# Attenuation modified by DIG and dust as seen in M31

## TL;DR

This study investigates the dust distribution and attenuation in M31, revealing a foreground screen model and emphasizing the importance of diffuse ionized gas in dust attenuation models, with results varying across galactic radii.

## Contribution

It demonstrates that M31's dust attenuation follows a foreground screen model and highlights the need to include diffuse ionized gas in dust/gas geometry models, differing from previous galaxy studies.

## Key findings

- M31's dust attenuation follows a foreground screen model
- Diffuse ionized gas significantly affects dust attenuation models
- Attenuation varies with galactic radius due to gas and dust distribution

## Abstract

The spatial distribution of dust in galaxies affects the global attenuation, and hence inferred properties, of galaxies. We trace the spatial distribution of dust in five fields (at 0.6-0.9 kpc scale) of M31 by comparing optical attenuation with the total dust mass distribution. We measure the attenuation from the Balmer decrement using Integral Field Spectroscopy and the dust mass from Herschel far-IR observations. Our results show that M31's dust attenuation closely follows a foreground screen model, contrary to what was previously found in other nearby galaxies. By smoothing the M31 data we find that spatial resolution is not the cause for this difference. Based on the emission line ratios and two simple models, we conclude that previous models of dust/gas geometry need to include a weakly or non-attenuated diffuse ionized gas (DIG) component. Due to the variation of dust and DIG scale heights with galactic radius, we conclude that different locations in galaxies will have different vertical distributions of gas and dust and therefore different measured attenuation. The difference between our result in M31 with that found in other nearby galaxies can be explained by our fields in M31 lying at larger galactic radii than the previous studies that focused on the centres of galaxies.