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 6-D phase space information for stars from LAMOST DR5. With high precision of 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 three Gaussian components, stellar halo, disk, and a counter-rotating hot population, we find that the local halo progradely rotates with $V_T=+27^{+4}_{-5}$ km s$^{-1}$ providing the local standard of rest velocity of $V_{LSR}=232$ km s$^{-1}$. Meanwhile, we obtain the dispersion of rotational velocity is $\sigma_{T}=72^{+4}_{-4}$ 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 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 counter-rotating stars with larger dispersion and lower metallicity. Finally, the disk component rotates with $V_T=+182^{+6}_{-6}$ km s$^{-1}$ and $\sigma_T=45^{+3}_{-3}$ km s$^{-1}$, which is quite consistent with the metal-weak thick disk population.