Dynamics of Warm-Absorbing Gas in Seyfert Galaxies: NGC 5548

Abstract

A hydromagnetic (MHD) wind from a clumpy molecular accretion disk is invoked to explain observations of warm absorbing (WA) gas in UVX from Sy galaxies. This paper focuses on two issues: (1) compatibility of kinematics and dynamics of MHD wind with the observed properties of WAs; and (2) relationship between the UVX absorptions. We provide an in-depth comparison between the MHD model and the Sy 1 galaxy NGC 5548, which at high spectral resolution exhibits a number of discrete UV absorption components. We find that: (1) the total column densities of Ovii, Oviii and H, are reproduced by constraining the UV ion column densities of Civ and Nv in each component to lie within a factor of 2 of their observed values and optimizing over the possible sets of component ionization states and Civ column densities; (2) the WA exists in the outer part of the wind and is not a continuation of the flow in the BLR; and (3) the WA extends in radial and polar directions and is ionization-stratified. X-ray absorption is found to be heavily biased towards smaller r, and UV absorption originates at larger distances from the central continuum source. We show that the discrete absorption components along the line-of-sight are intrinsically clumpy. Density differences between kinematic components result in a range of ionization and recombination timescales. We further test the applicability of the MHD wind to WAs in general, by constructing a quasi-continuous flow model, and extending it to arbitrary aspect angles. We estimate the fraction of Sy 1s having detectable WAs with larger Ovii column density than Oviii, and the range of total H column densities. We also find that the ratio of Ovii to Oviii optical depths can serve as a new diagnostic of AGN aspect angle.