Amata Mercurio

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% accuracy and enables the investigation of cosmological parameters. The number of known systems of this type is bound to increase in the coming 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 100 images of galaxy clusters that we 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; these were derived from state-of-the-art observations for modelling characteristics such as morphology, colour, and spatial distribution of the cluster-member population. This allowed us to reliably reproduce the complexity of real observations. The simulations were 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. We make the images, deflection maps, convergence maps, and catalogues of cluster members and background sources publicly available to the community at https://openscience.inaf.it/?page_id=724 .

Context . The physical properties of galaxies 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 wide range of galaxy properties from the cluster center to the periphery. Observations of large-scale structures, particularly filaments in cluster outskirts ( r ∼ 5 r 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. Aims . The Abell S1063 cluster ( z = 0.346) was 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 investigated galaxy evolution across a wide range of stellar masses and environments. We release a multiwavelength photometric catalog with photometric redshifts for 64 394 sources over a 1×1 deg ² field of view. The analysis of overdensity regions provides insights into future studies on galaxy properties in cluster outskirts. Methods . The dataset was obtained through deep ( r < 24.65 mag) and wide optical ( u, g, r, i , VST) and near-infrared ( Y, J, Ks , 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 measurements for cluster membership. The density field measurement enabled comparison of galaxy properties, colors, and masses across different cluster environments. Results . We detect a very dense structure near the cluster center. Thanks to a large field of view, we also identify another dense region to the northwest, opposite the direction of the cluster elongation. Filaments connecting the regions are also visible.

In this series of papers, we present dynamical models of cluster members in strong lensing galaxy clusters to independently probe the persistent discrepancy reported between strong lensing models and cosmological hydrodynamical simulations, in terms of total mass properties for the cluster subhalos. In this work, we focused our study on early-type galaxies within the clusters Abell 2744 (z=0.309) and MACS J0416.1-2403 (z=0.397). We took advantage of deep spectroscopic data from the Multi Unit Spectroscopic Explorer (MUSE), on the Very Large Telescope, complemented with Frontier Fields photometry. We used a pipeline based on spectral fitting to perform kinematic measurements of the line-of-sight velocity dispersion profiles of 109 cluster members, extending, in the best cases, up to approximately 15; Hubble kpc from the galactic centers. We modeled the cluster members assuming a dual pseudo-isothermal total mass density distribution and a Jaffe stellar mass density distribution. From the models, we inferred the values of the central stellar velocity dispersion, σ_0, and the truncation radius, r_t, for the galaxies in our sample. We found that σ_0 is accurately recovered for all of the cluster members and varies approximately from 40, to 370, , while r_t is reliably measured for a fraction of galaxies in our sample, with sufficiently extended radial kinematic coverage. Our dynamical models predicted line-of-sight velocity dispersion profiles that fit the measured ones better than those inferred from strong lensing models. To tackle the discrepancy from the strong lensing side, we exploited the σ_0 measurements obtained from the dynamical models to calibrate the Faber-Jackson scaling relations for the cluster members in both galaxy clusters. When comparing our relations to those obtained in previous kinematics and strong lensing works, we found systematically higher normalization and compatible slope and scatter values. We conclude that our dynamical measurements of σ_0 and r_t, along with calibrated scaling relations, are more robust than previous kinematic estimates (which are biased due to not taking into account the effects of the point spread function and of averaging inside the aperture) and should therefore be adopted as improved initial prescriptions in future strong lensing models. -1 -1

Context. Understanding the formation and evolution of late-type galaxies requires deep imaging for tracing the faintest stellar components in their outskirts (i.e. accreted material, diffuse halos, and tidal streams). Despite their crucial role in the build-up of stellar mass, these low-surface-brightness features remain largely unexplored due to observational limitations. The VST Survey of Mass Assembly and Structural Hierarchy (VST-SMASH) is designed to fill this gap, providing deep, wide-field optical imaging for a volume-limited sample of nearby spiral galaxies ( D < 11 Mpc), overlapping with the Euclid Wide Survey in the southern hemisphere. Aims. This paper aims to introduce the VST-SMASH survey and showcase its scientific potential through the analysis of IC 5332, a late-type spiral galaxy observed in the g , r , and i bands. The main goal is to demonstrate the depth, quality, and diagnostic power of the dataset in tracing low-surface-brightness features and structural components in galactic outskirts. Methods. We carried out detailed surface photometry of IC 5332, using elliptical isophotal fitting and growth curve analysis to extract radial surface brightness and colour profiles down to μ _r ∼ 30 mag arcsec ⁻² . We performed multi-component Sérsic decompositions in each band and constructed stellar mass surface density profiles. Furthermore, we identified and characterized faint stellar streams visible in the very deep and smoothed images, estimating their colours and comparing them with adjacent galactic regions. Results. While the internal (< 1 effective radius) negative colour gradients can be explained by dissipative (monolithic) collapses and supernovae outflows, the colour profiles at larger radii reveal a significant gradient towards redder g − r and g − i , spatially consistent with the presence of accreted populations in the outskirts. Nevertheless, we also find bluer r − i , which , together with the behaviour of g − r and g − i , could be explained by strong Hα emission. Single-component Sérsic fits fail to reproduce the observed light distribution, especially in the outer regions, requiring a two-component model. In the outskirts, multiple faint stellar streams are detected, with integrated g − r colours consistent with disrupted dwarf and/or low-surface-brightness satellites. However, spectroscopic follow-ups would be needed to detect possible strong emission lines, which could affect the r -band photometry. These findings support a scenario of ongoing stellar mass assembly through accretion and highlight the capability of VST-SMASH to uncover faint structures in nearby galaxies.

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 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 large sample of 723 red cluster members in the four clusters, selected spectroscopically and photometrically, 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 time) integral-field spectroscopic data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, available for the cores of all four clusters, we tested a pipeline based on the public spectral fitting code pPXF to reliably and systematically measure the line-of-sight stellar velocity dispersion σ of cluster members with a spectral S / N ≥ 10, with a statistical uncertainty consistently below 5%. The resulting catalogue contains 213 measured σ values across the four clusters. Combining stellar kinematics and multi-band galaxy morphology, 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 in comparison with large samples of early-type field galaxies, and noted 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 σ [km s ⁻¹ ] = 1.5. In spite of the intrinsic variability between the four clusters resulting from their assembly history, a Schechter function fit of the velocity functions suggests compatible parameters: a positive α slope with values in the range 0.55 − 1.60 and log σ ^* [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- σ regimes. We suggest that deeper insights on the evolution of member galaxies may be obtained with a larger cluster sample spanning a wider redshift range.

Context . The Magellanic Clouds (MCs) offer a unique laboratory for studying galaxy interaction and the evolution of dwarf galaxies. The star formation history (SFH), which traces when and how stars formed, provides powerful constraints for the dynamical modelling of the system’s past interactions and the processes of stripping and triggered star formation in tidally influenced environments. Aims . We aim to reconstruct the SFH of the Magellanic Bridge, the gaseous and stellar stream connecting the Magellanic Clouds. We used data from the deep optical STEP survey, which covers 54 deg ² across the Small Magellanic Cloud (SMC) and the Bridge, reaching stars below the oldest main-sequence turnoff at the distance of the MCs. Methods . We applied the synthetic colour-magnitude diagram technique to 14 deg ² of STEP data. We constructed two libraries of synthetic stellar populations based on the PARSEC-COLIBRI and BaSTI stellar evolutionary models, with metallicities in the range −2.0 ≤ [Fe/H] ≤ 0 across the whole Hubble time. Results . We find a clear peak of recent star formation ∼100 Myr ago in the Magellanic Bridge, which becomes increasingly pronounced towards the SMC. The low metallicity of this population suggests that it formed from gas stripped from the SMC during its most recent close encounter with the LMC. In the eastern part of the Bridge (LMC side), star formation peaks at earlier times, around 10 Gyr and 2 Gyr ago. We estimate a total stellar mass in the Bridge of (5.1 ± 0.2) × 10 ⁵ M _⊙ and a present-day stellar metallicity of [Fe/H]~-0.6 dex, close to SMC value.

We measured the velocity anisotropy profile, β ( r ), of different galaxy cluster member populations by analysing the stacked projected phase space of nine massive ( M _200c > 7 × 10 ¹⁴ M _⊙ ) galaxy clusters at intermediate redshifts (0.18 < z < 0.45). We selected our sample of galaxy clusters by choosing the most round and virialised objects among the targets of the CLASH-VLT spectroscopic programme, which offers a large spectral database. Complementary MUSE observations of most of these clusters allowed us to identify an unprecedented number of cluster members, strongly enhancing the precision of our measurement with respect to previous studies. Our sample of cluster members is divided into four classes: the first two are based on colour (red and blue galaxies), and the other two on stellar mass (high and low). To study the velocity anisotropy profile of each cluster-member population, we employed two parallel techniques, namely the MAMPOSSt method (parametric in β ( r )) and the inversion of the Jeans equation (non-parametric in β ( r )). The results from both techniques are found to be in agreement for any given cluster member population, and suggest that the orbital anisotropy in galaxy clusters grows from the centre (where β ≈ 0.2 − 0.4) to the virial radius ( β ≳ 0.8), and it is similar for the different cluster-member populations. We also find an interesting dynamical feature emerging from the Jeans inversion results, that is, a sudden drop in β ( r ) at a distance of ∼250 kpc from the cluster centre. We provide robust anisotropy estimates from our exploration of a highly significant number of model combinations: 72 with MAMPOSSt (varying the mass, surface number density, β ( r ) model, and galaxy population) and 18 (varying total mass model and galaxy population) in the Jeans inversion. Such an extensive investigation of the velocity anisotropy profile in galaxy clusters is a wide basis for future studies of cluster dynamical masses and cluster cosmology in the era of large spectroscopic surveys.

We provide a new measurement of the expansion history of the Universe at z = 0.54 based on the cosmic chronometer (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 enabled H ₀ measurements via time delay cosmography. 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 derived their age and physical parameters via full spectrum fitting. We used the code Bagpipes , which was specifically modified to remove the cosmological prior on ages. On average, the CC sample shows super-solar metallicities ( Z / Z _⊙ = 1.3 ± 0.7) and a low dust extinction ( A _V = 0.3 ± 0.3 mag) and seems to have formed in short bursts ( τ = 0.6 ± 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 ⁺⁸¹ ₋₂₉ (stat) ± 13 (syst) km/s/Mpc. We also simulated 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 ∼100 CCs and a slightly broader redshift range (d z ∼ 0.2).

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 z _SN = 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.