Warp and flare of the Galactic disc revealed with supergiants by Gaia EDR3

TL;DR

This study uses Gaia EDR3 data to analyze the warp and flare of the Milky Way's outer disc, revealing differences between young supergiants and the older population, with implications for understanding Galactic structure.

Contribution

It provides new measurements of the warp and flare in the Galactic disc using Gaia data, highlighting population-dependent differences in these features.

Findings

Supergiants exhibit an asymmetric warp with amplitudes up to ±0.7 kpc.

The entire disc shows a significant flare, with scale heights increasing with galactocentric distance.

The supergiant population has only a small flare compared to the older population.

Abstract

The outer Galactic disc contains some features such as the warp and flare, whose origin is still debated. The Gaia data provide an excellent opportunity to probe the Galactic disc at large distances and study these features. We derive the density distributions of the average (old) whole population and the supergiants (representative of a young population), and we use them to constrain their warp and flare. By comparing the results, we study how the properties of these phenomena depend on the studied population. We used Lucy's deconvolution method to recover corrected star counts as a function of distance, from which we derive the density distribution. We find that supergiants have an asymmetric warp, reaching a maximum amplitude of $z_w=0.658$ kpc and minimum amplitude of $z_w=-0.717$ kpc at a distance of $R=[19.5,20]$ kpc, which is almost twice as high as the amplitude of the whole population of the disc. We find a significant flare of the whole population, especially in the thick disc. The scale height increases from $h_{z,thick}\approx 0.8$ kpc and $h_{z,thin}\approx 0.3$ kpc in the solar neighbourhood, to $h_{z,thick} \approx 3$ kpc and $h_{z,thin}\approx 0.7$ kpc in the remote regions of the Milky Way ($R\approx 18$ kpc). The supergiants' population has only a small flare.