Photometry and astrometry with JWST – I. NIRCam point spread functions and the first JWST colour–magnitude diagrams of a globular cluster

Abstract

As the James Webb Space Telescope (JWST) has become fully operational, early-release data are now available to begin building the tools and calibrations for precision point-source photometry and astrometry in crowded cluster environments. Here, we present our independent reduction of NIRCam imaging of the metal-poor globular cluster M 92, which were collected under Director’s Discretionary Early Release Science programme ERS-1334. We derived empirical models of the Point Spread Function (PSF) for filters F090W, F150W, F277W, and F444W, and find that these PSFs: (i) are generally under-sampled (FWHM ∼ 2 pixel) in F150W and F444W and severely under-sampled (FWHM ∼ 1 pixel) in F090W and F277W; (ii) have significant variation across the field of view, up to ∼15–20 per cent; and (iii) have temporal variations of ∼ 3–4 per cent across multi-epoch exposures. We deployed our PSFs to determine the photometric precision of NIRCam for stars in the crowded, central regions of M 92, measured to be at the ∼0.01 mag level. We use these data to construct the first JWST colour-magnitude diagrams of a globular cluster. Employing existing stellar models, we find that the data reach almost the bottom of the M 92 main sequence (∼0.1 M⊙), and reveal 24 white dwarf candidate members of M 92 in the brightest portion of the white dwarf cooling sequence. The latter are confirmed through a cross-match with archival HST UV and optical data. We also detect the presence of multiple stellar populations along the low-mass main sequence of M 92.

Spatially Resolved Stellar Populations of 0.3 < z < 6.0 Galaxies in WHL 0137–08 and MACS 0647+70 Clusters as Revealed by JWST: How Do Galaxies Grow and Quench over Cosmic Time?

“…The JWST level-2 imaging data were then scaled with detector-dependent factors (Brammer 2022) based on a NIRCam flux calibration using the standard star J1743045. Our photometric zero-points described here are similar to those obtained by the JWST Resolved Stellar Populations ERS program (Boyer et al 2022;Nardiello et al 2022), which analyzed the M92 globular cluster. We also check the consistency of our calibration with the more recent one based on CAL program data jwst_0989.pmap and find out that they are consistent within 3% in all filters analyzed here.…”

Massive galaxy formation caught in action at z ∼ 5 with JWST

“…The Grizli pipeline masks various artifacts and models wisps, calibrates for a flat-field, subtracts large-scale sky background, and aligns the fully calibrated images to stars from the Gaia DR3 catalog (Gaia Collaboration et al 2022). The NIRCam photometric zero-point correction was applied and the derived photometric zeropoints are consistent with those derived by other JWST Early Release Science (ERS) teams (Boyer et al 2022;Nardiello et al 2022). The NIRCam images of short wavelength chan-nels were processed to a final pixel scale of 0.02 ′′ , and 0.04 ′′ per pixel for long wavelength channels.…”

Red Spiral Galaxies at Cosmic Noon Unveiled in the First JWST Image

“…As calibrations of NIRCam data progressed, studies reported that the NIRCam's photometric zero-points have uncertainties, which could be up to ∼20% different from the preflight measurements (Adams et al 2022;Morishita & Stiavelli 2022;Rigby et al 2022) and/or could be time variable (Nardiello et al 2022). In this study, we use the photometric zero-point of JWST NIRCam data based on Adams et al (2022), which is derived using updated JWST calibration references for the SMACS 0723 field using in-flight data.…”