Photoionization Modelling of the Giant Broad-Line Region in NGC 3998

Abstract

Prior high angular resolution spectroscopic observations of the Low-ionization nuclear emission-line region (Liner) in NGC 3998 obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST) revealed a rich UV-visible spectrum consisting of broad permitted and broad forbidden emission lines. The photoionization code XSTAR is employed together with reddening-insensitive emission line diagnostics to constrain a dynamical model for the broad-line region (BLR) in NGC 3998. The BLR is modelled as a large H${}^{+}$ region $\sim$ 7 pc in radius consisting of dust-free, low density $\sim$ 10${}^4$ cm${}^{-3}$, low metallicity $\sim$ 0.01 $Z/Z_{\odot }$ gas. Modelling the shape of the broad H$\alpha$ emission line significantly discriminates between two independent measures of the black hole mass, favouring the estimate of de Francesco et al. (2006). Interpreting the broad H$\alpha$ emission line in terms of a steady-state spherically symmetric inflow leads to a mass inflow rate of 1.4 $\times$ 10${}^{-2}$ M${}_{\odot }$/yr, well within the present uncertainty of calculations that attempt to explain the observed X-ray emission in terms of an advection-dominated accretion flow (ADAF). Collectively, the model provides an explanation for the shape of the H$\alpha$ emission line, the relative intensities and luminosities for the H Balmer, [O III], and potentially several of the broad UV emission lines, as well as refining the initial conditions needed for future modelling of the ADAF.

Accepted for publication in MNRAS, 10 pages, 8 figures

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