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The first James Webb Space Telescope (JWST) Near InfraRed Camera imaging in the field of the galaxy cluster PLCK G165.7+67.0 (z = 0.35) uncovered a Type Ia supernova (SN Ia) at z = 1.78, called “SN H0pe.” Three different images of this one SN were detected as a result of strong gravitational lensing, each one traversing a different path in spacetime, thereby inducing a relative delay in the arrival of each image. Follow-up JWST observations of all three SN images enabled photometric and rare spectroscopic measurements of the two relative time delays. Following strict blinding protocols which oversaw a live unblinding and regulated postunblinding changes, these two measured time delays were compared to the predictions of seven independently constructed cluster lens models to measure a value for the Hubble constant, H0 = 71.8 + 9.2 − 8.1 km s−1 Mpc−1. The range of admissible H0 values predicted across the lens models limits further precision, reflecting the well-known degeneracies between lens model constraints and time delays. It has long been theorized that a way forward is to leverage a standard candle, but this has not been realized until now. For the first time, the lens models are evaluated by their agreement with the SN absolute magnifications, breaking degeneracies and producing our best estimate, H0 = 75.7−5.5+8.1 km s−1 Mpc−1. This is the first precise measurement of H0 from a multiply imaged SN Ia and only the second from any multiply imaged SN.

A deep near-infrared spectroscopic survey of 43 photometrically-selected galaxies with redshift z  > 6.5 detects a near-infrared emission line from only a single galaxy; this line is likely to be Lyman α emission at a wavelength of 1.0343 μm, placing this galaxy at z = 7.51. Most distant star-forming galaxy confirmed Hubble Space Telescope data have yielded hundreds of candidates for galaxies with redshifts observed less than one billion years from the Big Bang, but so far distances have been confirmed for only a few of them. Using the newly commissioned MOSFIRE spectrograph on the Keck I telescope, Steven Finkelstein and co-workers have detected a galaxy with an emission line that can be confirmed at a redshift of 7.51, placing it at an epoch 700 million years after the Big Bang. That makes it the most distant spectroscopically confirmed galaxy, This galaxy's colours are consistent with a significant metal content, and it has a surprisingly high star-formation rate of about 330 solar masses per year, more than 100-fold greater than that seen in the Milky Way. The authors suggest that there may be many more such sites of intense star formation in the early Universe than previously expected. Of several dozen galaxies observed spectroscopically that are candidates for having a redshift ( z ) in excess of seven, only five have had their redshifts confirmed via Lyman α emission, at z = 7.008, 7.045, 7.109, 7.213 and 7.215 (refs 1 , 2 , 3 , 4 ). The small fraction of confirmed galaxies may indicate that the neutral fraction in the intergalactic medium rises quickly at z  > 6.5, given that Lyman α is resonantly scattered by neutral gas 3 , 5 , 6 , 7 , 8 . The small samples and limited depth of previous observations, however, makes these conclusions tentative. Here we report a deep near-infrared spectroscopic survey of 43 photometrically-selected galaxies with z  > 6.5. We detect a near-infrared emission line from only a single galaxy, confirming that some process is making Lyman α difficult to detect. The detected emission line at a wavelength of 1.0343 micrometres is likely to be Lyman α emission, placing this galaxy at a redshift z = 7.51, an epoch 700 million years after the Big Bang. This galaxy’s colours are consistent with significant metal content, implying that galaxies become enriched rapidly. We calculate a surprisingly high star-formation rate of about 330 solar masses per year, which is more than a factor of 100 greater than that seen in the Milky Way. Such a galaxy is unexpected in a survey of our size 9 , suggesting that the early Universe may harbour a larger number of intense sites of star formation than expected.

While supermassive black holes are ubiquitous features of galactic nuclei, only a small minority are observed during episodes of luminous accretion. The physical mechanism(s) driving the onset of fueling and ignition in these active galactic nuclei (AGN) are still largely unknown for many galaxies and AGN-selection criteria. Attention has focused on AGN triggering by means of major galaxy mergers gravitationally funneling gas toward the galactic center, with evidence both for and against this scenario. However, several recent studies have found that radio-loud AGN overwhelmingly reside in ongoing or recent major galaxy mergers. In this study, we test the hypothesis that major galaxy mergers are important triggers for radio-loud AGN activity in powerful quasars during cosmic noon (1 ≲ z ≲ 2). To this end, we compare Hubble Space Telescope WFC3/IR observations of the z > 1 3CR radio-loud broad-lined quasars to three matched radio-quiet quasar control samples. We find strong evidence for major-merger activity in nearly all radio-loud AGN, in contrast to the much lower merger fraction in the radio-quiet AGN. These results suggest major galaxy mergers are key ingredients in launching powerful radio jets. Given many of our radio-loud quasars are blue, our results present a possible challenge to the “blowout” paradigm of galaxy evolution models in which blue quasars are the quiescent end result following a period of red quasar feedback initiated by a galaxy merger. Finally, we find a tight correlation between black hole mass and host galaxy luminosity for these different high-redshift AGN samples that is inconsistent with those observed for local elliptical galaxies.

SN H0pe is a triply imaged supernova (SN) at redshift z = 1.78 discovered using the James Webb Space Telescope. In order to classify the SN spectroscopically and measure the relative time delays of its three images (designated A, B, and C), we acquired NIRSpec follow-up spectroscopy spanning 0.6–5 μm. From the high signal-to-noise spectra of the two bright images B and C, we first classify the SN, whose spectra most closely match those of SN 1994D and SN 2013dy, as a Type Ia SN. We identify prominent blueshifted absorption features corresponding to Si ii λ6355 and Ca ii H λ3970 and K λ3935. We next measure the absolute phases of the three images from our spectra, which allow us to constrain their relative time delays. The absolute phases of the three images, determined by fitting the three spectra to Hsiao07 SN templates, are 6.5−1.8+2.4 days, 24.3−3.9+3.9 days, and 50.6−15.3+16.1 days for the brightest to faintest images. These correspond to relative time delays between Image A and Image B and between Image B and Image C of −122.3−43.8+43.7 days and 49.3−14.7+12.2 days, respectively. The SALT3-NIR model yields phases and time delays consistent with these values. After unblinding, we additionally explored the effect of using Hsiao07 template spectra for simulations through 80 days instead of 60 days past maximum, and found a small (11.5 and 1.0 days, respectively) yet statistically insignificant (∼0.25σ and ∼0.1σ) effect on the inferred image delays.

With its unprecedented sensitivity and spatial resolution, the James Webb Space Telescope (JWST) has opened a new window for time-domain discoveries in the infrared. Here we report observations in the only field that has received four epochs (spanning 126 days) of JWST NIRCam observations in Cycle 1. This field is toward MACS J0416.1−2403, which is a rich galaxy cluster at redshift z = 0.4 and is one of the Hubble Frontier Fields. We have discovered 14 transients from these data. Twelve of these transients happened in three galaxies (with z = 0.94, 1.01, and 2.091) crossing a lensing caustic of the cluster, and these transients are highly magnified by gravitational lensing. These 12 transients are likely of a similar nature to those previously reported based on the Hubble Space Telescope (HST) data in this field, i.e., individual stars in the highly magnified arcs. However, these 12 could not have been found by HST because they were too red and too faint. The other two transients are associated with background galaxies (z = 2.205 and 0.7093) that are only moderately magnified, and they are likely supernovae. They indicate a demagnified supernova surface density, when monitored at a time cadence of a few months to a ∼3–4 μm survey limit of AB ∼28.5 mag, of ∼0.5 arcmin−2 integrated to z ≈ 2. This survey depth is beyond the capability of HST but can be easily reached by JWST.

Four ultraluminous infrared galaxies (ULIRGs) observed with JWST/NIRcam in the Cosmos Evolution Early Release Science program offer an unbiased preview of the z ∼ 2 ULIRG population. The objects were originally selected at 24 μm and have strong polycyclic aromatic hydrocarbon emission features observed with Spitzer/Infrared Spectrometer. The four objects have similar stellar masses of ∼1011 M ⊙ but otherwise are quite diverse. One is an isolated disk galaxy, but it has an active nucleus as shown by X-ray observations and by a bright point-source nucleus. Two others are merging pairs with mass ratios of 6–7:1. One has active nuclei in both components, while the other has only one active nucleus: the one in the less-massive neighbor, not the ULIRG. The fourth object is clumpy and irregular and is probably a merger, but there is no sign of an active nucleus. The intrinsic spectral energy distributions for the four active galactic nuclei in these systems are typical of type-2 QSOs. This study is consistent with the idea that even if internal processes can produce large luminosities at z ∼ 2, galaxy merging may still be necessary for the most luminous objects. The diversity of these four initial examples suggests that large samples will be needed to understand the z ∼ 2 ULIRG population.

JWST/NIRCam 0.9 to 2.0 μm images reveal a population of point sources around the major galaxies in the El Gordo cluster at redshift z = 0.87. Their distribution in the color–magnitude diagrams shows a narrow sequence well separated from field-galaxy contamination and consistent with their identification as ultracompact dwarf galaxies (UCDs) or luminous globular clusters (GCs). The point-source sequence is more luminous by almost a magnitude than the corresponding sequence in Abell 2744 at z = 0.31, matching the predicted evolutionary change for GC/UCDs over the 4 Gyr difference in look-back time between these two clusters. Deeper observations should allow direct JWST imaging of GC/UCD populations, even without the help of lensing, up to z ∼ 1.4, a look-back time of more than 9 Gyr. Such observations would directly reveal the evolution of these compact stellar systems two-thirds of the way back to the Big Bang.

We determine the mid-infrared (MIR, ∼5–22 μm) extinction toward the Galactic center using MIRI/Medium-Resolution Spectrometer (MRS) integral field unit observations of the central 3″ × 3″ region (near 5 μm) to 7″ × 7″ region (near 22 μm). To measure the MIR extinction, we employ two approaches: modeling the intrinsic-to-observed dust thermal spectrum and assessing the differential extinction between hydrogen recombination lines. Expanding on prior work, we directly model the dust-opacity distribution along the line of sight, and we make available a Python code that provides a flexible tool for deriving intrinsic dust emission spectra. We confirm the spatial variability of extinction across the field, demonstrating that dusty sources—such as IRS 29N—exhibit higher local extinction. Furthermore, we verify the absence of emission features from polycyclic aromatic hydrocarbons in the MIR spectra of the Galactic center. Using the two complementary methods, we derive a refined “best guess” MIR extinction law for Sgr A* and the surrounding Galactic-center region. By applying the extinction law to an MIR flare measurement discussed in a companion paper, we estimate a residual relative extinction uncertainty for the short MIRI/MRS grating of the order of 0.2 mag from ∼5 to ∼18 μm and ∼0.3 mag from ∼18 to ∼22 μm, consistent with our uncertainty estimate.

S. D. von Fellenberg et al. reported the first mid-infrared detection of a flare from Sgr A*. The JWST/MIRI/Medium Resolution Spectrometer observations were consistent with an orbiting hotspot undergoing electron injection with a spectrum that subsequently breaks from synchrotron cooling. However, mid-infrared extinction measurements appropriate for these data were not yet determined, and, therefore, the temporal evolution of the absolute spectral index remained unknown. This work applies new Sgr A* extinction measurements to the flare observations. The evolution of the spectral index after the peak is fully consistent with that reported in Paper I with a maximum absolute mid-infrared spectral index αMIR = 0.45 ± 0.01stat ± 0.08sys during the second mid-infrared flare peak, matching the known near-infrared spectral index during bright states (αNIR ≈ 0.5). There was a near-instantaneous change in the mid-infrared spectral index of ΔαMIR = 0.33 ± 0.06stat ± 0.11sys at the flare onset. We propose this as a quantitative definition for this infrared flare’s beginning, physically interpreted as the underlying electron distribution’s transition into a hard power-law distribution. This paper also reports the Submillimeter Array millimeter polarization during the flare, which shows a small, distorted, but overall CW-oriented Stokes Q–U loop during the third mid-infrared peak. Extrapolating the mid-infrared flux power law to the millimeter yields a variable flux consistent with the observed 220 GHz emission. These results, together with the Paper I modeling, plausibly suggest a single hotspot produced both the mid-infrared and millimeter variability during this event. However, additional flares are required to make a general statement about the millimeter and mid-infrared connection.

We present first results from James Webb Space Telescope Near-Infrared Spectrograph, Mid-Infrared Instrument, and Keck Cosmic Webb Imager integral field spectroscopy of the powerful but highly obscured host galaxy of the jetted radio source Cygnus A. We detect 169 infrared emission lines at 1.7–27 μm and explore the kinematics and physical properties of the extended narrow-line region (NLR) in unprecedented detail. The density-stratified NLR appears to be shaped by the initial blow-out and ongoing interaction of the radio jet with the interstellar medium, creating a multiphase bicone with a layered structure composed of molecular and ionized gas. The NLR spectrum, with strong coronal emission at kiloparsec scale, is well modeled by active galactic nucleus photoionization. We find evidence that the NLR is rotating around the radio axis, perhaps mediated by magnetic fields and driven by angular momentum transfer from the radio jet. The overall velocity field of the NLR is well described by 250 km s−1 outflow along biconical spiral flow lines, combining both rotation and outflow signatures. There is particularly bright [Fe ii] λ1.644 μm emission from a dense, high-velocity dispersion, photoionized clump of clouds found near the projected radio axis. Outflows of 600–2000 km s−1 are found in bullets and streamers of ionized gas that may be ablated by the radio jet from these clouds, driving a local outflow rate of 40M⊙ yr−1.