The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope II. Multi-object spectroscopy (MOS)

TL;DR

This paper presents a comprehensive assessment of JWST/NIRSpec MOS capabilities enabled by the Micro Shutter Array, detailing the hardware design, detector mapping, and operational strategies needed to maximize simultaneous spectroscopy of faint sources across 0.6–5.3 μm. It outlines the three primary disperser configurations (PRISM and two sets of gratings) and their baseline mappings, as well as the complex target-planning and data-processing pipelines (MPT/eMPT and the MOS pipeline) required to place many targets within non-overlapping shuttered spectra. Through analytic modeling and Monte Carlo simulations, the authors quantify multiplexing limits and sensitivity, discuss wavelength calibration and path-loss corrections for off-centered sources, and demonstrate the approach with simulated deep-field observations (e.g., JADES). The work highlights the unprecedented potential of NIRSpec MOS for large spectroscopic surveys, while acknowledging instrument- and schedule-driven constraints (e.g., ~55% effective field due to non-operational shutters and the need for precise target acquisition). Overall, the paper establishes the methodology, expected performance, and observing strategies that will enable transformative near-infrared spectroscopic studies with JWST.

Abstract

We provide an overview of the capabilities and performance of the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) when used in its multi-object spectroscopy (MOS) mode employing a novel Micro Shutter Array (MSA) slit device. The MSA consists of four separate 98 arcsec $\times$ 91 arcsec quadrants each containing $365\times171$ individually addressable shutters whose open areas on the sky measure 0.20 arcsec $\times$ 0.46 arcsec on a 0.27 arcsec $\times$ 0.53 arcsec pitch. This is the first time that a configurable multi-object spectrograph has been available on a space mission. The levels of multiplexing achievable with NIRSpec MOS mode are quantified and we show that NIRSpec will be able to observe typically fifty to two hundred objects simultaneously with the pattern of close to a quarter of a million shutters provided by the MSA. This pattern is fixed and regular, and we identify the specific constraints that it yields for NIRSpec observation planning. We also present the data processing and calibration steps planned for the NIRSpec MOS data. The significant variation in size of the mostly diffraction-limited instrument point spread function over the large wavelength range of 0.6-5.3 $μ$m covered by the instrument, combined with the fact that most targets observed with the MSA cannot be expected to be perfectly centred within their respective slits, makes the spectrophotometric and wavelength calibration of the obtained spectra particularly complex. These challenges notwithstanding, the sensitivity and multiplexing capabilities anticipated of NIRSpec in MOS mode are unprecedented, and should enable significant progress to be made in addressing a wide range of outstanding astrophysical problems.

Figures (14)

• Figure 1: Geometry of the NIRSpec aperture plane, including the four MSA quadrants (frame in blue, shutters as a grey checkerboard), the fixed slits and IFU aperture (all in black). Each quadrant, labelled Q1 through Q4, contains 365$\times$171 shutters. The projected locations of the detector arrays are shown in green, with green dotted lines indicating their extent and hence where the gap between detectors is hidden in the figure by the MSA. The red dashed rectangle marks the $\sim$3 6$\times$3 4 on-sky field-of-view, constrained by a field stop at the entrance of the instrument.
• Figure 2: Schematic showing the detailed construction of the shutters of the MSA. The width $\times$ length of 105 $\mu$m $\times$ 204 $\mu$m corresponds on average to 268 mas $\times$ 530 mas on the sky. Image credit: Valérian Ferruit.
• Figure 3: Monochromatic footprints of the functional shutters of the MSA, incident on the detector plane (the two outer squares), for the blue and red ends of the nominal wavelength range in three representative filter/disperser combinations (top down, respectively): CLEAR/PRISM (blue $=$ 0.6 $\mu$m, red $=$ 5.3 $\mu$m), F100LP/G140M (0.97 and 1.94 $\mu$m) and F290LP/G395H (2.87 and 5.3 $\mu$m). These plots demonstrate that when using the prism and medium resolution gratings (the example here is representative of all three $R\!\simeq \!1000$ gratings), all the spectral traces connecting the two extreme monochromatic images fall within the outer bounds of the detectors. The plots also reveal the impact of the detector gap, as only spectra from shutters appearing in both colours on a single detector are uninterrupted by the gap (i.e. $\sim$80% of shutters for the prism, but $<$10% for G140M). The F290LP/G395H plot also shows that for this, and indeed all high resolution gratings, only the left-most shutters will have the red end of their spectra falling on the detectors.
• Figure 4: Representation of all non-operational shutters as measured at the end of the final cryo-vacuum ground test in 2017 Rawle+2018: Failed closed (dark green colour), vignetted shutters (light green colour, and transparent to show failed shutters behind the field stop), and lines and rows of shutters removed from service by the short masks (olive green colour). Failed open shutters are highlighted by red circles. Quadrant orientation is the same as in Fig. \ref{['fig:slit_plane']}.
• Figure 5: Average number of spectra $\bar{n}_\mathrm{SP}$ that can be accommodated on the NIRSpec detector array in MOS mode without overlap as a function of $n_\mathrm{VS}$, the number of candidate targets located within Viable Slitlets on the MSA. The points show the results of 10 000 Monte Carlo trials carried out at the shown values of $n_\mathrm{VS}$ for the PRISM and the G235M grating. The continuous curves show the fits to Eq. (\ref{['eq:nsp']}).