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 ($\rm RV$) 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 ($σ_{\rm RV_{Gaia}} \leq$ 2 km s$^{-1}$ or $\leq$ 2%, and $σ_{\rm RV_{RAVE}}\leq$ 2 km s$^{-1}$ or $\leq$ 2%). The difference between $\rm RV_{Gaia}$ and $\rm RV_{RAVE}$ for a majority of the sample follows normal distribution with mean = 0.28 km s$^{-1}$ and $σ$ = 1.49 km s$^{-1}$. However, we found a very small group of stars ($\approx 0.08\%$ 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 $σ$ = 4.92 km s$^{-1}$. Kinematics based on $\rm RV_{ Gaia }$ 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.