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

TL;DR

The paper tackles mapping the Milky Way’s old stellar disc by exploiting an all-sky RC star sample selected from Gaia DR3 and AllWISE, with distances derived from MIR photometry and extinction maps. It develops a forward-modeling approach that embeds a two-disc density model with flare and warp, together with a three-layer selection function and mock-data validation, to fit 3D stellar densities while accounting for observational biases. The main result is that RC stars are best described by a two-disc system: a long, flared disc with $R_d ext{(disc1)}=4.24 ext{ kpc}$, $h_{z, ext{⊙}}=0.18 ext{ kpc}$, $ ext{log}_{10}R_{fl}=0.36$, and a shorter, thicker disc with $R_{d2}=2.66 ext{ kpc}$, $h_{z2, ext{⊙}}=0.48 ext{ kpc}$, comprising about 66% of RC mass beyond 3 kpc. The analysis finds evidence for a warp in the outer disc ($R ightarrow 9$–$12$ kpc) and residuals consistent with a broad two-armed spiral perturbation in the outer disc, aligning with NIR spiral models. By explicitly modeling selection effects and extinction, the work demonstrates how RCs can map large-scale Galactic structure and constrain the disc’s radial, vertical, warp, and spiral features with robust, bias-aware inferences.

Abstract

We present an exploration of the Milky Way's structural parameters using an all-sky sample of 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, allowing us to estimate their distances with reasonable confidence. We exploit 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 10 million sources, for which we estimate photometric distances based on the WISE W1 photometry. We then derive 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 are able to fit a two-disc, multi-parameter model to constrain the scale height (hz), 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=$4.24\pm0.32$ kpc and scale height at the Sun of hz(at Sun)=$0.18\pm0.01$ kpc, and a shorter and thicker disc with rd=$2.66\pm0.11$ kpc, hz(at Sun)=$0.48\pm0.11$ 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 center. 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.

Figures (42)

• Figure 1: All sky spatial distribution of our dataset. Initial cross-match of Gaia DR3 and AllWISE, i.e. gdr3wise shown in the left panel, and the red clump candidates selected from this are shown in the right panel, i.e. gdr3wise[RC] for the region $|$$Z_{GC}$$|<$2 kpc.
• Figure 2: Procedure to obtain the distribution of the absolute magnitude starting from distance priors illustrated for an example source. The top inset shows the distance prior used, which is turned into a grid of absolute magnitude using 3D extinction maps. The bottom figures show a Gaussian fit to the resulting absolute magnitude distributions in the $G$ and $W1$ bands used to select the RC candidates.
• Figure 3: Distribution of giants on the $CaMD$ diagram in a mock realisation of the Milky Way using the Galaxia code (see Appendix \ref{['app:rc_feh_cdf']}). The top panel shows $M_{G}$ vs Gaia colours, while the bottom panel shows $M_{W1}$ versus AllWISE colours. The RC is marked by the black contours. The marginalised histogram (normalised) for the x (top sub panel) and y (right sub panel) axes are also shown. For the RC, both $M_{G}$ and $M_{W1}$ can be approximated by a quasi-Gaussian as is shown by the red curves on the right insets.
• Figure 4: Coordinate system adopted in this paper.
• Figure 5: Panel (a): Typical distance uncertainty for the red clump as a function of distance. The red curve shows the expectation from a naive inverse parallax estimation, the blue curve shows the predicted uncertainties in the photometric distances in $G$ band and for $W1$ this is shown by the green curve. The two vertical dotted lines indicate roughly the distances beyond which the photometric distances become more informative than parallax inversion for the two bands. Panel (b): Distribution of photometric uncertainty in $W1$ for the RC stars shows that $\sigma_{W1}<<0.1$, i.e. lower than the intrinsic dispersion in the absolute magnitude of RC stars in $W1$.