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

TL;DR

This study models the broad-line region in NGC 3998 using photoionization simulations, constraining its physical properties and black hole mass, and linking the emission line features to accretion processes.

Contribution

It provides a detailed dynamical model of the BLR in NGC 3998, incorporating dust-free, low-density gas and constraining black hole mass and inflow rate.

Findings

The BLR is a large, dust-free, low-density region about 7 pc in radius.

The broad Hα line shape favors a specific black hole mass estimate.

The inflow rate is consistent with ADAF models for X-ray emission.

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.