LAMOST Medium-Resolution Spectral Survey of Galactic Nebulae (LAMOST-MRS-N): Subtraction of Geocoronal Halpha Emission

TL;DR

This paper presents a new method to subtract geocoronal Halpha emission from LAMOST spectra of Galactic nebulae, improving the accuracy of nebulae analysis by reducing sky contamination.

Contribution

A novel linear relation-based technique for subtracting geocoronal Halpha emission from spectral data, enhancing data quality for Galactic nebulae studies.

Findings

Halpha sky line flux ratio correlates with solar altitude.

The subtraction method reduces contamination from 40% to less than 10%.

Improved spectral accuracy aids in nebulae classification and parameter measurement.

Abstract

We introduce a method of subtracting geocoronal Halpha emissions from the spectra of LAMOST medium-resolution spectral survey of Galactic nebulae (LAMOST-MRS-N). The flux ratios of the Halpha sky line to the adjacent OH lambda6554 single line do not show a pattern or gradient distribution in a plate. More interestingly, the ratio is well correlated to solar altitude, which is the angle of the sun relative to the Earth's horizon. It is found that the ratio decreases from 0.8 to 0.2 with the decreasing solar altitude from -17 to -73 degree. Based on this relation, which is described by a linear function, we can construct the Halpha sky component and subtract it from the science spectrum. This method has been applied to the LAMOST-MRS-N data, and the contamination level of the Halpha sky to nebula is reduced from 40% to less than 10%. The new generated spectra will significantly improve the accuracy of the classifications and the measurements of physical parameters of Galactic nebulae.