We present the results from performing spectral energy distribution (SED) fitting on 121 variable active galactic nuclei (AGN) candidates in the Hubble Ultra Deep Field using photometry from both the Hubble Space Telescope and the James Webb Space Telescope (JWST) covering 0.2–4.8 μm. We designed a bespoke SED fitting code, which decomposes the total SED into its stellar and AGN contributions. Our SED fitting retrieves a significant contribution to the total SED from an AGN template for 26 of our variable sources with 0 < z < 7. We leverage the model AGN spectrum to estimate black hole masses (M_BH) using the measured luminosity at 5100 Å and local empirical calibrations. Common with recently discovered JWST broad-line AGN, we observe a trend in the M_BH–M_* plane where low-redshift sources have M_BH that agrees with local relations while high-redshift sources have increasingly overmassive black holes with respect to the stellar mass (M_*) of their host galaxies. Within our sample, we identify two IMBH candidates hosted by dwarf galaxies at z < 1 featuring overmassive black holes in the M_BH–M_* plane, similarly to our high-redshift sources. Finally, our SED fitter successfully retrieves the AGN nature of one source at z > 6. This object has z_phot = 6.74, and we estimate a lower limit on its black hole mass of log₁₀(M_BH/M_☉) > 7.36.

We present an analysis of archival JWST NIRSpec IFS and HST imaging observations of the z = 3 Lyman continuum emitter (LCE) candidate LACES104037. We show that a nearby galaxy, denoted LACES104037-S, has a redshift offset from the main galaxy by only ∼ 450 km s⁻¹. Together with the identification of a tidal bridge between the galaxies, this indicates that the galaxies are interacting and most likely in the early stages of a merger. We show that the rest-frame LCE cluster sits ∼ 2.7 kpc from the galaxy core in the tidal bridge. It is faint in the nonionizing stellar continuum and shows faint but nonnegligible Hα and [O III] emission, suggesting that much of the gas surrounding the LCE cluster has been dispersed by feedback in the shallower gravitational potential of the tidal bridge. Comparing the direct LyC escape and the local Hα emission, we find a total ionizing escape fraction of f_esc^LyC =57 ± 8% from the LCE cluster. We estimate the age of the LCE cluster to ≲ 6.5 Myr, indicating that the cluster must have formed in situ in the tidal bridge well after the time of closest interaction. LyC escape from tidal stripping or in situ formed stars in tidal features would depend less on intrinsic galaxy properties than typically observed in low-z LCE surveys and could help explain the higher cosmic escape fraction and enhanced diversity of LCE galaxy properties observed at cosmic noon.

One of the current challenges in galaxy evolution studies is to establish the mechanisms that govern the escape of ionizing radiation from galaxies. Here, we investigate the connection between Lyman continuum (LyC) escape and the conditions of the circumgalactic medium (CGM), as probed by Lyα halos (LAHs) in emission. We use Lyα and UV continuum imaging data from the Lyα and Continuum Origins Survey (LaCOS), targeting 42 nearby (z≃0.3), star-forming galaxies with LyC observations (escape fractions of ). LaCOS galaxies show extended Lyα emission ubiquitously, with LyC emitters (LCEs) having more compact Lyα morphologies than non-LCEs, and Lyα spatial offsets that do not exceed the extent of the UV continuum. We model the diffuse LAHs using a combined Sérsic plus exponential 2D profile, and find that the characteristic scale length of the Lyα halo is 10 times larger than the UV, on average. We unveil a significant anticorrelation between and the Lyα halo fraction (HF; or contribution of the halo to the total Lyα luminosity), which we propose as a new LyC indicator. Our observations show that halo scale lengths and HFs both scale positively with the optical depth of the neutral gas in the interstellar medium, revealing a picture in which Lyα and LyC photons in LCEs either emerge directly from the central starbursts or escape isotropically, and in the case of Lyα, minimize the number of scattering interactions in a less-extended CGM.

Characterizing the mechanisms and galaxy properties conducive to the escape of ionizing (LyC) emission is necessary to accurately model the Epoch of Reionization and identify the sources that powered it. Using Hubble Space Telescope data, the Ly alpha and Continuum Origins Survey (LaCOS) is the first program to obtain uniform, multiwavelength subkiloparsec imaging for a large sample (42) of galaxies observed in LyC and enable statistically robust studies between LyC and resolved galaxy properties. Here, we characterize the morphology and galaxy merger properties of LaCOS galaxies and investigate their connection with the escape fraction of LyC emission fescLyC . We find strong anticorrelations between fescLyC and size (r20, r50, and r80) measured in filters containing emission from star-forming regions, and with the asymmetry and clumpiness in F150LP, a filter tracing UV continuum and Ly alpha. We find that >= 48% of LaCOS galaxies, and >= 41% of LaCOS LyC-emitters are visually classified as galaxy mergers. Galaxies robustly identified as mergers in LaCOS are at advanced stages of interaction, close to coalescence. The fescLyC properties of robust mergers and low-probability mergers cannot be differentiated statistically, and we only find significant difference between the two populations in terms of their sizes and LyC luminosity: robust mergers having larger values. We conclude that (i) fescLyC tends to be larger in galaxies with a small number of compact, centrally located, UV-emitting star-forming regions, (ii) mergers at advanced stages of interaction represent a sizable fraction of LyC-emitting samples at z similar to 0.3, and (iii) mergers can facilitate the escape of LyC photons from galaxies.

We present a comprehensive analysis of the MIRI Extremely Red Object Virgil, a Lyα emitter at zspec = 6.6379 ± 0.0035 with the photometric properties of a Little Red Dot. Leveraging new JWST/MIRI imaging from the MIDIS and PAHSPECS programs, we confirm Virgil’s extraordinary nature among galaxies in JADES/GOODS-South, exhibiting a strikingly red NIRCam-to-MIRI color (F444W–F1500W = 2.84 ± 0.04 mag). Deep NIRSpec/PRISM spectroscopy from the OASIS program offers key insights into the host galaxy, revealing properties of an average star-forming galaxy during Cosmic Reionization, such as a subsolar metallicity, low-to-moderate dust content, and a relatively high ionization parameter and electron temperature. By estimating the star formation rate of Virgil from UV and Hα, we find evidence that the galaxy is either entering or fading out of a bursty episode. Although line-ratio diagnostics employed at high z would classify Virgil as an active galactic nucleus (AGN), this classification becomes ambiguous once redshift evolution is considered. Nonetheless, Virgil occupies the same parameter space as recently confirmed AGNs at similar redshifts. The new deep MIRI data at 15 μm reinforce the AGN nature of Virgil, as inferred from multiple spectral energy distribution (SED) fitting codes. Virgil’s rising infrared SED and UV excess resemble those of Dust-Obscured Galaxies (DOGs) studied with Spitzer at Cosmic Noon, particularly blue-excess HotDOGs. Our results highlight the need for a multiwavelength approach incorporating MIRI to uncover such extreme sources at z ≳ 6 and to shed light on the interplay between galaxy evolution and early black hole growth during Cosmic Reionization.

This paper presents a deep MIRI/JWST medium-resolution spectroscopy (MRS) covering the rest-frame optical spectrum of the GN-z11 galaxy. The [O III] 5008 Å and Hα emission lines are detected and spectroscopically resolved. The line profiles are well modeled by a narrow Gaussian component with intrinsic full widths at half maximum of 189 ± 25 and 231 ± 52 km s-1, respectively. We do not find any evidence of a dominant broad Hα emission line component tracing a broad-line region in a type 1 active galactic nucleus (AGN). The existence of an accreting black hole dominating the optical continuum and emission lines of GN-z11 is not compatible with the measured Hα and [O III] 5008 Å luminosities. If the well-established relations for low-z AGNs apply in GN-z11, the [O III] 5008 Å and Hα luminosities would imply extremely high super-Eddington ratios (λE > 290), and bolometric luminosities ∼20 times those derived from the UV/optical continuum. However, a broad (∼430-470 km s-1) and weak (< 20-30%) Hα line component, tracing a minor AGN contribution in the optical, cannot be completely ruled out with the sensitivity of the current data. The physical and excitation properties of the ionized gas are consistent with a low-metallicity starburst with a star formation rate of 24 ± 3 M⊙ yr-1. The electron temperature of the ionized gas is Te (O++) = 14 000 ± 2100 K, while the direct-Te gas-phase metallicity is 12 + log(O/H) = 7.91 ± 0.07 (Z = 0.17 ± 0.03 Z⊙). The optical line ratios locate GN-z11 in the starburst or AGN region, but they are more consistent with those of local low-metallicity starbursts and high-z luminous galaxies detected at redshifts similar to GN-z11. We conclude that the MRS optical spectrum of GN-z11 is consistent with that of a massive, compact, and low-metallicity starburst galaxy. Its high star formation and stellar mass surface densities are close to those of the densest stellar clusters, and we therefore speculate that GN-z11 might undergo a feedback-free, highly efficient starburst phase. Additional JWST data are needed to validate this scenario and other recently proposed alternatives to explain the existence of bright compact galaxies in the early Universe.

We present Virgil, a Mid-Infrared Instrument (MIRI) extremely red object detected with the F1000W filter as part of the MIRI Deep Imaging Survey observations of the Hubble Ultra Deep Field. Virgil is an Lyα emitter (LAE) at zspec = 6.6312 ± 0.0019 (from the Very Large Telescope/MUSE) with a rest-frame UV-to-optical spectral energy distribution (SED) typical of LAEs at similar redshifts. However, MIRI observations reveal an unexpected extremely red color at rest-frame near-infrared (NIR) wavelengths, F444W − F1000W = 2.33 ± 0.06. Such a steep rise in the NIR, completely missed without MIRI imaging, is poorly reproduced by models including only stellar populations and hints toward the presence of an active galactic nucleus, although alternative explanations such as extreme dust obscuration and strong nebular continuum and emission lines contribution due to young stellar ages cannot be completely ruled out. According to the shape of its overall SED, Virgil belongs to the recently discovered population of little red dots but displays an extended rest-frame UV-optical wavelength morphology following a 2D-Sérsic profile with an average index of n = (Formula presented) pkpc. Only at MIRI wavelengths, Virgil is unresolved due to the coarser point-spread function. This discovery demonstrates the crucial importance of deep MIRI surveys to reveal the true nature and properties of high-z galaxies that otherwise would be misinterpreted and raises the question of how common Virgil-like objects could be in the early Universe.

Context. Thanks to decades of observations using the Hubble Space Telescope (HST), the structure of galaxies at redshift z>2 has been widely studied in the rest-frame ultraviolet regime, which traces recent star formation from young stellar populations. However, we still have little information about the spatial distribution of the older, more evolved stellar populations, constrained by the rest-frame infrared portion of the galaxies’ spectral energy distribution.

Aims. We present the morphological characterization of a sample of 49 massive galaxies (log(M_⋆/M_⊙)>9) at redshift 3<z<5. These galaxies are observed as part of the MIRI Deep Imaging Survey (MIDIS), the guaranteed time observations program with the MIRI instrument on board the James Webb Space Telescope (JWST). Deep MIRI 5.6 μm imaging (28.64 mag 5σ depth) allows us to characterize the rest-frame near-infrared structure of galaxies beyond cosmic noon, at higher redshifts than possible with NIRCam, tracing their older and dust-insensitive stellar populations.

Methods. We derived the nonparametric morphology of galaxies, focusing on the Gini, M₂₀, concentration, asymmetry, and deviation statistics. Furthermore, we modeled the light distribution of galaxies with a single Sérsic component and derived their parametric morphology (i.e., effective radius and Sérsic index).

Results. We find that at z>3 massive galaxies show a smooth distribution of their rest-infrared light, strongly supporting the increasing number of regular disk galaxies already in place at early epochs. These results are further reinforced by the analysis of JWST/NIRCam data at 4.4 μm. On the contrary, the ultraviolet structure obtained from HST/WFC3 and JWST/NIRCam observations at ∼1.5 μm is generally more irregular, catching the most recent episodes of star formation. Importantly, we find a segregation of morphologies across cosmic time, where galaxies at redshift z>3.75 show later-type morphologies compared to z∼3 galaxies. These findings suggest a transition phase in galaxy assembly and central mass build-up, which takes place already at z∼3−4.

Conclusions. The combined analysis of NIRCam and MIRI imaging datasets allows us to prove that the rest-frame near-infrared morphology of massive galaxies at cosmic noon is typical of compact disk galaxies with a smooth mass distribution.

X-ray and infrared surveys provide efficient, and to some degree complementary, means of detecting and characterising active galactic nuclei (AGNs), with the infrared also providing an important probe of the host galaxies. To this end we combine the deepest X-ray survey from the Chandra Deep Field-South (CDF-S) ‘7-Ms’ survey with the deepest mid-infrared (5.6 μm) image from the JWST/MIRI Deep Imaging Survey (MIDIS) in the Hubble Ultra-Deep Field (HUDF) to study the infrared counterparts and point-source emission of 31 X-ray sources with a median, intrinsic, rest-frame X-ray luminosity of log₁₀(L_Xc^{0.5−7 keV}) = 42.04 ± 0.22 erg s⁻¹. The sample includes 24 AGNs with a redshift range, set by the X-ray detectability, of z ≃ 0.5−3, with the bulk of the sources lying at z ≃ 1−2, i.e. around the epoch of cosmic noon. Through a multi-wavelength morphological decomposition, employing three separate classifications (visual, parametric, and non-parametric), we separate (where present) the luminosity of the point-like AGN component from the remainder of the host-galaxy emission. The unprecedented mid-infrared sensitivity and imaging resolution of MIRI allows, in many cases, for the direct characterisation of point-like (i.e. unresolved) components in the galaxies’ emission. We establish a broad agreement between the three morphological classifications. At least 70% of the X-ray sources, including some classified as galaxies, show unresolved emission in the MIRI images, with the unresolved-to-total flux fraction at rest-frame 2 μm ranging from ∼0.2 to ∼0.9. At high X-ray luminosities (log₁₀(L_Xc) > 43 erg s⁻¹), we derive a consistent rest-frame near-infrared 2 μm point-source luminosity to that derived for local AGNs, whilst at lower X-ray luminosities we identify an excess in the 2 μm emission compared to pre-JWST studies. We speculate that this offset may be driven by a combination of Compton-thick AGN components and nuclear starburst, merger-driven activity. Our observations highlight the complex nature of X-ray sources in the distant Universe and demonstrate the power of JWST/MIRI in quantifying their nuclear infrared emission.

Context. This paper investigates the star formation histories (SFHs) of a sample of massive galaxies (M_⋆ ≥ 10¹⁰ M_⊙) in the redshift range 1 < z < 4.5.

Methods. We analyzed spectro-photometric data, combining broadband photometry from HST and JWST with low-resolution grism spectroscopy from JWST/NIRISS, obtained as part of the MIRI Deep Imaging Survey program. SFHs were derived through spectral energy distribution fitting using two independent codes, BAGPIPES and synthesizer, under various SFH assumptions. This approach enables a comprehensive assessment of the biases introduced by different modeling choices.

Results. The inclusion of NIRISS spectroscopy, even with its low resolution, significantly improves constraints on key physical parameters, such as the mass-weighted stellar age (t_M) and formation redshift (z_form), by narrowing their posterior distributions. The massive galaxies in our sample exhibit rapid stellar mass assembly, forming 50% of their mass between 3 ≤ z ≤ 9. The highest inferred formation redshifts are compatible with elevated star formation efficiencies (ϵ) at early epochs. Nonparametric SFHs generally imply an earlier and slower mass assembly compared to parametric forms, highlighting the sensitivity of inferred formation timescales to the chosen SFH model–particularly for galaxies at z < 2. We find that quiescent galaxies are, on average, older (t_M ∼ 1.1 Gyr) and assembled more rapidly at earlier times than their star-forming counterparts. These findings support the “downsizing” scenario, in which more massive and passive systems form earlier and more efficiently.