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

Abstract We present a kinematical study of the outer halo ( r GC ∼ 60–160 kpc) of the Milky Way (MW) based on spectroscopy of 55 RR Lyrae stars using the Echelle Spectrograph and Imager instrument on the Keck II telescope. Our spectroscopic targets were selected from three photometric surveys: NGVS, DES, and Pan-STARRS 1. We derive center-of-mass radial velocities with uncertainties of 6–35 km s −1 . The halo velocity dispersion measured with our sample is 70 ± 7 km s −1 . The velocity field shows a possible dipole-like structure, with redshifted northern and blueshifted southern hemispheres. Fitting a MW–Large Magellanic Cloud (LMC) dipole perturbation model yields a weak/marginal dipole signal, with an amplitude of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>20</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>16</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> km s −1 and an apex direction <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mi>l</mml:mi> <mml:mo>,</mml:mo> <mml:mi>b</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mo>−</mml:mo> <mml:mn>38</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>31.5</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>42.4</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>,</mml:mo> <mml:mo>−</mml:mo> <mml:mn>41</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>23.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>27.9</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo stretchy="false">)</mml:mo> </mml:math> deg, along with a bulk compression velocity of −16 ± 11 km s −1 . While 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. Aside from the 55 RR Lyrae stars, our spectroscopic analysis reveals that 10 additional phometrically selected RR Lyrae candidates are, in fact, quasar/blazar contaminants; this provides a cautionary tale about the presence of 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 like Rubin/LSST and the Dark Energy Spectroscopic Instrument (DESI-II) have the potential to significantly advance this effort.