arXiv subjects

P. Bergamini

Publications and source records attributed to P. Bergamini.

At least 19 recordsLinked to original sources

arXiv2026

Observation-driven simulations of strong lensing galaxy clusters

Galaxy clusters are the most powerful strong lenses: they greatly magnify the flux of distant and faint sources. Strong lensing also allows for the reconstruction of their mass distribution with-1% level accuracy and enables investigating cosmological parameters. The number of known systems of this type is bound to increase in the next years thanks to wide imaging surveys. Using simulations in this context is crucial to validate the analysis methods before they are applied to real data, and to train machine learning algorithms that can handle large volumes of images. In this work, we present a simulated set of one hundred images of galaxy clusters that we have produced with a novel code for simulating cluster-scale strong lenses. One of the main novelties of our approach, distinguishing it from other existing codes, is the use of empirical relations, derived from state-of-the-art observations, for modelling the characteristics, such as morphology, color, and spatial distribution of the cluster member population. This allows us to reliably reproduce the complexity of real observations. The simulations are partly carried out with the latest version of SkyLens, a code that creates mock observations of strong lensing events in different systems and observational setups. The main improvements we introduce are the use of the message passage interface (MPI) paradigm, a standard for parallel programming that leverages the use of several processors to perform some given task, and the implementation of score-based diffusion models to augment the images of the background sources. Together, they lead to more efficient and realistic image simulations. We also present the validation of the code and simulations by comparing the properties of the mock clusters to those of real ones. [Abridged]

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arXiv2026

A new $H_0$ measurement with SNe Requiem and Encore using $\texttt{Gravity.jl}$

We present a strong-lensing (SL) analysis of the galaxy cluster MACS J0138.0-2155 (z=0.336), the first known lens cluster discovered to host two distinct multiply imaged Type Ia supernovae (SNe): SN Requiem and SN Encore. Both SNe are located in the massive, multiply imaged red galaxy MRG-M0138 at z=1.949. The projected total mass of this cluster has been investigated with several independent lens models (Suyu+26; Pierel+26), using a sample of 23 spectroscopically confirmed multiple images from 8 background sources (0.767 10%). The forthcoming reappearance of SN Requiem offers an immediate opportunity to significantly improve constraints on H0, provided that lens-model systematics are controlled. These results establish M0138 as a premier anchor for high-precision cluster-scale TDC.

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arXiv2026

A Cosmic Archipelago of lensed metal-poor galaxies at $z\sim6$

The Cosmic Archipelago is an ensemble of galaxies, strongly lensed by the cluster MACSJ0416, showing extreme physical properties at $z\sim6.14$. We combine JWST/NIRCam with deep VLT/X-Shooter and JWST/NIRSpec IFU to perform a joint spectrophotometric analysis from the far ultraviolet to red optical rest-frame. We focus on CA4, a UV-faint ($M_{UV}=-17.7$), compact ($r_e=81\pm11$ pc) galaxy at $z=6.1446$, magnified by a factor $\mu=3.73$. CA4 is a young, low-mass ($M_\star =4.3\times10^6$ M$\odot$), star-forming (${\rm SFR}=0.46$ M$\odot$/yr), and metal-poor ($Z\sim0.02$ Z$\odot$) galaxy, and an efficient producer of ionizing photons ($\log(\xi_{ion}/{\rm erg^{-1} Hz})\sim25.5$). Its properties place CA4 at the poorly explored interface between massive stellar clusters and dwarf galaxies during the epoch of reionization. Moreover, CA4 shows large Ly$\alpha$ ($f_{esc}^{\rm Ly\alpha}\sim43\%$) and Lyman-continuum ($f_{esc}\sim47\%$) escape fractions, consistent with its small Ly$\alpha$ velocity offset ($\Delta v\sim100$ km/s) and extremely blue UV-continuum slope ($\beta=-3.10$). These characteristics suggest that such UV-faint, metal-poor galaxies may contribute significantly to cosmic reionization. We also confirm five additional systems at the redshift of the Cosmic Archipelago, magnified by factors up to 12.5. They are all young (mass-weighted ages $<11$ Myr) and metal-poor ($Z<0.05$ Z$_\odot$), spanning a wide range of stellar masses and SFRs. Given the large number of these bursty star-forming galaxies in a small cosmic volume, we estimate that the currently known members of the Cosmic Archipelago result in a significant overdensity at $z\sim6$ ($\Delta z\sim0.08$), with $\delta_{gal}=12.3^{+6.6}_{-4.6}$. These results highlight the Cosmic Archipelago as an unprecedented laboratory for studying the earliest groups of low-mass, low-metallicity galaxies during the epoch of reionization.

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arXiv2026

A New Perspective on Galactic Evolution: Studying the Outskirts of the Abell S1063 Galaxy Cluster

Galaxy physical properties are influenced by their environments, but the processes responsible for mass and environmental quenching and structural transformations remain debated. Galaxy clusters are ideal laboratories for investigating galaxy formation and evolution, offering a full range of galaxy properties and environments. Observations of large-scale structures, particularly filaments in cluster outskirts ($r \sim5r_{200}$), are currently constrained to the low-redshift Universe. To explore galaxy evolution at intermediate redshifts, deep photometric data, ideally combined with spectroscopic redshifts, are essential. Abell S1063 cluster ($z$ = 0.346) is observed within the Galaxy Assembly as a function of the Mass and Environment program with the VLT Survey Telescope (VST-GAME) combined VISTA Public Survey program Galaxy Cluster At Vircam. We investigate galaxy evolution across a wide range of stellar masses and environments. We release a multiwavelength photometric catalog with photometric redshifts for 64394 sources in $1x1 deg^2$. The analysis of overdensity regions provides insights for future studies on galaxy properties in cluster outskirts. The dataset is obtained through deep ($r<$24.65 mag) and wide optical ($u$, $g$, $r$, $i$, VST) and near-infrared ($Y$, $J$, $K_s$, VISTA) observations. The photometric catalog includes all detected sources, excluding nearby or overlapping objects, saturated stars, and image artifacts. The multiwavelength catalog enabled photometric redshift estimates and identification of cluster members. The density field allowed comparison of galaxy properties, colors, and masses across environments. We detect a very dense structure near the cluster center, and with such a large field of view, we find another dense region to the north-west, in the opposite direction to the cluster elongation. Filaments connecting the regions are also visible.

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arXiv2026

The velocity dispersion function of red galaxies in four Hubble Frontier Fields galaxy clusters

We present a detailed study of the stellar kinematic properties of red member galaxies in the cores of four strong lensing galaxy clusters at intermediate redshifts included in the the Hubble Frontier Fields programme: Abell 2744 ($z=0.307$), Abell S1063 ($z=0.346$), MACS J0416.1$-$2403 ($z=0.397$), and MACS J1149.6$+$2223 ($z=0.542$). We focussed on a sample of 723 red cluster members in the four clusters and we measured their structural parameters using MORPHOFIT for all Hubble Frontier Fields bands. Taking advantage of deep (3.1 h to 17 h of exposure) integral-field spectroscopy from MUSE on the Very Large Telescope, we tested a pipeline based on the public spectral fitting code pPXF to systematically measure the line-of-sight stellar velocity dispersion $\sigma$ of cluster members with a spectral $S/N\geq 10$, with a statistical uncertainty consistently below 5%. The resulting catalogue contains 213 measured $\sigma$ values across the four clusters. We calibrated the Fundamental Plane relation in the rest-frame $r$ band for the early-type cluster members, selected from their colour and morphology; we found compatible parameters both across the clusters, and noting hints of zero-point evolution with redshift. Finally, we used the calibrated Fundamental Plane relations to assign a velocity dispersion value to all 723 red cluster members and studied the velocity dispersion function for each cluster, down to $\log \sigma \, \mathrm{[km \, s^{-1}] = 1.5}$. A Schechter function fit of the velocity functions suggests compatible parameters: a positive $\alpha$ slope with values in the range $0.55-1.60$, and $\log\sigma^*\, [\mathrm{km\,s^{-1}}]$ between $2.18$ and $2.47$. Unlike previous works, we extended the systematic study of the central velocity dispersion of cluster galaxies to lower-$\sigma$ regimes.

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arXiv2026

JWST Reveals Two Overmassive Black Hole Candidates in Dwarf Galaxies at z $\approx$ 0.7: Pushing Black Hole Searches into the Dwarf-Galaxy Regime

We report the discovery and characterization of two compact galaxies, Pelias and Neleus, at z ~ 0.71 and z ~ 0.75, identified in MACS J0416.1-2403 and GOODS-North. Both exhibit unusual spectral energy distributions (SEDs), with very blue rest-frame UV-optical emission and a steep rise toward near- and mid-infrared wavelengths. JWST/NIRISS and JWST/NIRSpec spectroscopy show strong rest-frame optical lines ([O III] 4959,5007 and Halpha) with extreme equivalent widths (>= 1000 Angstrom), indicating young burst-dominated populations with low metallicities (Z ~ 0.1-0.4 Zsun), low dust attenuation (Av ~ 0.2 mag), and stellar masses of Mstar ~ 10^7 Msun. Nonetheless, JWST/MIRI photometry reveals a strong mid-infrared excess that cannot be explained by stellar populations or star-formation-heated dust alone, requiring a hot-dust component most naturally associated with a deeply embedded active galactic nucleus (AGN). SED modelling yields log10(Lbol [erg/s]) ~ 43.7-44.0, implying black hole masses of log10(MBH [Msun]) ~ 5.7-6.7 under the assumption of Eddington-limited accretion. Given the very low stellar masses of the hosts, this corresponds to black-hole-to-stellar mass ratios of about 6-60%, well above the extrapolation of local scaling relations. The lack of X-ray detections suggests that the accretion may be either heavily obscured or intrinsically X-ray weak. Their SEDs also resemble those of Blue Excess Hot Dust Obscured Galaxies and show the characteristic V-shaped continuum seen in Little Red Dots, although with the inflection occurring at redder wavelengths.

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arXiv2026

A boost in the precision of cluster-mass models: Exploiting the extended surface brightness of the lensed supernova Refsdal host galaxy

Combining deep Hubble Space Telescope (HST) images and extensive data from the Multi-Unit Spectroscopic Explorer, we present new mass models of the cluster MACS J1149.5+2223, strongly lensing the supernova (SN) Refsdal, fully exploiting the source surface-brightness distribution of the SN host for the first time. In detail, we incorporated 77,000 HST pixels, in addition to the known 106 point-like multiple images, in our modeling. We considered four different models to explore the effect of the relative weighting of the point-like multiple image positions and flux distribution of the SN host on the model optimization. When the SN host's extended image is included, we find that the statistical uncertainties of all 34 free model parameters are reduced by factors ranging from one to two orders of magnitude compared to the statistical uncertainty of the point-like only model, irrespective of the adopted different image weights. We quantified the remarkably increased level of precision with which the cluster's total mass and the predicted time delays of the SN Refsdal multiple image positions can be reconstructed. We also show the delensed image of the SN host, a spiral galaxy at zSN = 1.49, in multiple HST bands. In all those applications, we obtain a significant reduction of the statistical uncertainty, which is now below the level of even the small systematic uncertainty on the mass model that could be assessed by the different approaches. These results demonstrate that with extended image models of lensing clusters it is possible to measure the cluster's total mass distribution, the values of the cosmological parameters, and the physical properties of high-redshift sources with an unparalleled precision, making the typically not-quantified systematic uncertainties now crucial.

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arXiv2025

A faint M$_{\rm UV} = -14.5$ Lyman-continuum leaker candidate in the epoch of reionization: Unprecedented Ly$\alpha$ properties at z=5.725

We report the unprecedented Ly$\alpha$ properties of AMORE6, an extremely metal-poor ($12+\log({\rm O/H}) < 6$), low-mass ($M_\star = 4.4\times10^{5}\,M_\odot$), and ultracompact (effective radius $\sim30$ pc) dwarf galaxy at $z=5.7253$, which is gravitationally lensed by the cluster A2744. A prominent, narrow, and nearly symmetric Ly$\alpha$ emission line is detected at the systemic redshift (the latter traced by H$\beta$, from JWST/NIRCam slitless spectroscopy), with a rest-frame equivalent width of $150 \pm 10$ \AA, a full width at half maximum of $\rm 58\pm1$ km s$^{-1}$, and a slight asymmetry, resulting in a flux excess of $\rm \sim10\%$ in the red wing of the line. The negligible velocity offset from systemic ($dv = 4\pm67$ km s$^{-1}$, $3\sigma$ uncertainty), together with the sharpness and symmetry of the profile, indicates minimum radiative transfer effects, which implies a neutral hydrogen column density consistent with an optically thin medium that in turn is compatible with a nonzero ionizing photon escape fraction. If indirect spectral diagnostics calibrated at $z<4.5$ remain the only viable tools for identifying LyC leakers during reionization, then based on its strongest indicator (Ly$\alpha$), AMORE6 stands out as one of the most compelling LyC-leaking candidates yet discovered in the epoch of reionization.

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arXiv2025

The Early Maturity of High-Redshift Galaxies: Insights from sSFR, M/L and SFHs at z~7-14

The James Webb Space Telescope (JWST) has revealed an unexpected excess of UV-bright galaxies at z>10, unaccounted for by extrapolations from pre-JWST observations and theoretical models. Understanding the physical properties and star formation histories (SFHs) of high-redshift systems is key to distinguishing between the proposed scenarios. We identified and analysed a sample of 2420 robust candidates at z~7-14 drawn from the ASTRODEEP-JWST dataset over ~0.2 deg^2, and modelled their properties with non-parametric SFHs to derive the specific star formation rate (sSFR) and stellar population properties. We find that the median sSFR and M/L remain roughly constant across the probed redshift range. We show that this result is robust against potential systematics unless a hidden population of dust-enshrouded starbursts, undetectable in current data, exists at these redshifts. In any case, the absence of observed high-sSFR systems at the highest redshifts suggests that any dust-free starburst phase must be short-lived. The observed sSFR evolution is in tension with most theoretical models, making it a key quantity for discriminating among competing scenarios. The sample shows a wide range of physical conditions and galaxy classes, including systems with low sSFRs and high mass-to-light ratios (M/L) up to z~10, indicative of already-evolved galaxies only a few hundred Myr after the Big Bang, and different degrees of dust attenuation. We finally reconstructed the assembly histories of two sub-samples, namely the highest-M/L galaxies at z~7-8, which appear to have formed the bulk of their stars at least 500 Myr before observation, implying progenitors observable as UV-bright sources at z>20, and the full sample of z>11 galaxies, which formed through stochastic SFHs, remaining UV-faint for most of their early evolution, before undergoing recent (~50 Myr old) episodes of major growth.

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arXiv2025

Cosmic chronometers with galaxy clusters: a new avenue for multi-probe cosmology

We provide a new measurement of the expansion history of the Universe at $z=0.54$ with the cosmic chronometers (CC) method, exploiting the high-quality spectroscopic VLT/MUSE data for three galaxy clusters in close-by redshift bins: SDSS J2222+2745 ($z=0.49$), MACS J1149.5+2223 ($z=0.54$), and SDSS J1029+2623 ($z=0.59$). The central one, MACS J1149.5+2223, hosts the well-known supernova 'Refsdal', which allowed for $H_0$ measurements via time delay cosmography (TDC). This represents the first step for a self-consistent probe combination, where different methods are applied to the same data sample. After selecting the most passive and massive cluster members (38 CCs), we derive their age and physical parameters via full spectrum fitting. We use the code Bagpipes, specifically modified to remove the cosmological prior on ages. On average, the CC sample shows super-solar metallicities $Z/Z_{\odot} = 1.3 \pm 0.7$, low dust extinction $A_{\rm{V}} = 0.3 \pm 0.3$ mag and to have formed in short bursts $\tau = 0.6 \pm 0.2$ Gyr. We also observe both an ageing trend in redshift and a mass-downsizing pattern. From the age-redshift trend, implementing the CC method through a bootstrap approach, we derive a new $H(z)$ measurement: $H$($z$=0.542) = $66_{-29}^{+81}$ (stat) $\pm$13 (syst) km/s/Mpc. We also simulate the impact of increased statistics and extended redshift coverage, finding that $H$($z$) uncertainties can be reduced by up to a factor of 4 with $\sim$100 CCs and a slightly broader redshift range (d$z\sim$0.2).

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arXiv2025

Cosmology with supernova Encore in the strong lensing cluster MACS J0138$-$2155: photometry, cluster members, and lens mass model

The strongly lensed Supernova (SN) Encore at a redshift of $z = 1.949$, discovered behind the galaxy cluster MACS J0138$-$2155 at $z=0.336$, provides a rare opportunity for time-delay cosmography and studies of the SN host galaxy, where previously another SN, called SN Requiem, had appeared. To enable these studies, we combine new James Webb Space Telescope (JWST) imaging, archival Hubble Space Telescope (HST) imaging, and new Very Large Telescope (VLT) spectroscopic data to construct state-of-the-art lens mass models that are composed of cluster dark-matter (DM) halos and galaxies. We determine the photometric and structural parameters of the galaxies across six JWST and five HST filters. We use the color-magnitude and color-color relations of spectroscopically-confirmed cluster members to select additional cluster members, identifying a total of 84 galaxies belonging to the galaxy cluster. We construct seven different mass models using a variety of DM halo mass profiles, and explore both multi-plane and approximate single-plane lens models. As constraints, we use the observed positions of 23 multiple images from eight multiply lensed sources at four distinct spectroscopic redshifts. In addition, we use stellar velocity dispersion measurements to obtain priors on the galaxy mass distributions. We find that six of the seven models fit well to the observed image positions. Mass models with cored-isothermal DM profiles fit well to the observations, whereas the mass model with a Navarro-Frenk-White cluster DM profile has an image-position $χ^2$ value that is four times higher. We build our ultimate model by combining four multi-lens-plane mass models and predict the image positions and magnifications of SN Encore and SN Requiem. Our work lays the foundation for building state-of-the-art mass models of the cluster for future cosmological analysis and SN host galaxy studies.

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arXiv2025

Euclid Quick Data Release (Q1). Searching for giant gravitational arcs in galaxy clusters with mask region-based convolutional neural networks

Strong gravitational lensing (SL) by galaxy clusters is a powerful probe of their inner mass distribution and a key test bed for cosmological models. However, the detection of SL events in wide-field surveys such as Euclid requires robust, automated methods capable of handling the immense data volume generated. In this work, we present an advanced deep learning (DL) framework based on mask region-based convolutional neural networks (Mask R-CNNs), designed to autonomously detect and segment bright, strongly-lensed arcs in Euclid's multi-band imaging of galaxy clusters. The model is trained on a realistic simulated data set of cluster-scale SL events, constructed by injecting mock background sources into Euclidised Hubble Space Telescope images of 10 massive lensing clusters, exploiting their high-precision mass models constructed with extensive spectroscopic data. The network is trained and validated on over 4500 simulated images, and tested on an independent set of 500 simulations, as well as real Euclid Quick Data Release (Q1) observations. The trained network achieves high performance in identifying gravitational arcs in the test set, with a precision and recall of 76% and 58%, respectively, processing 2'x2' images in a fraction of a second. When applied to a sample of visually confirmed Euclid Q1 cluster-scale lenses, our model recovers 66% of gravitational arcs above the area threshold used during training. While the model shows promising results, limitations include the production of some false positives and challenges in detecting smaller, fainter arcs. Our results demonstrate the potential of advanced DL computer vision techniques for efficient and scalable arc detection, enabling the automated analysis of SL systems in current and future wide-field surveys. The code, ARTEMIDE, is open source and will be available at github.com/LBasz/ARTEMIDE.

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arXiv2025

Parametric strong lensing model of the galaxy cluster Abell 2390 from Euclid and MUSE observations

We present a new high precision parametric strong lensing total mass reconstruction of the Euclid Early Release Observations (ERO) galaxy cluster Abell 2390, at redshift z = 0.231. We include in this analysis 35 multiple images from 13 background sources, of which 25 are spectroscopically confirmed thanks to observations from the MUSE, spanning a redshift range from z = 0.535 to z = 4.877. After fully reanalysing the MUSE spectroscopy, we combine it with archival spectroscopic catalogues allowing us to select 65 secure cluster members. This sample is further complemented with 114 photometric member galaxies, identified within the Euclid VIS and NISP imaging down to magnitude H = 23. We also measure the stellar velocity dispersions for 22 cluster members, in order to calibrate the Faber-Jackson relation and hence the scaling relations for the subhalo mass components. We test and compare 11 total mass parametrisations of the galaxy cluster with increasing complexity. To do so, we employ the new parametric strong lensing modelling code Gravity.jl. Our best fit total mass parametrisation is characterised by a single large scale halo, 179 subhalo components, and an external shear term. The reference model yields a mean scatter between the model predicted and observed positions of the multiple images of 0.32 arcseconds. We are able to quantify the systematics arising from our modelling choices by taking advantage of all the different explored total mass parametrisations. When our results are compared with those from other lensing studies, we notice an overall agreement in the reconstructed cluster total mass profile in the outermost strong lensing regime: the discrepancy in the innermost region of the cluster (few kiloparsec from the brightest cluster galaxy, where few or no strong lensing features are observed) could possibly be ascribed to the different data and modelling choices.

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arXiv2025

Cosmology with supernova Encore in the strong lensing cluster MACS J0138-2155: Time delays & Hubble constant measurement

Multiply-imaged supernovae (SNe) provide a novel means of constraining the Hubble constant ($H_0$). Such measurements require a combination of precise models of the lensing mass distribution and an accurate estimate of the relative time delays between arrival of the multiple images. Only two multiply-imaged SNe, Refsdal and H0pe, have enabled measurements of $H_0$ thus far. Here we detail the third such measurement for SN Encore, a $z=1.95$ SNIa discovered in JWST/NIRCam imaging. We measure the time delay, perform simulations of additional microlensing and millilensing systematics, and combine with the mass models of Suyu et al. in a double-blind analysis to obtain our $H_0$ constraint. Our final time-delay measurement is $Δt_{1b,1a}=-39.8_{-3.3}^{+3.9}$ days, which is combined with seven lens models weighted by the likelihood of the observed multiple image positions for a result of $H_0=66.9_{-8.1}^{+11.2} \rm{km} \rm{s}^{-1}\rm{Mpc}^{-1}$. The uncertainty on this measurement could be improved significantly if template imaging is obtained. Remarkably, a sibling to SN Encore (SN "Requiem") was discovered in the same host galaxy, making the MACS J0138.0-2155 cluster the first system known to produce more than one observed multiply-imaged SN. SN Requiem has a fourth image that is expected to appear within a few years, providing an unprecedented decade-long baseline for time-delay cosmography and an opportunity for a high-precision joint estimate of $H_0$.

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arXiv2025

Cosmology with supernova Encore in the strong lensing cluster MACS J0138-2155: Lens model comparison and H0 measurement

MACS J0138-2155 is the only known cluster to strongly lens two supernovae (SNe), Requiem and Encore, from the same host galaxy at z=1.949. We present seven independent mass models of the galaxy cluster built using six software packages. By conducting a blind analysis (no exchanges of results between modeling teams), we quantified uncertainties due to modeling and software. Through HST, JWST and MUSE observations, we assembled high-quality data products, including eight "gold" lensed image systems consisting of 23 images with secure spectroscopic redshifts, and one "silver" system with a likely redshift value. Restricting to the gold images, we obtain overall consistent model predictions of the positions, magnifications and time delays of SN Encore and SN Requiem images, especially for models with $\chi^2 \leq 25$. We predict the appearance of the next images of SNe Encore and Requiem with a time delay of >~3000 days and of ~3700 to 4000 days, respectively, based on a fiducial cosmological model of $H_0 = 70 {\rm\ km\ s^{-1}\ Mpc^{-1}}$ and $\Omega_{\rm m} = 0.3$. We obtain relations between $H_0$ and the time delays of SNe Encore and Requiem. In particular, for $H_0 = 73 {\rm\ km\ s^{-1}\ Mpc^{-1}}$, the four lowest $\chi^2$ models predict SN Requiem to reappear in ~Apr-Dec 2026; for $H_0 = 67 {\rm\ km\ s^{-1}\ Mpc^{-1}}$, in ~Mar-Nov 2027. Using the newly measured time delay between the two detected images of SN Encore by Pierel et al. (2026) and our mass models, we jointly infer $H_0 = {\rm 66.9^{+11.2}_{-8.1}\ km\ s^{-1}\ Mpc^{-1}}$, where the uncertainty is dominated by that of the time delay. The long delays of the next-appearing SN Requiem and SN Encore images provide excellent opportunities to measure $H_0$ with an uncertainty of 2-3%. Our mass models form the basis for cosmological inference from this unique lens cluster with two strongly lensed SNe. (Abridged)

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arXiv2025

A Pristine Star-Forming Complex at z=4.19

We report the discovery of a faint (M_1700 ~ -12.2), oxygen-deficient strongly-lensed ionizing source -- dubbed LAP2 (Lensed And Pristine 2) -- at a spectroscopic redshift of z=4.19. LAP2 appears to be isolated and lies very close to the caustic produced by the lensing galaxy cluster Abell 2744. It was observed with the James Webb Space Telescope (JWST) NIRSpec MSA in prism mode as part of the UNCOVER program. The NIRSpec spectrum reveals prominent Lya emission (7.1 sigma), clear Ha emission (6.2 sigma), tentative Hb emission (2.8 sigma), and no detectable [OIII]4959,5007 (~ 7 times fainter than Ha). The inferred [OIII] 2 sigma upper limit corresponds to an R3 index <0.85 (assuming the Ha/Hb ~ 2.86 case~B recombination ratio), which, under high-ionization conditions, implies a metallicity of Z < 6 X 10^(-3) Z_sun. The combination of faint ultraviolet luminosity, large rest-frame Ha equivalent width (~ 650 A), and extremely compact size (< 10 pc) suggests that LAP2 is being caught in an early, pristine formation phase consistent with an instantaneous-burst scenario, with an estimated stellar mass of at most a few 10^4 Msun. Deep VLT/MUSE observations further reveal copious Lya emission forming an arclet that straddles the critical line. LAP2 thus joins the rare class of extremely metal-poor star-forming complexes that JWST has started to unveil at redshifts 3 - 7, and provides a rare glimpse into a still very poorly explored faint-luminosity regime.

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arXiv2025

Is there a tilt in the fundamental (hyper)plane?

We investigate the fundamental plane (FP) of selected early-type (ETG) member galaxies of the galaxy cluster PLCK G287.0+32.9 ($ z_c = 0.3833 $), exploring also four-dimensional hyperplane extensions. We measure ETGs structural parameters and photometry from Hubble Space Telescope (HST) observations. We use high-quality spectroscopic data from the Multi Unit Spectroscopic Explorer (MUSE) to measure the galaxy central stellar velocity dispersions and stellar population properties. With this data, we construct the FP through a robust fitting procedure and analyze its tilt and scatter. We then introduce two hyperplane extensions, one including the stellar mass ($M^\star$-HP) and another including the stellar over total mass fraction ($f_{\mathrm{e}}^\star$-HP), and compare their coefficients and scatter to those of the FP. The FP of PLCK G287.0+32.9 is found to have best-fit parameter values consistent with those in the literature ($α= 1.2 \pm 0.1$ and $β= -0.75 \pm 0.05$), with a scatter of $0.09$ dex. The ($f_{\mathrm{e}}^\star$-HP) shows no tilt compared to the theoretical plane ($α= 2.1 \pm 0.2$ and $β= -1.12 \pm 0.07$), with a scatter of $0.042$ dex, and the ($M^\star$-HP) reveals an even tighter relation, with a scatter of only $0.023$. Our findings support the idea that the FP is a lower-dimensional projection of a more complex hyperplane and confirm that the variations in the dark matter content contribute significantly to the tilt of the FP. Future studies incorporating larger samples of galaxies and additional physical parameters may further refine our understanding of the FP and its higher-dimensional extensions.

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arXiv2025

Euclid preparation. LXXIV. Euclidised observations of Hubble Frontier Fields and CLASH galaxy clusters

We present HST2EUCLID, a novel Python code to generate Euclid realistic mock images in the $H_{\rm E}$, $J_{\rm E}$, $Y_{\rm E}$, and $I_{\rm E}$ photometric bands based on panchromatic Hubble Space Telescope observations. The software was used to create a simulated database of Euclid images for the 27 galaxy clusters observed during the Cluster Lensing And Supernova survey with Hubble (CLASH) and the Hubble Frontier Fields (HFF) program. Since the mock images were generated from real observations, they incorporate, by construction, all the complexity of the observed galaxy clusters. The simulated Euclid data of the galaxy cluster MACS J0416.1$-$2403 were then used to explore the possibility of developing strong lensing models based on the Euclid data. In this context, complementary photometric or spectroscopic follow-up campaigns are required to measure the redshifts of multiple images and cluster member galaxies. By Euclidising six parallel blank fields obtained during the HFF program, we provide an estimate of the number of galaxies detectable in Euclid images per ${\rm deg}^2$ per magnitude bin (number counts) and the distribution of the galaxy sizes. Finally, we present a preview of the Chandra Deep Field South that will be observed during the Euclid Deep Survey and two examples of galaxy-scale strong lensing systems residing in regions of the sky covered by the Euclid Wide Survey. The methodology developed in this work lends itself to several additional applications, as simulated Euclid fields based on HST (or JWST) imaging with extensive spectroscopic information can be used to validate the feasibility of legacy science cases or to train deep learning techniques in advance, thus preparing for a timely exploitation of the Euclid Survey data.

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