How Dark the Sky: The JWST Backgrounds

Abstract

We describe the sources of stray light and thermal background that affect JWST observations, report actual backgrounds as measured from commissioning and early-science observations, compare these background levels to prelaunch predictions, estimate the impact of the backgrounds on science performance, and explore how the backgrounds probe the achieved configuration of the deployed observatory. We find that for almost all applications, the observatory is limited by the irreducible astrophysical backgrounds, rather than scattered stray light and thermal self-emission, for all wavelengths λ < 12.5 μm, thus meeting the level 1 requirement. This result was not assured given the open architecture and thermal challenges of JWST, and it is the result of meticulous attention to stray light and thermal issues in the design, construction, integration, and test phases. From background considerations alone, JWST will require less integration time in the near-infrared compared to a system that just met the stray-light requirements; as such, JWST will be even more powerful than expected for deep imaging at 1–5 μm. In the mid-infrared, the measured thermal backgrounds closely match prelaunch predictions. The background near 10 μm is slightly higher than predicted before launch, but the impact on observations is mitigated by the excellent throughput of MIRI, such that instrument sensitivity will be as good as expected prelaunch. These measured background levels are fully compatible with JWST’s science goals and the Cycle 1 science program currently underway.

Characterizing the Near-infrared Spectra of Flares from TRAPPIST-1 during JWST Transit Spectroscopy Observations

“…The Space Telescope Science Institute (STScI) JDox SOSS SUBSTRIP256 background model is used to correct for the zodiacal background (Rigby et al 2023). A region in the images without significant contamination from any of the orders is identified, and a scaling factor is defined from the median flux of the image region and the median flux of the comparison region of the background model and applied to the background model.…”

JWST's TEMPLATES for Star Formation: The First Resolved Gas-phase Metallicity Maps of Dust-obscured Star-forming Galaxies at z ∼ 4

“…The launch of JWST and its successful commissioning (Rigby et al 2023a(Rigby et al , 2023bGardner et al 2023;McElwain et al 2023;Menzel et al 2023) have transformed our ability to study galaxy formation and evolution, and we are now entering a new era for extragalactic astronomy. JWSTʼs Near-Infrared Spectrograph (NIRSpec; Böker et al 2023) can study highredshift galaxies at sub-kpc spatial resolution, offering a vast improvement over previous ground-based (and therefore seeing-limited) studies.…”

First Results from the JWST Early Release Science Program Q3D: Ionization Cone, Clumpy Star Formation, and Shocks in a z = 3 Extremely Red Quasar Host

“…We achieve a final 2σ flux sensitivity of 2.87 × 10 −19 erg s −1 cm −2 Å −1 arcsec −2 and AB magnitude/arcsec 2 of 22.45 at 1.995 μm, near redshifted [O III] 5007 Å and 2.45 × 10 −19 erg s −1 cm −2 Å −1 arcsec −2 and AB magnitude/arcsec 2 of 22.09 at 2.549 μm, near redshifted Hα. Our 2σ surface brightness sensitivities are at least a factor of two higher than the measured JWST background at 2-3 μm (Rigby et al 2023), hence our observations are limited by the detector noise and systematics of the NIRSpec IFU.…”