Identifying signatures of inflow onto face-on galaxies using the Balmer decrement

TL;DR

This study proposes using the Balmer decrement as a diagnostic tool to identify inflowing gas onto face-on galaxies, supported by simulations that show systematic differences in the decrement based on gas location.

Contribution

It introduces a novel method leveraging the Balmer decrement to distinguish inflowing gas in face-on galaxies, validated through detailed radiation-hydrodynamics simulations.

Findings

Gas in front of the disk shows lower Balmer decrements than behind.

Mean offset in Balmer decrement between front and back gas is approximately -0.14.

Dust distribution causes scatter, limiting the differentiation ability.

Abstract

Isolated star-forming galaxies require inflows of fresh gas from the surrounding medium to sustain episodes of star formation over time. However, there are very few direct detections of accretion onto external galaxies. Studies in absorption can only observe along limited sightlines, while those in emission can have difficulty distinguishing inflowing gas in the foreground of the galactic disk from similarly Doppler-shifted outflowing gas in the background. We explore the possibility of using the Balmer decrement (H$\alpha$/H$\beta$) in low-inclination systems as a diagnostic for disentangling the flow geometry in disk-like galaxies. We leverage mock spatial-spectral observations of an isolated Milky Way-mass galaxy simulated using the radiation-hydrodynamics code AREPO-RT and post-processed with the Monte Carlo radiative transfer code COLT. We find that gas components located in front of the disk exhibit systematically lower Balmer decrements than gas embedded in or behind the disk, with a mean front-back offset of $\Delta(\text{H}\alpha/\text{H}\beta) \approx -0.14$. The ability to differentiate between the disk and far-side components is limited by the extremely clumpy, multiphase dust distribution along the line of sight introducing substantial scatter. Overall, the results provide a useful observational diagnostic of inflow and outflow in dusty face-on galaxies.