The Size, Structure and Ionization of the Broad Line Region in NGC 3227

TL;DR

This study uses HST spectroscopy to analyze the size, structure, and ionization of the broad line region in NGC 3227, revealing an inflow model and challenging traditional views of BLRs in AGNs.

Contribution

It provides a detailed model of the BLR in NGC 3227, favoring a spherical inflow over a thin disk and clarifying the true dimensions of the ionized gas region.

Findings

Broad H-alpha flux decreased by ~11% between 1999 and 2000.

The BLR has an outer radius of ~90 light-days and an inner radius of ~3 light-days.

The mass inflow rate is estimated at ~10^{-2} solar masses per year.

Abstract

Hubble Space Telescope (HST) spectroscopy of the Seyfert 1.5 galaxy, NGC 3227, confirms previous reports that the broad H-alpha emission line flux is time variable, decreasing by a modest ~ 11% between 1999 and 2000 in response to a corresponding ~ 37% decrease in the underlying continuum. Modeling the gas distribution responsible for the broad H-alpha, H-beta and H-gamma emission lines favors a spherically symmetric inflow as opposed to a thin disk. Adopting a central black hole mass of 7.6 x 10^{6} Msun, determined from prior reverberation mapping, leads to the following dimensions for the size of the region emitting the broad H-alpha line; an outer radius ~ 90 l.d and an inner radius ~ 3 l.d. Thus, the previously determined reverberation size for the broad line region (BLR) consistently coincides with the inner radius of a much larger volume of ionized gas. However, the perceived size of the BLR is an illusion, a consequence of the fact that the emitting region is ionization bounded at the outer radius and diminished by Doppler broadening at the inner radius. The actual dimensions of the inflow remain to be determined. Nevertheless, the steady state mass inflow rate is estimated to be ~ 10^{-2} Msun/yr which is sufficient to explain the X-ray luminosity of the AGN in terms of radiatively inefficient accretion. Collectively, the results challenge many preconceived notions concerning the nature of BLRs in active galactic nuclei.