Rotating halo traced by the K giant stars from LAMOST and Gaia

TL;DR

This study uses Gaia and LAMOST data to analyze the rotation of the local stellar halo, revealing a prograde rotation that is largely independent of metallicity and suggesting a secular evolution origin.

Contribution

It provides the first detailed measurement of the local stellar halo's prograde rotation using combined Gaia and LAMOST data, including velocity distribution fitting and metallicity analysis.

Findings

The local stellar halo progradely rotates with V_T=+27 km/s.

The rotational velocity shows little correlation with metallicity.

A small fraction of counter-rotating, metal-poor stars are identified.

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.