This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.

Galaxy clusters magnify background objects through strong gravitational lensing. Typical magnifications for lensed galaxies are factors of a few but can also be as high as tens or hundreds, stretching galaxies into giant arcs 1;2 . Individual stars can attain even higher magnifications given fortuitous alignment with the lensing cluster. Recently, several individual stars at redshift z ∼ 1 − 1.5 have been discovered, magnified by factors of thousands, temporarily boosted by microlensing 3;4;5;6 . Here we report observations of a more distant and persistent magnified star at redshift z phot = 6.2 ± 0.1, 900 Myr after the Big Bang. This star is magnified by a factor of thousands by the foreground galaxy cluster lens

We present Hubble Space Telescope WFC3-IR imaging in the fields of six apparently bright dusty star-forming galaxies (DSFGs) at z = 2 -4 identified by their rest-frame far-infrared colors using the Planck and Herschel space facilities. We detect near-infrared counterparts for all six submillimeter sources, allowing us to undertake strong-lensing analyses. One field in particular stands out for its prominent giant arcs, PLCK G165.7+67.0 (G165). After combining the color and morphological information, we identify 11 sets of image multiplicities in this one field. We construct a strong-lensing model constrained by this lensing evidence, which uncovers a bimodal spatial mass distribution, and from which we measure a mass of (2.6±0.11) × 10 14 M within ∼250 kpc. The bright (S 350 ≈ 750 mJy) DSFG appears as two images: a giant arc with a spatial extent of 4. 5 that is merging with the critical curve, and a lower-magnification counterimage that is detected in our new longer-wavelength groundand space-based imaging data. Using our ground-based spectroscopy, we calculate a dynamical mass of 1.3 +0.04 −0.70 × 10 15 M to the same fixed radius, although this value may be inflated relative to the true value if the velocity distribution is enhanced in the line-of-sight direction. We suggest that the bimodal mass taken in combination with the weak X-ray flux and low SZ decrement may be explained as a pre-merger for which the intracluster gas is diluted along the line of sight, while the integrated surface mass density is supercritical to strong-lensing effects.

Recent observations have revealed the presence of strong CIII] emission (EW CIII] > 20Å) in z > 6 galaxies, the origin of which remains unclear. In an effort to understand the nature of these line emitters, we have initiated a survey targeting CIII] emission in gravitationally-lensed reionization era analogs identified in HST imaging of clusters from the RELICS survey. Here we report initial results on four galaxies selected to have low stellar masses (2-8×10 7 M ) and J 125 -band flux excesses indicative of intense [OIII]+Hβ emission (EW [OIII]+Hβ =500-2000Å), similar to what has been observed at z > 6. We detect CIII] emission in three of the four sources, with the CIII] EW reaching values seen in the reionization era (EW CIII] 17 − 22Å) in the two sources with the strongest optical line emission (EW [OIII]+Hβ 2000Å). We have obtained a Magellan/FIRE near-infrared spectrum of the strongest CIII] emitter in our sample, revealing gas that is both metal poor and highly ionized. Using photoionization models, we are able to simultaneously reproduce the intense CIII] and optical line emission for extremely young (2-3 Myr) and metal poor (0.06-0.08 Z ) stellar populations, as would be expected after a substantial upturn in the SFR of a low mass galaxy. The sources in this survey are among the first for which CIII] has been used as the primary means of redshift confirmation. We suggest that it should be possible to extend this approach to z > 6 with current facilities, using CIII] to measure redshifts of objects with IRAC excesses indicating EW [OIII]+Hβ 2000Å, providing a method of spectroscopic confirmation independent of Lyα.