Medium to ultra-deep mid-infrared imaging surveys with the James Webb Space Telescope (JWST)'s Mid-Infrared Instrument (MIRI) are reframing our view of the early Universe, from the emergence of ultra-red dusty and quiescent galaxies to the epoch of reionization to the first galaxies. Here we present the MIRI coordinated parallels component of the JADES program, which obtained ultra-deep (155 ks) imaging at 7.7 μm over ∼10 arcmin^2 as well as medium depth (∼5-15 ks) imaging at 7.7, 12.8, and 15 μm over ∼36, 25, and 22 arcmin^2, respectively, in the GOODS-S and GOODS-N fields. This paper describes the data reduction, which combines the official JWST Calibration Pipeline with custom steps to optimize flagging of warm/hot pixels and optimize background subtraction. We further introduce a new step to address artifacts caused by persistence from saturating sources. The final, fully reduced JADES/MIRI mosaics are being released as part of JADES Data Release 5, along with prior-based forced photometry using NIRCam detection images, providing critical rest-frame near-infrared and optical constraints on early galaxy populations.

We present a snapshot of the ongoing efforts to obtain background-subtracted, wavelength-, and flux-calibrated spectra taken with the new Wide-Field Slitless Spectroscopy (WFSS) mode for the MIRI instrument on the James Webb Space Telescope (JWST), offered for the first time in JWST Cycle 5 (starting July 2026). We describe here the capabilities of the new mode, the operational concept, and an overview of the calibration and pipeline development activities that are currently ongoing.

We present the second data release of the Systematic Mid-Infrared Instrument Legacy Extragalactic Survey (SMILES), focusing on JWST/NIRSpec medium-resolution spectroscopy of galaxies across cosmic time. This release includes spectroscopic observations of 166 galaxies spanning 0 < z < 7.5, sampling star-forming galaxies, quiescent systems, and active galactic nuclei (AGNs), with an emphasis on galaxies at cosmic noon ( z ∼ 1–3). We describe the target selection strategy, the observational setup with the G140M/F100LP and G235M/F170LP gratings, and the data calibration process. The final data products include the reduced spectra, redshift catalog, emission-line catalogs produced with GELATO for emission-line galaxies and pPXF fits for quiescent systems, and ancillary spectral energy distribution fit results derived from multiband photometry. The SMILES NIRSpec dataset enables investigations of obscured AGNs, multiphase outflows, ionizing properties, and the role of environment in galaxy evolution. All data products are publicly available through STScI/MAST at https://archive.stsci.edu/hlsp/smiles .

Context. The Medium Resolution Spectrometer (MRS) of the Mid-InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST) is affected by interferometric spectral fringing, due to scattering within the detector and dichroic layers. The amplitude of these oscillations on the spectrum can be up to 30%. Correcting them is non-trivial, since the depth and phase of the fringes depend strongly on the illumination pattern and the way the pixels sample it. By default the JWST pipeline uses static fringe flats to divide out the fringes. These flats are representative for a spatially homogeneous extended source, but not for point sources. The significant residuals in the data are removed by using a self-calibrating correction step which can alter physical features in the spectra in a non-systematic way. Aims. We build on our corrections from Paper I (Gasman et al., 2024, A&A, 688, A226) in this series, to derive a library of detector-based fringe flats for unresolved sources in a nine-point mosaic around all nominal MRS point source dither positions. We provide users with either an absolute or interpolated fringe flat that can correct the fringes without the need for self-calibration, hence mitigating the risk of altering astrophysical features of interest. Methods. We used the data of 10 Lac from the Cycle 2 calibration programme 3779 to create the library of fringe flats. By removing the continuum and spectral features from the data at the detector-plane level, each of the nine mosaic points around the eight dither positions resulted in a pointing specific fringe flat. By assessing the difference in response between the individual pointings, we found correction factors to bring all the spectra to the same level, and used these to derive a single spectrophotometric calibration curve per band. Results. The library of fringe flats is able to reduce the remaining power of the fringe frequencies on the detector by up to two orders of magnitude compared to the current pipeline flats tailored to extended sources. This improvement carries over to the residuals in the cube spaxels, where the contrast is reduced from >10% to <1-2%. This becomes less apparent after extracting a spectrum from the cube, where in channel 1 averaging of fringe phases in the current pipeline case can reduce its residual contrast. The spectrophotometric calibration curves have a root-mean squared variation of less than a percent in all bands except bands 4B and 4C, while channels 2 and 3 have a stability within 0.5%. Sources taken without target acquisition (TA) fall outside the mosaic grid, but our correction improves the defringing depending on the source location. Conclusions. The improvements in fringe residual found are significant on the detector and spectrum-level. The corrections derived here are directly compatible with the current JWST pipeline infrastructure, and work best for unresolved sources observed with TA in one of the nominal point-source dither patterns.

The majority of most luminous quasars during the epoch of reionization accrete near or above the Eddington limit, marking the vigorous growth of primitive supermassive black holes (SMBHs). However, their subsequent evolution and environmental impact remain poorly characterized. We present JWST/NIRSpec prism integral field unit observations of HSC J2239+0207, a low-luminosity quasar at z ∼ 6.25 likely in a late stage of mass assembly with an overmassive SMBH relative to its host galaxy. Using H β and H α broad emission lines, we estimate an SMBH mass M _BH ∼ 3 × 10 ^8 M _⊙ and confirm its sub-Eddington accretion at λ _Edd ∼ 0.4. Strong Fe ii emission and a proximity zone of typical size suggest a metal-rich, highly evolved system. In the far-UV, this quasar presents strong broad absorption line features, indicative of high-velocity winds ( ν ∼ 10 ^4 km s ^−1 ). Meanwhile, minimal dust reddening is inferred from the quasar continuum and broad-line Balmer decrement, suggesting little dust along the polar direction. Most interestingly, we identify a gas companion ∼5 kpc from the quasar with a high [O iii ]/H β ratio (≳10), likely representing outflowing gas blown away by active galactic nucleus (AGN) feedback. These results highlight HSC J2239+0207 as a likely fading quasar in transition, providing rare insights into SMBH evolution, AGN feedback, and AGN–galaxy interactions in the early Universe.

We explore the accretion rates of supermassive black holes (SMBHs) in late-stage galaxy mergers by observing three ultraluminous infrared galaxies (ULIRGs), IRAS 14378-3651, IRAS 17208-0014 and IRAS 23365+3604, using the JWST/MIRI Medium Resolution Spectrometer and JWST/NIRSpec integral field unit. In all three cases, we fail to detect [Ne vi ] λ 7.65 μ m, a robust active galactic nuclei (AGN) tracer lying in a low-opacity interstellar window, nor do we detect any other lines that might indicate AGNs. The only detected high-excitation emission line, [Mg iv ] ( λ 4.488 μ m), arises from shocks associated with supernovae. Our new, deep flux limits on AGN tracers in the near- and mid-infrared indicate that the nuclear obscuration of any purported AGNs in our sample is isotropic, i.e., the far-infrared luminosities of these galaxies are unlikely to be driven by escaping AGN power. This allows us to show that the Eddington ratios of their SMBHs are low. We then assemble an unbiased sample of 19 ULIRGs (from the IRAS Bright Galaxy Sample with L (TIR) ≥ 10 ^12 L _⊙ ) in late-stage mergers and show that their dynamically measured black hole masses are consistent with the values from scaling from their stellar masses. On this basis, we show that the Eddington ratios of any AGNs in 15 of these galaxies are also very low, ≲10%. This indicates that any black holes are in a relatively quiescent state. That is, high levels of accretion are found in only a minority of late-merger-phase ULIRGs.

The topology of reionization and the environments where galaxies efficiently produce ionizing photons are key open questions. For the first time, we investigate the trend between ionizing photon production efficiency, ξ ion , and galaxy overdensity, log ( 1 + δ ) . We analyze the ionizing properties of 79 galaxies between 1.0 < z < 5.2 using JWST NIRSpec medium-resolution spectra from the Systematic Mid-infrared Instrument Legacy Extragalactic Survey (SMILES) program. Among these, 67 galaxies have H α coverage, spanning 1.0 < z < 3.1. The galaxy overdensity, log ( 1 + δ ) , is measured using the JADES photometric catalog, which covers the SMILES footprint. For the subset with H α coverage, we find that log ξ ion is positively correlated with log ( 1 + δ ) , with a slope of 0.9 4 − 0.46 + 0.46 . Additionally, the mean ξ ion for galaxies in overdense regions ( log ( 1 + δ ) > 0.1 ) is 2.43 times that of galaxies in lower density regions ( log ( 1 + δ ) < 0.1 ). This strong trend is found to be independent of redshift evolution. Furthermore, our results confirm the robust correlations between ξ ion and the rest-frame equivalent widths of the [O iii ] or H α emission lines. Our results suggest that galaxies in high-density regions are efficient producers of ionizing photons.

Understanding when and how circumstellar disks disperse is crucial to constrain planet formation and migration. Thermal winds powered by high-energy stellar photons have long been theorized to drive disk dispersal. However, evidence for these winds is currently based only on small (~3-6 km/s) blue-shifts in [Ne II] 12.81 um lines, which does not exclude MHD winds. We report JWST MIRI MRS spectro-imaging of T Cha, a disk with a large dust gap (~30 au in radius) and blue-shifted [Ne II] emission. We detect four forbidden noble gas lines, [Ar II], [Ar III], [Ne II], and [Ne III], of which [Ar III] is the first detection in any protoplanetary disk. We use line flux ratios to constrain the energy of the ionizing photons and find that Argon is ionized by EUV whereas Neon is most likely ionized by X-rays. After performing continuum and Point Spread Function (PSF) subtraction on the IFU cube, we discover a spatial extension in the [Ne II] emission off the disk continuum emission. This is the first spatially resolved [Ne II] disk wind emission. The mostly ionic spectrum of T Cha, in combination with the extended [Ne II] emission, points to an evolved stage for any inner MHD wind and is consistent with the existence of an outer thermal wind ionized and driven by high-energy stellar photons. This work acts as a pathfinder for future observations aiming at investigating disk dispersal using JWST.

Aims. This paper utilises the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) to extend the observational studies of dust and polycyclic aromatic hydrocarbon (PAH) emission to a new mass and star formation rate (SFR) parameter space beyond our local Universe. The combination of fully sampled spectral energy distributions (SEDs) with multiple mid-infrared (mid-IR) bands and the unprecedented sensitivity of MIRI allows us to investigate dust obscuration and PAH behaviour from z=0.7 up to z=2 in typical main-sequence galaxies. Our focus is on constraining the evolution of PAH strength and the dust-obscured luminosity fraction before and during cosmic noon, the epoch of peak star formation activity in the Universe. Methods. In this study, we utilise MIRI multi-band imaging data from the SMILES survey (5 to 25 mu m), complemented with NIRCam photometry from the JADES survey (1 to 5 mu m), available HST photometry (0.4 to 0.9 mu m), and spectroscopic redshifts from the FRESCO and JADES surveys in GOODS-S for 443 star-forming (without dominant active galactic nucleus (AGN)) galaxies at z=0.7-2.0. This redshift range was chosen to ensure that the MIRI data cover mid-IR dust emission. Our methodology involved employing ultraviolet (UV) to IR energy balance SED fitting to robustly constrain the fraction of dust mass in PAHs and dust-obscured luminosity. Additionally, we inferred dust sizes from MIRI 15 mu m imaging data, enhancing our understanding of the physical characteristics of dust within these galaxies. Results. We find a strong correlation between the fraction of dust in PAHs (PAH fraction, q(PAH)) with stellar mass. Moreover, the sub-sample with robust q(PAH) measurements (N=216) shows a similar behaviour between q(PAH) and gas-phase metallicity to that at z similar to 0, suggesting a universal relation: q(PAH) is constant (similar to 3.4%) above a metallicity of Z similar to 0.5 Z(circle dot) and decreases to < 1% at metallicities less than or similar to 0.3 Z(circle dot). This indicates that metallicity is a good indicator of the interstellar medium properties that affect the balance between the formation and destruction of PAHs. The lack of a redshift evolution from z similar to 0-2 also implies that above Z similar to 0.5 Z(circle dot) the PAH emission effectively traces obscured luminosity and the previous locally calibrated PAH-SFR calibrations remain applicable in this metallicity regime. We observe a strong correlation between the obscured UV luminosity fraction (ratio of obscured to total luminosity) and stellar mass. Above the stellar mass of M-* > 5x10(9)M(circle dot), on average, more than half of the emitted luminosity is obscured, while there exists a non-negligible population of lower-mass galaxies with > 50% obscured fractions. At a fixed mass, the obscured fraction correlates with SFR surface density. This is a result of higher dust covering fractions in galaxies with more compact star-forming regions. Similarly, galaxies with high IRX (IR to UV luminosity) at a given mass or UV continuum slope (beta) tend to have higher Sigma(SFR) and shallower attenuation curves, owing to their higher effective dust optical depths and more compact star-forming regions.

The Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST) provides the observatory with a huge advance in mid-infrared imaging and spectroscopy covering the wavelength range of 5 to 28 microns. This paper describes the performance and characteristics of the MIRI imager as understood during observatory commissioning activities, and through its first year of science operations. We discuss the measurements and results of the imager's point spread function, flux calibration, background, distortion and flat fields as well as results pertaining to best observing practices for MIRI imaging, and discuss known imaging artefacts that may be seen during or after data processing. Overall, we show that the MIRI imager has met or exceeded all its pre-flight requirements, and we expect it to make a significant contribution to mid-infrared science for the astronomy community for years to come.

This paper utilizes the JWST MIRI multi-band imaging data from the SMILES survey (5-25micron), complemented with HST and NIRCam photometric and spectroscopic data from the JADES and FRESCO surveys for 443 star-forming (non-AGN) galaxies at z=0.7-2.0 to extend the study of dust and PAH emission to a new mass and SFR parameter space beyond our local universe. We find a strong correlation between the fraction of dust in PAHs (PAH fraction, q_PAH) with stellar mass. Moreover, the PAH fraction behavior as a function of gas-phase metallicity is similar to that at z 0 from previous studies, suggesting a universal relation: q_PAH is constant ( 3.4 0.5Z_⊙ and decreases to <1 indicates that metallicity is a good indicator of the ISM properties that affect the balance between the formation and destruction of PAHs. The lack of a redshift evolution from z 0-2 also implies that above 0.5 Z_⊙, the PAH emission effectively traces obscured luminosity and the previous locally-calibrated PAH-SFR calibrations remain applicable in this metallicity regime. We observe a strong correlation between obscured UV luminosity fraction (ratio of obscured to total luminosity) and stellar mass. Above the stellar mass of >5× 10^9M_⊙, on average, more than half of the emitted luminosity is obscured, while there exists a non-negligible population of lower mass galaxies with >50 fraction correlates with SFR surface density. This is a result of higher dust covering fractions in galaxies with more compact star forming regions. Similarly, galaxies with high IRX (IR to UV luminosity) at a given mass or UV continuum slope tend to have higher SFR surface density and shallower attenuation curves, owing to their higher effective dust optical depths and more compact star forming regions.

The James Webb Space Telescope (JWST) is revolutionizing our view of the Universe through unprecedented sensitivity and resolution in the infrared, with some of the largest gains realized at its longest wavelengths. We present the Systematic Mid-infrared Instrument (MIRI) Legacy Extragalactic Survey (SMILES), an eight-band MIRI survey with Near-Infrared Spectrograph (NIRSpec) spectroscopic follow-up in the GOODS-S/HUDF region. SMILES takes full advantage of MIRI's continuous coverage from $5.6-25.5\,\mu$m over a $\sim34$ arcmin$^2$ area to greatly expand our understanding of the obscured Universe up to cosmic noon and beyond. This work, together with a companion paper by Rieke et al., covers the SMILES science drivers and technical design, early results with SMILES, data reduction, photometric catalog creation, and the first data release. As part of the discussion on early results, we additionally present a high-level science demonstration on how MIRI's wavelength coverage and resolution will advance our understanding of cosmic dust using the full range of polycyclic aromatic hydrocarbon (PAH) emission features from $3.3-18\,\mu$m. Using custom background subtraction, we produce robust reductions of the MIRI imaging that maximize the depths reached with our modest exposure times ($\sim0.6 - 2.2$ ks per filter). Included in our initial data release are (1) eight MIRI imaging mosaics reaching depths of $0.2-18\,\mu$Jy ($5\sigma$) and (2) a $5-25.5\,\mu$m photometric catalog with over 3,000 sources. Building upon the rich legacy of extensive photometric and spectroscopy coverage of GOODS-S/HUDF from the X-ray to the radio, SMILES greatly expands our investigative power in understanding the obscured Universe.

The late-stage galaxy merger and prototypical ultraluminous infrared galaxy (ULIRG) Arp 220 is probed using the JWST Mid Infrared Instrument medium-resolution spectrometer to search for multiple diagnostic active galactic nuclei (AGN) emission lines in the low-interstellar-opacity spectral ranges of 5.3-7.9 mu m and 12.9-16.0 mu m. We detect none of the characteristic high-excitation AGN emission lines. The most diagnostic nondetections are of the [Ne vi] lambda 7.652 mu m and [Ne v] lambda 14.32 mu m lines, for which the combined upper limit is similar to 10-20 times fainter than the equivalent limit from Spitzer data. We also use X-ray measurements of the reflected Fe K alpha line to suggest a relatively low X-ray luminosity. These results suggest that the high obscuration observed along the line of sight to the nuclei of Arp 220 is isotropic and keeps any high-excitation emission from a putative AGN from escaping. An implication is that the observed mid-infrared output of the nuclei is essentially their bolometric luminosity, i.e., no significant additional energy escapes. We utilize this result to show that any embedded AGNs are accreting at no more than similar to 5% of the Eddington rate, lower than many theoretical predictions for this stage of ULIRG evolution.

Context. The James Webb Space Telescope (JWST) has been collecting scientific data for over two years now. The Medium Resolution Spectrometer (MRS) of the Mid-InfraRed Instrument (MIRI) has been one of the telescope's most popular modes, and has already produced ground-breaking results. Scientists are now looking deeper into the data for new exciting discoveries, which introduces the need to characterise and correct known systematic effects to reach the photon noise limit. Five important limiting factors for the MRS are the pointing accuracy, non-linearity, detector charge migration, detector scattering - resulting in both spatial broadening and spectral interferometric fringing - the accuracy of the point-spread function (PSF) model, and the complex interplay between these. Aims. The Cycle 2 calibration programme 3779, entitled 'The MIRI/MRS Library', proposed a 72-point intra-pixel dither raster of the calibration star 10-Lac, which provides a unique dataset tailored for the purpose of addressing the limiting factors on the MRS data accuracy. In this first work of the paper series, we aim to address the degeneracy between the non-linearity and charge migration (brighter-fatter effect) that affect the pixel voltage integration ramps of the MRS. Due to the low flux in the longer wavelengths, we only do this in the 4.9-11.7 micron region (spectral channels 1 and 2). Methods. We fitted the ramps individually per pixel and dither, in order to fold in the deviations from classical non-linearity that are caused by charge migration. The ramp shapes should be repeatable depending on the part of the PSF that is sampled. By doing so, we defined both a grid-based linearity correction, and an interpolated linearity correction. Results. Including the change in ramp shape due to charge migration yields significant improvements compared to the uniform illumination assumption that is currently used by the standard JWST calibration pipeline. The standard deviation on the pixel ramp residual non-linearity is between 70 and 90% smaller than the current standard pipeline when self-calibrating with the grid. We are able to interpolate these coefficients to apply to any unresolved source not on the grid points, resulting in an up to 70% smaller standard deviation on the residual deviation from linearity. After applying the correction, the full-width at half maximum is up to 20% narrower for sources that cover the full pixel dynamic range. Furthermore, the depth of the fringes is now consistent up the ramp, improving the standard deviation on the difference in fringe depth between the start and ends of integrations by similar to 60%. Conclusions. Pointing-specific linearity corrections allow us to accurately model the pixel ramps across the PSF, and for the first time, fix the systematic deviation in the slopes. In this work we demonstrated this for unresolved sources. The discovered trends with PSF sampling suggest that, in the future, we may be able to model ramps for spatially extended and resolved illumination as well.

[Ne ii ] 12.81 μ m emission is a well-used tracer of protoplanetary disk winds due to its blueshifted line profile. Mid-Infrared Instrument (MIRI)-Medium Resolution Spectrometer (MRS) recently observed T Cha, detecting this line along with lines of [Ne iii ], [Ar ii ], and [Ar iii ], with the [Ne ii ] and [Ne iii ] lines found to be extended while the [Ar ii ] was not. In this complementary work, we use these lines to address long-debated questions about protoplanetary disk winds regarding their mass-loss rate, the origin of their ionization, and the role of magnetically driven winds as opposed to photoevaporation. To this end, we perform photoionization radiative transfer on simple hydrodynamic wind models to map the line emission. We compare the integrated model luminosities to those observed with MIRI-MRS to identify which models most closely reproduce the data and produce synthetic images from these to understand what information is captured by measurements of the line extents. Along with the low degree of ionization implied by the line ratios, the relative compactness of [Ar ii ] compared to [Ne ii ] is particularly constraining. This requires Ne ii production by hard X-rays and Ar ii production by soft X-rays (and/or EUV) in an extended (≳10 au) wind that is shielded from soft X-rays, necessitating a dense wind with material launched on scales down to ∼1 au. Such conditions could be produced by photoevaporation, whereas an extended magnetohydrodynamic (MHD) wind producing equal shielding would likely underpredict the line fluxes. However, a tenuous inner MHD wind may still contribute to shielding the extended wind. This picture is consistent with constraints from spectrally resolved line profiles.

Understanding the coevolution of supermassive black holes and their host systems requires a comprehensive census of active galactic nuclei (AGNs) behavior across a wide range of redshift, luminosity, obscuration level, and galaxy properties. We report significant progress with JWST toward this goal from the Systematic Mid-infrared Instrument Legacy Extragalactic Survey (SMILES). Based on comprehensive spectral energy distribution (SED) analysis of 3273 MIRI-detected sources, we identify 217 AGN candidates over a survey area of similar to 34 arcmin(2), including a primary sample of 111 AGNs in normal massive galaxies (M-* > 10(9.5)M(circle dot)) at z similar to 0-4, an extended sample of 86 AGN candidates in low-mass galaxies (M-* < 10(9.5)M(circle dot)), and a high-z sample of 20 AGN candidates at z similar to 4-8.4. Notably, about 80% of our MIRI-selected AGN candidates are new discoveries despite the extensive pre-JWST AGN searches. Even among the massive galaxies where the previous AGN search is believed to be thorough, 34% of the MIRI AGN identifications are new, highlighting the impact of obscuration on previous selections. By combining our results with the efforts at other wavelengths, we build the most complete AGN sample to date and examine the relative performance of different selection techniques. We find the obscured AGN fraction increases from L-AGN,L-bol similar to 10(10)L(circle dot) to 10(11)L(circle dot) and then drops toward higher luminosity. Additionally, the obscured AGN fraction gradually increases from z similar to 0 to z similar to 4 with most high-z AGNs obscured. We discuss how AGN obscuration, intrinsic SED variations, galaxy contamination, survey depth, and selection techniques complicate the construction of a complete AGN sample.

We describe an algorithm for application of the classic “drizzle” technique to produce 3D spectral cubes using data obtained from the slicer-type integral field unit (IFU) spectrometers on board the James Webb Space Telescope. This algorithm relies upon the computation of overlapping volume elements (composed of two spatial dimensions and one spectral dimension) between the 2D detector pixels and the 3D data cube voxels, and is greatly simplified by treating the spatial and spectral overlaps separately at the cost of just 0.03% in spectrophotometric fidelity. We provide a matrix-based formalism for the computation of spectral radiance, variance, and covariance from arbitrarily dithered data and comment on the performance of this algorithm for the Mid-Infrared Instrument’s Medium Resolution IFU Spectrometer. We derive a series of simplified scaling relations to account for covariance between cube spaxels in spectra extracted from such cubes, finding multiplicative factors ranging from 1.5–3 depending on the wavelength range and kind of data cubes produced. Finally, we discuss how undersampling produces periodic amplitude modulations in the extracted spectra in addition to those naturally produced by fringing within the instrument; reducing such undersampling artifacts below 1% requires a four-point dithering strategy and spectral extraction radii of 1.5 times the point-spread function FWHM or greater.

The Mid-Infrared Instrument (MIRI) extends the reach of the James Webb Space Telescope (JWST) to 28.5 μ m. It provides subarcsecond-resolution imaging, high sensitivity coronagraphy, and spectroscopy at resolutions of λ /Δ λ ∼ 100–3500, with the high-resolution mode employing an integral field unit to provide spatial data cubes. The resulting broad suite of capabilities will enable huge advances in studies over this wavelength range. This overview describes the history of acquiring this capability for JWST. It discusses the basic attributes of the instrument optics, the detector arrays, and the cryocooler that keeps everything at approximately 7 K. It gives a short description of the data pipeline and of the instrument performance demonstrated during JWST commissioning. The bottom line is that the telescope and MIRI are both operating to the standards set by pre-launch predictions, and all of the MIRI capabilities are operating at, or even a bit better than, the level that had been expected. The paper is also designed to act as a roadmap to more detailed papers on different aspects of MIRI.

Context. The Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) uses three Si:As impurity band conduction (IBC) detector arrays. The output voltage level of each MIRI detector pixel is digitally recorded by sampling up the ramp. For uniform or low-contrast illumination, the pixel ramps become nonlinear in a predictable way, but in areas of high contrast, the nonlinearity curve becomes much more complex. The origin of the effect is poorly understood and currently not calibrated out of the data. Aims. We provide observational evidence of the brighter-fatter effect (BFE) in MIRI conventional and high-contrast coronagraphic imaging, low-resolution spectroscopy, and medium-resolution spectroscopy data, and we investigate the physical mechanism that gives rise to the effect on the MIRI detector pixel raw voltage integration ramps. Methods. We used public data from the JWST/MIRI commissioning and Cycle 1 phase. We also developed a numerical electrostatic model of the MIRI detectors using a modified version of the public Poisson_CCD code. Results. We find that the physical mechanism behind the BFE manifesting in MIRI data is fundamentally different to that of charge-coupled devices and photodiode arrays such as the Hawaii-XRG near-infrared detectors used by the NIRISS, NIRCam, and NIRSpec instruments on board JWST. Observationally, the BFE makes the JWST MIRI data yield 10–25% larger point sources and spectral line profiles as a function of the relative level of de-biasing of neighboring detector pixels. This broadening impacts the MIRI absolute flux calibration, time-series observations of faint companions, and point spread function modeling and subtraction. We also find that the intra-pixel 2D profile of the shrinking Si:As IBC detector depletion region directly impacts the accuracy of the pixel ramp nonlinearity calibration model.

The Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) uses three Si:As impurity band conduction (IBC) detector arrays. The output voltage level of each MIRI detector pixel is digitally recorded by sampling-up-the-ramp. For uniform or low-contrast illumination, the pixel ramps become non-linear in a predictable way, but in areas of high contrast, the non-linearity curve becomes much more complex. The origin of the effect is poorly understood and currently not calibrated. We provide observational evidence of the Brighter-Fatter Effect (BFE) in MIRI conventional and high-contrast coronographic imaging, low-resolution spectroscopy, and medium-resolution spectroscopy data and investigate the physical mechanism that gives rise to the effect on the MIRI detector pixel raw voltage integration ramps. We use public data from the JWST MIRI commissioning and Cycle 1 phase. We also develop a numerical electrostatic model of the MIRI detectors using a modified version of the public Poisson_CCD code. We find that the physical mechanism behind the BFE manifesting in MIRI data is fundamentally different to that of CCDs and photodiode arrays such as the Hawaii-XRG (HXRG) near-infrared detectors used by the NIRISS, NIRCam, and NIRSpec instruments on board JWST. Observationally, the BFE makes the JWST MIRI data yield 10-25 % larger point sources and spectral line profiles as a function of the relative level of debiasing of neighboring detector pixels. This broadening impacts the MIRI absolute flux calibration, time-series observations of faint companions, and PSF modeling and subtraction. We also find that the intra-pixel 2D profile of the shrinking Si:As IBC detector depletion region directly impacts the accuracy of the pixel ramp non-linearity calibration model.