An accurate measurement of the spectral resolution of the JWST Near Infrared Spectrograph

TL;DR

This paper provides a precise measurement of the JWST NIRSpec's spectral resolution, revealing it exceeds pre-launch estimates and enabling more accurate kinematic studies in cosmology and galaxy evolution.

Contribution

The study offers the first robust, in-flight spectral resolution measurements of JWST NIRSpec, correcting for intrinsic line widths and demonstrating higher-than-expected resolution across wavelengths.

Findings

NIRSpec's in-flight resolution exceeds pre-launch estimates by up to 53%.

Resolution increases with wavelength within each configuration.

The results enable more accurate kinematic measurements in astrophysical research.

Abstract

The spectral resolution ($R \equiv \lambda / \Delta \lambda$) of spectroscopic data is crucial information for accurate kinematic measurements. In this letter, we present a robust measurement of the spectral resolution of the JWST's Near Infrared Spectrograph (NIRSpec) in fixed slit (FS) and integral field spectroscopy (IFS) modes. Due to the similarity of the utilized slit dimension if the FS mode to that of the shutters in the multi-object spectroscopy (MOS) mode, our resolution measurements in the FS mode can also be used for the MOS mode in principle. We modeled H and He lines of the planetary nebula SMP LMC 58 using a Gaussian line spread function (LSF) to estimate the wavelength-dependent resolution for multiple disperser and filter combinations. We corrected for the intrinsic width of the planetary nebula's H and He lines due to its expansion velocity by measuring it from a higher-resolution X-shooter spectrum. We find that NIRSpec's in-flight spectral resolutions exceed the pre-launch estimates provided in the JWST User Documentation by 11-53% in the FS mode and by 1-24% in the IFS mode across the covered wavelengths. We recover the expected trend that the resolution increases with the wavelength within a configuration. The robust and accurate LSFs presented in this letter will enable high-accuracy kinematic measurements using NIRSpec for applications in cosmology and galaxy evolution.