On the radial velocity calibrations in the LAMOST medium-resolution spectroscopic survey of nebulae

Abstract

Abstract Accurate radial velocity determinations of optical emission lines (i.e., [Nii] λλ 6548, 6584, Hα and [S ii] λλ 6717, 6731) are very important for investigating the kinematic and dynamic properties of nebulae. The second stage survey program of Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) has started a sub-survey of nebulae (MRS-N) which will spectroscopically observe the optical emission lines of a large sample of nebulae near the Galactic plane. Until now, 15 MRS-N plates have been observed from September 2017 to June 2019. Based on fitting the sky emission lines in the red band spectra of MRS-N, we investigate the precision of wavelength calibration and find there are systematic deviations of radial velocities (RVs) from ∼ 0.2 to 4 km s−1 for different plates. Especially for the plates obtained in March 2018, the systematic deviations of RVs can be as large as ∼ 4 km s−1, which then go down to ∼ 0.2 – 0.5 km s−1 at the end of 2018 and January 2019. An RV calibration function is proposed for these MRS-N plates, which can simultaneously and successfully calibrate the systematic deviations and improve the precision of RVs.