Rotating Halo Traced by the K-giant Stars from LAMOST and Gaia

Abstract

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.