Self-Consistent Grain Depletions and Abundances II: Effects on strong-line diagnostics of extragalactic H II regions

TL;DR

This study investigates how element depletion onto dust grains affects strong-line diagnostics in extragalactic H II regions, revealing complex dependencies and degeneracies among parameters influencing emission lines and thermal balance.

Contribution

It introduces a method to calculate element depletions and their impact on emission lines, highlighting parameter degeneracies and providing observational constraints on depletion strength.

Findings

Depletion strength significantly influences emission line predictions.

Degeneracies exist among metallicity, ionization parameter, and depletion strength.

Best-fit depletion strength varies across different metallicities and requires further observational validation.

Abstract

The depletion of elements onto dust grains is characterized using a generalized depletion strength $F_*$ for any sightline, and trend-line parameters $A_X, B_X$ and $z_X$. The parameters $A_X, B_X$ and $z_X$ define the relative depletion pattern, for which values are published in previous works. The present study uses these parameters to calculate post-depleted gas-phase abundances of 15 different elements while varying $F_*$ from 0 to 1. An analysis of emergent strong spectral line intensities, obtained by inputting the calculated abundances into a cloudy model, shows that the depletion strength has a non-trivial effect on predicted emission lines and the thermal balance of the ionized cloud. The amount by which elements deplete also affects the coolant abundances in the gas. Furthermore, it was found that each of the parameters - metallicity, ionization parameter U and depletion strength $F_*$ have degenerate effects on the emission-line strengths, and thermal balance of the interstellar medium (ISM). Finally, comparing our results to a sample of H II regions using data obtained from the Mapping Nearby Galaxies at Apache Point Observatory survey (MaNGA) revealed that the best-fit $F_*$ was approximately 0.5. However, this best-fit value does not work well for all metallicities. Removing the sulfur depletion and changing the nitrogen abundance pattern can improve the fit. As a result, extra observational evidence is required to verify the choices of parameters and better constrain the typical depletion strength in galaxies.