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

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 (hɀ), scale length (Rd), 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 Rd=4.24 ± 0.32 kpc and scale height at the Sun of hɀ,⊙=0.18 ± 0.01 kpc, and a shorter and thicker disc with Rd=2.66 ± 0.11 kpc, hɀ,⊙=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.