Investigating the Narrow-line Region Dynamics in Nearby Active Galaxies

Abstract

We present dynamical models of the narrow-line region (NLR) outflows in the nearby Seyfert galaxies Mrk 3, Mrk 78, NGC 1068, and NGC 4151 using observations from the Hubble Space Telescope and Apache Point Observatory. We employ long-slit spectroscopy to map the spatially resolved outflow and rotational velocities of the ionized gas. We also perform surface brightness decompositions of host galaxy images to constrain the enclosed stellar mass distributions as functions of distance from the supermassive black holes (SMBHs). Assuming that the NLR gas is accelerated by active galactic nuclei (AGN) radiation pressure, and subsequently decelerated by the host galaxy and SMBH gravitational potentials, we derive outflow velocity profiles where the gas is launched in situ at multiple distances from the SMBH. We find a strong correlation between the turnover (from acceleration to deceleration) radii from our models, with the turnovers seen in the observed velocities and spatially resolved mass outflow rates for the AGN with bolometric luminosities > 1044 erg s−1. This consistency indicates that radiation pressure is the dominant driving mechanism behind the NLR outflows in these moderate-luminosity AGNs, with a force multiplier ∼500 yielding the best agreement between the modeled and observed turnover radii. However, in Meena et al. we found that this trend may not hold at lower luminosities, where our modeled turnover distance for NGC 4051 is much smaller than in the observed kinematics. This result may indicate that either additional force(s) are responsible for accelerating the NLR outflows in low-luminosity AGNs, or higher spatial resolution observations are required to quantify their turnover radii.