2019ApJ...871..184T

With the help of Gaia DR2, we are able to obtain the full 6D phase space information for stars from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope DR5. With high precision of the position, velocity, and metallicity, the rotation of the local stellar halo is presented using the K-giant stars with [Fe/H] < -1 dex within 4 kpc from the Sun. By fitting the rotational velocity distribution with the three-Gaussian components, stellar halo, disk, and counterrotating hot population, we find that the local halo progradely rotates with V_T=+27_–5⁺⁴ km s^–1, providing the local standard of the rest velocity of V_LSR = 232 km s^–1. Meanwhile, we obtain the dispersion of rotational velocity is σ_T=72_–4⁺⁴ km s^–1. Although the rotational velocity strongly depends on the choice of V_LSR, the trend of prograde rotation is substantial even when V_LSR is set at as low as 220 km s^–1. Moreover, we derive the rotation for subsamples with different metallicities and find that the rotational velocity is essentially not correlated with [Fe/H]. This may hint at a secular evolution origin of the prograde rotation. It shows that the metallicity of the progradely rotating halo is peaked within -1.9 < [Fe/H] < -1.6 without considering the selection effect. We also find a small fraction of counterrotating stars with larger dispersion and a lower metallicity. Finally, the disk component rotates with V_T=+182_–6⁺⁶ km s^–1 and σ_T=45_–3⁺³ km s^–1, which is quite consistent with the metal-weak thick disk population.