Determining the Scale Length and Height of the Milky Way's Thick Disc Using RR~Lyrae

TL;DR

This study precisely measures the Milky Way's thick disc scale length and height, as well as the inner halo's density profile, using RR Lyrae data and Bayesian methods, providing key structural parameters of our galaxy.

Contribution

It introduces a Bayesian approach with two techniques to determine the Milky Way's thick disc and halo parameters from RR Lyrae surveys, considering different vertical density profiles.

Findings

Thick disc scale length ~2.1-2.14 kpc

Thick disc scale height ~0.64-1.02 kpc

Inner halo density follows a power law with exponent -2.35

Abstract

Using the RR Lyrae surveys Gaia DR3 Specific Objects Study, PanSTARRS1 and ASAS-SN-II, we determine the Milky Way's thick disc scale length and scale height as well as the radial scale length of the galaxy's inner halo. We use a Bayesian approach to estimate these values using two independent techniques: Markov chain Monte Carlo sampling, and importance nested sampling. We consider two vertical density profiles for the thick disc. In the exponential model, the scale length of the thick disc is $h_R=2.14_{-0.17}^{+0.19}$ kpc, and its scale height is $h_z=0.64_{-0.06}^{+0.06}$ kpc. In the squared hyperbolic secant profile $sech^2$, those values are correspondingly $h_R=2.10_{-0.17}^{+0.19}$ kpc and $h_z=1.02_{-0.08}^{+0.09}$ kpc. The density distribution of the inner halo can be described as a power law function with the exponent $n =-2.35_{-0.05}^{+0.05}$ and flattening $q =0.57_{-0.02}^{+0.02}$. We also estimate the halo to disc concentration ratio as $\gamma=0.19_{-0.02}^{+0.02}$ for the exponential disc and $\gamma=0.32_{-0.03}^{+0.03}$ for the $sech^2$ disc.