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

Abstract

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