The Photo-Astrometric Vertical Tracer Density of the Milky Way II: Results from Gaia

TL;DR

This study uses Gaia data to precisely model the vertical structure of the Milky Way, accounting for systematic uncertainties, and provides detailed parameters for the thin and thick discs and stellar halo.

Contribution

It offers the first comprehensive Gaia-based measurement of the Milky Way's vertical structure, including systematic error analysis and refined density parameters.

Findings

Thin disc scale height: 260 ± 3 (stat) ± 26 (sys) pc

Thick disc scale height: 693 ± 7 (stat) ± 121 (sys) pc

Halo power-law index: 3.543 ± 0.023 (stat) ± 0.259 (sys)

Abstract

We use Gaia photometry and astrometry to estimate the vertical spatial structure of the Milky Way at the Solar radius, formally accounting for sample incompleteness (the selection function) and parallax measurement uncertainty. Our results show impressive precision demonstrating the power of the Gaia data. However, systematic errors dominate the parameter value uncertainties. We thoroughly test and quantify the impacts of all systematic uncertainties. The vertical tracer density is modelled as a sum of two exponential profiles for the thin and thick discs, together with a spherically symmetric power-law for the stellar halo. We constrain the thin disc scale height as ${h_\mathrm{Tn}=260 \pm 3\, (\mathrm{stat}) \pm 26\,\mathrm{pc}\, (\mathrm{sys})}$ and thick disc ${h_\mathrm{Tk}=693 \pm 7 \,(\mathrm{stat}) \pm 121\,\mathrm{pc}\, (\mathrm{sys})}$. For the halo, we obtain a power law profile with $n_\mathrm{H}=3.543\pm0.023 \,(\mathrm{stat}) \pm0.259\, (\mathrm{sys})$. We infer a local stellar mass density for non-compact object stars of ${\rho_\mathrm{local}^* = 3.66\pm0.03\,(\mathrm{stat})\pm0.52 \times10^{-2}\,\mathrm{M}_\odot/\mathrm{pc}^3\,(\mathrm{sys})}$ and surface density of ${\Sigma_\mathrm{local}^* = 23.17\pm0.08\,(\mathrm{stat})\pm2.43\,\mathrm{M}_\odot/\mathrm{pc}^2\,(\mathrm{sys})}$. We find asymmetries above and below the disc with longer disc scale heights in the north but a flatter halo in the south at the $\lesssim 10$ per cent level.