# GaiaUnlimited: The old stellar disc of the Milky Way as traced by the Red Clump

**Authors:** Shourya Khanna, Jie Yu, Ronald Drimmel, Eloisa Poggio, Tristan Cantat-Gaudin, Alfred Castro-Ginard, Evgeny Kurbatov, Vasily Belokurov, Anthony Brown, Morgan Fouesneau, Andrew Casey, Hans-Walter Rix  
**arXiv:** [2410.22036](https://arxiv.org/abs/2410.22036)  
**Date:** 2024-10-29  
**Summary page:** https://www.summarizepaper.com/en/paper/2410.22036v1/

## Key points

- Study by Khanna et al. (2024) explores Milky Way's structural parameters using Red Clump (RC) giants
- RC giants are standard candles with minimal intrinsic variability, enabling accurate distance estimations
- Data from AllWISE mid-infrared survey, Gaia survey, Gaia Data Release 3 astrometry, and 3D extinction maps used to map stellar density across Galactic disc
- Approximately 10 million \rc{}-like stars identified with high confidence in curated catalogue
- Two-disc model fitted to constrain parameters like scale height (\hz{}) and scale length (\rd{}) for thin and thick discs
- Results show flared thin disc with \rd{}=$3.56\pm0.32$ kpc and \hzsun{}=$0.17\pm0.01$ kpc; shorter, thicker disc with \rd{}=$2.59\pm0.11$ kpc and \hzsun{}=$0.45\pm0.11$ kpc identified
- Thicker disc accounts for 64% of \rc{} stellar mass beyond 3 kpc; thin disk shows warping beyond 9 kpc from Galactic center
- Discrepancies between predicted RC star number densities and actual counts suggest potential two-armed spiral perturbations within Milky Way's disc structure

## Summary

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In their study, Khanna et al. (2024) present a comprehensive exploration of the structural parameters of the Milky Way by utilizing an all-sky sample of Red Clump (RC) giants. These RC giants are considered standard candles due to their minimal intrinsic variability in absolute luminosities, allowing for accurate distance estimations. The researchers use data from the AllWISE mid-infrared survey and Gaia survey, along with astrometry from Gaia Data Release 3 and recent 3D extinction maps, to map the stellar density across the Galactic disc from its inner to outer regions. Through a probabilistic selection scheme, they identify \rc{}-like stars with high confidence and create a curated catalogue comprising approximately 10 million sources. By estimating photometric distances based on WISE $W1$ photometry and deriving the selection function for their sample—combining \gaia{} and \allwise{} photometry—the team fits a two-disc, multi-parameter model to constrain key parameters such as scale height (\hz{}) and scale length (\rd{}) for both thin and thick discs. Their methodology is validated using mock catalogues of \rc{} stars. The results indicate that the RC population is best described by a flared thin disc with a scale length of \rd{}=$3.56\pm0.32$ kpc and a scale height at the Sun of \hzsun{}=$0.17\pm0.01$ kpc. Additionally, they identify a shorter and thicker disc with \rd{}=$2.59\pm0.11$ kpc and \hzsun{}=$0.45\pm0.11$ kpc, lacking flare characteristics. The thicker disc accounts for 64% of the \rc{} stellar mass beyond 3 kpc, while evidence suggests that the thin disk exhibits warping beyond 9 kpc from the Galactic center. Moreover, discrepancies between predicted RC star number densities from their axisymmetric model and actual counts hint at potential two-armed spiral perturbations within the Milky Way's disc structure—an intriguing finding that warrants further investigation into possible effects of incorrect dust models on these observations. Overall, this study sheds light on the intricate dynamics of our Galaxy's old stellar disc through meticulous analysis of RC giants' distribution across different Galactic regions, offering valuable insights into its structural properties and potential spiral perturbations within its disc morphology.