Ionized outflows in local luminous AGN: what are the real densities and outflow rates?

Abstract

We report on the determination of electron densities, and their impact on the outflow masses and rates, measured in the central few hundred parsecs of 11 local luminous active galaxies. We show that the peak of the integrated line emission in the AGN is significantly offset from the systemic velocity as traced by the stellar absorption features, indicating that the profiles are dominated by outflow. In contrast, matched inactive galaxies are characterised by a systemic peak and weaker outflow wing. We present three independent estimates of the electron density in these AGN, discussing the merits of the different methods. The electron density derived from the [SII] doublet is significantly lower than than that found with a method developed in the last decade using auroral and transauroral lines, as well as a recently introduced method based on the ionization parameter. The reason is that, for gas photoionized by an AGN, much of the [SII] emission arises in an extended partially ionized zone where the implicit assumption that the electron density traces the hydrogen density is invalid. We propose ways to deal with this situation and we derive the associated outflow rates for ionized gas, which are in the range 0.001–0.5 M⊙ yr−1 for our AGN sample. We compare these outflow rates to the relation between $\dot{M}_{\rm out}$ and LAGN in the literature, and argue that it may need to be modified and rescaled towards lower mass outflow rates.

Ionized outflows in local luminous AGN: Density and outflow rate

“…The reason for this stems from the high energy photons produced by an AGN, which can penetrate deep into clouds, creating a partially ionized zone. As explained by Davies et al (2020), the ionic fraction of S + remains high even at depths where the fraction of H + is already greatly reduced. As an illustration, in a cloud with N H = 10 3 cm −3 , only about 25% of the [SII] emission arises from the fully ionized gas; about half comes from the narrow region around the ionization front where the electron density descreases by a factor 10; and the rest arises from mostly neutral gas deeper in the cloud, where the electron density is only 1-10% that of the fully ionized gas.…”

“…The filled blue circles denote our adjustment to those points, in which we have used our best estimate of the density. The green crosses are from Baron & Netzer (2019); and the filled red circles are the data for our local luminous AGN in Davies et al (2020). It can be seen that the scatter of this proposed revision to the relation is rather larger, and the outflow rates are lower by about a factor three.…”

“…In this section we compare the densities (n e ∼ n p ∼ n H in fully ionized gas) we find for the ionized gas measured in three rather different ways. Additional details about the methods, and of the line fitting and modelling can be found in Davies et al (2020).…”

“…Filled blue circles are our modficiation based on our best estimate of the density. Green crosses are from Baron & Netzer (2019); and the filled red circles are for the AGN in this study and Davies et al (2020).…”

“…This is applicable up to ∼10 4 cm −3 for stellar HII regions; but may be biased in AGN photoionized gas because the high energy photons penetrate deep into the clouds and create a partially ionized zone, which is responsible for the enhanced emisson from low excitation lines around AGN. The impact of this on the density one derives has recently been assessed by Davies et al (2020).…”