# The Panchromatic Hubble Andromeda Treasury XX: The Disk of M31 is Thick

**Authors:** Julianne J. Dalcanton, Eric F. Bell, Yumi Choi, Andrew E. Dolphin, Morgan Fouesneau, Léo Girardi, David W. Hogg, Anil C. Seth, Benjamin F. Williams  
**arXiv:** [2304.08613](https://arxiv.org/abs/2304.08613)  
**Date:** 2023-04-17  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.08613v1/

## Key points

- Novel method for determining disk thickness using dust geometry
- Systematic variation in fraction of reddened stars across M31
- M31's red giant branch stars have substantial thickness compared to Milky Way
- Deviation from expected 50% reddening in thicker disks due to varying radii projection
- Importance of considering dust geometry when assessing stellar populations in galaxies
- Absence of significant thin disk in M31 due to interaction history and elevated disk velocity dispersion
- Proposal of using asymmetric reddening as a criterion for identifying systems dominated by a "thick disk"
- Future possibilities for conducting three-dimensional tomographic mapping of gas and stars within M31

## Summary

In their study titled "The Panchromatic Hubble Andromeda Treasury XX: The Disk of M31 is Thick," Julianne J. Dalcanton and colleagues present a novel method for determining the thickness of a partially face-on stellar disk by utilizing dust geometry. They investigate the fraction of reddened stars in a moderately-inclined disk galaxy and observe a systematic variation in this fraction across M31. This finding suggests that M31's red giant branch (RGB) stars possess an exponential scale height (h_z) that is approximately 0.14 times its exponential scale length (h_r~5.5kpc), indicating a substantial thickness compared to the Milky Way's thin disk but similar to its thick disk. The presence of dust in a thin midplane typically results in an expected 50% reddening across all regions in moderately-inclined disks. However, as the disk thickness increases, the projection of a wide range of radii onto the line of sight leads to varying numbers of stars on the near and far sides of the dust layer. This deviation from the predicted 50% reddening becomes more pronounced in thicker disks. This research sheds light on the structural properties of galactic disks and highlights the importance of considering dust geometry when assessing stellar populations in galaxies. The absence of a significant thin disk in M31 aligns with its interaction history and elevated disk velocity dispersion. The authors propose using asymmetric reddening as a criterion for identifying systems dominated by a "thick disk" and discuss future possibilities for conducting three-dimensional tomographic mapping of gas and stars within M31.