Kinematics of Distant Milky Way Halo RR Lyrae Stars out to 160 kpc

TL;DR

This study addresses the kinematics of the Milky Way's outer stellar halo to detect the dynamical imprint of the Large Magellanic Cloud (LMC) through reflex motion, using RR Lyrae stars as distant tracers. It relies on single-epoch Keck/ESI spectroscopy to derive center-of-mass radial velocities for RR Lyrae in the 60–160 kpc range and to characterize the velocity distribution of the distant halo. Key results include a halo velocity dispersion of $70 m \,km\,s^{-1}$ and a weak dipole signal with amplitude $-30^{+16}_{-20} m \,km\,s^{-1}$ and apex $(l,b)=(-38.2^{+42.4}_{-31.5}, -41.3^{+27.9}_{-23.8})$ deg, plus a bulk compression velocity of $-16 \,\pm\,11 m \,km\,s^{-1}$, all of which are consistent with LMC-induced disequilibrium beyond $r_{ m GC} \gtrsim 100$ kpc. The authors also identify contaminants among photometric RR Lyrae candidates and demonstrate the viability of single-epoch RR Lyrae spectroscopy for outer-halo kinematics, with future surveys like Rubin LSST and DESI-II expected to substantially expand the data set.

Abstract

We present a kinematical study of the outer halo (r_GC approximately 60 to 160 kpc) of the Milky Way based on spectroscopy of 55 RR Lyrae stars obtained with the ESI instrument on the Keck II telescope. Our spectroscopic targets were selected from three photometric surveys: NGVS, DES, and Pan-STARRS1. We derive center-of-mass radial velocities with uncertainties of 6 to 35 km s^-1. The halo velocity dispersion measured from our sample is 70 plus/minus 7 km s^-1. The velocity field shows a possible dipole-like structure, with redshifted northern and blueshifted southern hemispheres. Fitting a Milky Way - Large Magellanic Cloud dipole perturbation model yields a weak or marginal dipole signal with amplitude -30 (+16, -20) km s^-1 and apex direction (l, b) = (-38.2 (+42.4, -31.5), -41.3 (+27.9, -23.8)) deg, along with a bulk compression velocity of -16 plus/minus 11 km s^-1. Although limited by sky coverage and sample size, our results are consistent with the presence of LMC-induced disequilibrium in the distant halo beyond 100 kpc. In addition to the 55 RR Lyrae stars, our spectroscopy reveals that 10 additional photometrically selected RR Lyrae candidates are actually quasar or blazar contaminants, highlighting the need for caution regarding such contaminants in sparsely sampled photometric surveys. Our study demonstrates that single-epoch spectroscopy of RR Lyrae stars is a viable method for probing the kinematics of the outer halo, and future surveys such as Rubin LSST and DESI-II have the potential to significantly advance this effort.