milky Way, ∼ 4.5 × 1025 m−2 mag−1. The column density and low dust content of a typical cloud are consistent with the properties of the warm absorbers seen in the X-ray spectra of Seyfert 1 galaxies. Our results thus favour models in which the narrow-line clouds start life close to the nucleus and flow out. An emission line from the lensing system has allowed us to confirm its redshift as z ≈ 0.9.
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