The Panchromatic Hubble Andromeda Treasury. XX. The Disk of M31 is Thick

Abstract

We present a new approach to measuring the thickness of a partially face-on stellar disk, using dust geometry. In a moderately-inclined disk galaxy, the fraction of reddened stars is expected to be 50% everywhere, assuming that dust lies in a thin midplane. In a thickened disk, however, a wide range of radii project onto the line of sight. Assuming stellar density declines with radius, this geometrical projection leads to differences in the numbers of stars on the near and far sides of the thin dust layer. The fraction of reddened stars will thus differ from the 50% prediction, with a deviation that becomes larger for puffier disks. We map the fraction of reddened red giant branch (RGB) stars across M31, which shows prominent dust lanes on only one side of the major axis. The fraction of reddened stars varies systematically from 20% to 80%, which requires that these stars have an exponential scale height h z that is 0.14 ± 0.015 times the exponential scale length (h r ≈ 5.5 kpc). M31's RGB stars must therefore have h z = 770 ± 80 pc, which is far thicker than the Milky Way’s thin disk, but comparable to its thick disk. The lack of a significant thin disk in M31 is unexpected, but consistent with its interaction history and high disk velocity dispersion. We suggest that asymmetric reddening be used as a generic criterion for identifying “thick disk”-dominated systems, and discuss prospects for future 3D tomographic mapping of the gas and stars in M31.

Dust around Massive Stars Is Agnostic to Galactic Environment: New Insights from PHAT/BEAST

“…Using groups of ∼100 red giant branch (RGB) stars, D15 modeled the NIR colormagnitude diagram (CMD) as a combination of an unreddened foreground and a background population of stars observed through a complex layer of dusty gas. The reddened RGB stars are assumed to sample a log-normal distribution of dust columns, confined to a thin midplane embedded in a much thicker distribution of RGB stars (see Dalcanton et al 2023). As such, the resulting extinction maps characterize the statistical properties of the integrated column density of dust but do not inform the exact extinction of any individual star.…”