Quantifying Feedback from Narrow Line Region Outflows in Nearby Active Galaxies. IV. The Effects of Different Density Estimates on the Ionized Gas Masses and Outflow Rates

TL;DR

This study compares different methods of estimating ionized gas masses in AGN outflows, finding that models allowing ionization parameter variation best match detailed multi-component models, aiding larger sample analyses.

Contribution

It introduces simplified techniques for estimating gas masses in AGN outflows, comparing their effectiveness to detailed multi-component models, especially when spectral data is limited.

Findings

Constant density assumptions overestimate gas masses.

[S II]-based densities marginally match total masses.

Variable ionization parameter models best fit observed profiles.

Abstract

Active galactic nuclei (AGN) can launch outflows of ionized gas that may influence galaxy evolution, and quantifying their full impact requires spatially resolved measurements of the gas masses, velocities, and radial extents. We previously reported these quantities for the ionized narrow-line region (NLR) outflows in six low-redshift AGN, where the gas velocities and extents were determined from Hubble Space Telescope long-slit spectroscopy. However, calculating the gas masses required multi-component photoionization models to account for radial variations in the gas densities, which span $\sim$6 orders of magnitude. In order to simplify this method for larger samples with less spectral coverage, we compare these gas masses with those calculated from techniques in the literature. First, we use a recombination equation with three different estimates for the radial density profiles. These include constant densities, those derived from [S II], and power-law profiles based on constant values of the ionization parameter ($U$). Second, we use single-component photoionization models with power-law density profiles based on constant $U$, and allow $U$ to vary with radius based on the [O III]/H$\beta$ ratios. We find that assuming a constant density of $n_\mathrm{H} =$ 10$^2$ cm$^{-3}$ overestimates the gas masses for all six outflows, particularly at small radii where the outflow rates peak. The use of [S II] marginally matches the total gas masses, but also overestimates at small radii. Overall, single-component photoionization models where $U$ varies with radius are able to best match the gas mass and outflow rate profiles when there are insufficient emission lines to construct detailed models.