The Profile of the Galactic Halo from Pan-STARRS1 3$\pi$ RR Lyrae

TL;DR

This study uses Pan-STARRS1 RR Lyrae stars to map the Galactic halo's structure out to 141 kpc, revealing its density profile, flattening variations, and asymmetries with improved precision over previous work.

Contribution

It provides the first detailed characterization of the Galactic halo's shape and density profile using a large, precise RR Lyrae sample extending to large distances.

Findings

Halo density follows a power law with index ~4.4

Halo flattening varies with radius, with inner halo q~0.8

South hemisphere is more flattened than north

Abstract

We characterize the spatial density of the Pan-STARRS1 (PS1) sample of RR Lyrae stars, to study the properties of the old Galactic stellar halo as traced by RRab stars. This sample of 44,403 sources spans Galactocentric radii of $0.55 \; \mathrm{kpc} \leq R_{\mathrm{gc}} \leq 141 \; \mathrm{kpc}$ with a distance precision of 3\% and thus is able to trace the halo out to larger distances than most previous studies. After excising stars that are attributed to dense regions such as stellar streams, the Galactic disc and bulge as well as halo globular clusters, the sample contains ${\sim}11,000$ sources within $20 \; \mathrm{kpc} \leq R_{\mathrm{gc}} \leq 131 \; \mathrm{kpc}$. We then apply forward modeling using ellipsoidal stellar density models $\rho(l,b,R_{\mathrm{gc}})$ both with a constant and a radius-dependent halo flattening $q(R_{\mathrm{gc}})$. Assuming constant flattening $q$, the distribution of the sources is reasonably well fit from $20 \; \mathrm{kpc}$ to $131 \; \mathrm{kpc}$ by a single power law with $n=4.40^{+0.05}_{-0.04}$ and $q=0.918^{+0.016}_{-0.014}$. The distance distribution is fit comparably well by an Einasto profile with $n=9.53^{+0.27}_{-0.28}$, an effective radius $r_{\mathrm{eff}}=1.07 \pm 0.10 \; \mathrm{kpc}$ and a halo flattening of $q=0.923 \pm 0.007$. If we allow for a radius-dependent flattening $q(R_{\mathrm{gc}})$, we find evidence for a distinct flattening of $q{\sim}0.8$ of the inner halo at ${\sim} 25 \; \mathrm{kpc}$. Additionally, we find that the south Galactic hemisphere is more flattened than the north Galactic hemisphere. The results of our work are largely consistent with many earlier results, e.g. \cite{Watkins2009}, \cite{Iorio2017}. We find that the stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of further significant over- and underdensities, even after all known overdensities have been masked.