GaiaUnlimited: The old stellar disc of the Milky Way as traced by the red clump

¹ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese (TO), Italy
² School of Astronomy and Space Science, Nanjing University, Nanjing 210023, PR China
³ Key Laboratory of Modern Astronomy and Astrophysics, Ministry of Education, Nanjing 210023, PR China
⁴ Research School of Astronomy & Astrophysics, Australian National University, Cotter Rd., Weston, ACT 2611, Australia
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB30HA, UK
⁸ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
⁹ Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York City, NY, USA

^★ Corresponding authors: shourya.khanna@inaf.it; jie.yu@nju.edu.cn

Received: 29 October 2024
Accepted: 25 July 2025

Abstract

We present an exploration of the Milky Way’s structural parameters using an all-sky sample of red clump (RC) giants to map the stellar density from the Galactic disc beyond 3 kpc. These evolved giants are considered to be standard candles due to their low intrinsic variance in their absolute luminosities, and this allows us to estimate their distances with reasonable confidence. We exploited all-sky photometry from the AllWISE mid-infrared survey and the Gaia survey along with astrometry from Gaia Data Release 3 and recent 3D extinction maps to develop a probabilistic scheme in order to select with high confidence RC-like stars. Our curated catalogue contains about ten million sources, for which we estimated photometric distances based on the WISE W1 photometry. We derived the selection function for our sample, which is the combined selection function of sources with both Gaia and AllWISE photometry. Using the distances and accounting for the full selection function of our observables, we were able to fit a two-disc, multi-parameter model to constrain the scale height (h_ɀ), scale length (R_d), flaring, and the relative mass ratios of the two-disc components. We illustrate and verify our methodology using mock catalogues of RC stars. We find that the RC population is best described by a flared disc with scale length R_d=4.24 ± 0.32 kpc and scale height at the Sun of h_ɀ,⊙=0.18 ± 0.01 kpc, and a shorter and thicker disc with R_d=2.66 ± 0.11 kpc, h_ɀ,⊙=0.48 ± 0.11 kpc, with no flare. The thicker disc constitutes 66% of the RC stellar mass beyond 3 kpc, while the flared disc shows evidence of being warped beyond 9 kpc from the Galactic centre. The residuals between the predicted number density of RC stars from our axisymmetric model and the measured counts show possible evidence of a two-armed spiral perturbation in the disc of the Milky Way.