2018ApJ...859...31H

We characterize the spatial density of the Pan-STARRS1 (PS1) sample of Rrab stars to study the properties of the old Galactic stellar halo. This sample, containing 44,403 sources, spans galactocentric radii of 0.55 kpc <= R_gc <= 141 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 disk and bulge, and halo globular clusters, the sample contains ∼11,000 sources within 20 kpc <= R_gc <= 131 kpc. We then apply forward modeling using Galactic halo profile models with a sample selection function. Specifically, we use ellipsoidal stellar density models ρ(l, b, R_gc) with a constant and a radius-dependent halo flattening q(R_gc). Assuming constant flattening q, the distribution of the sources is reasonably well fit by a single power law with n=4.40_–0.04^+0.05 and q=0.918_–0.014^+0.016 and comparably well fit by an Einasto profile with n=9.53_–0.28^+0.27, an effective radius r_eff = 1.07 ± 0.10 kpc, and a halo flattening of q = 0.923 ± 0.007. If we allow for a radius-dependent flattening q(R_gc), we find evidence for a distinct flattening of q ∼ 0.8 of the inner halo at ∼25 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., Watkins et al.; Iorio et al.). We find that the stellar halo, as traced in RR Lyrae stars, exhibits a substantial number of further significant over- and underdensities, even after masking all known overdensities.