The Galactic Thick Disc density profile traced with RR Lyrae stars

TL;DR

This study uses RR Lyrae stars and Bayesian analysis to determine the structure of the Milky Way's Thick Disc and inner Halo, revealing a short scale height and length for the Thick Disc and constraining halo flattening.

Contribution

It provides the first detailed structural parameters of the Milky Way's Thick Disc using RR Lyrae stars and Bayesian methods, especially at low galactic latitudes.

Findings

Thick Disc has short scale height (~0.65 kpc) and length (~2.1 kpc).

Inner Halo has a flattening of ~0.90 and a power law index of -2.78.

A disc flare is mildly constrained; flare onset within 11 kpc is disfavoured.

Abstract

We used a combination of public RR Lyrae star catalogs and a Bayesian methodology to derive robust structural parameters of the inner Halo (<25 kpc) and Thick Disc of the Milky Way. RR Lyrae stars are an unequivocal tracer of old metal-poor populations, for which accurate distances and extinctions can be individually estimated and so, are a reliable independent means of tracing the population of the old high-[\alpha/Fe] disc usually associated to the Thick Disc. In particular, the chosen RR Lyrae sample spans regions at low galactic latitude toward the anti-center direction, allowing to probe the outermost parts of the disc. Our results favour a Thick Disc with short scale height and short scale length, $h_z=0.65_{-0.05}^{+0.09}$ kpc, $h_R=2.1_{-0.25}^{+0.82}$ kpc, for a model in which the inner Halo has a constant flattening of $q=0.90_{-0.03}^{+0.05}$ and a power law index of $n=-2.78_{-0.05}^{+0.05}$. Similar short scales for the Thick Disc are also found when considering an inner Halo with flattening dependent on radius. We also explored a model in which the Thick Disc has a flare and, although this is only mildly constrained by our data, a flare onset in the inner $\sim11$ kpc is highly disfavoured.