Letter to the Editor

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

¹ Center for Astronomy, Space Science and Astrophysics, Independent University, Bangladesh, Dhaka 1229, Bangladesh
² Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
³ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
⁴ Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
⁵ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, Garching 85748, Germany
⁶ Technical University of Munich, TUM School of Natural Sciences, Physics Department, James-Franck-Straße 1, 85748 Garching, Germany
⁷ Department of Physics & Astronomy, University College London, London WC1E 6BT, UK
⁸ Sub-Department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ SLAC National Laboratory, 2575 Sand Hill Rd, Menlo Park, CA 94025, USA

^⋆ Corresponding author: ajshajib@uchicago.edu

Received: 7 July 2025
Accepted: 1 September 2025

Abstract

The spectral resolution (R ≡ λ/Δλ) 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 Near Infrared Spectrograph (NIRSpec) in fixed slit (FS) and integral field spectroscopy (IFS) modes. Due to the similarity of the utilized slit dimension in 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.