Radial velocity comparison of Gaia DR2 and RAVE DR5 survey: a systematic offset in radial velocities among a group of highly accurate radial velocity stars

Here, we report comparative study of radial velocity ($\mathrm{R}\mathrm{V}$) data of two major surveys: Gaia Data Release 2 and RAVE Data Release 5. We restricted the sample to stars with relatively accurate radial velocities ($\sigma_{\rm RV_{Gaia}} \leq$2kms$^{-1}$or$\leq$2$\sigma_{\rm RV_{RAVE}}\leq$ 2 km s${}^{-1}$ or $\le$ 2%). The difference between $\mathrm{R}{\mathrm{V}}_{\mathrm{G}\mathrm{a}\mathrm{i}\mathrm{a}}$ and $\rm RV_{RAVE}$ for a majority of the sample follows normal distribution with mean = 0.28 km s${}^{-1}$ and $\sigma$ = 1.49 km s${}^{-1}$. However, we found a very small group of stars ($\approx 0.08\mathrm{\%}$ of the total) for which the difference in radial velocities between the two surveys is significantly larger with an offset of $-$104.50 km s${}^{-1}$ with $\sigma$ = 4.92 km s${}^{-1}$. Kinematics based on $\mathrm{R}{\mathrm{V}}_{\mathrm{G}\mathrm{a}\mathrm{i}\mathrm{a}}$ suggest that most of the group members belong to the Galactic thin disk which agrees with the group’s metallicity range of $-$1.2 to $+$0.5 dex suggesting the offset in radial velocity is probably due to RAVE velocity data for this particular group.