Aims. We investigate the formation and evolution of stellar halos (SHs) around bright central galaxies (BCGs), focusing on their scaling relations, colors, and metallicities across cosmic time, and we compare model predictions with ultra-deep imaging data. Methods. We used the semianalytic model FEGA25, applied to merger trees from high-resolution dark matter simulations, including an updated treatment of intracluster light (ICL) formation. SHs are defined as the stellar component within a physically motivated transition radius, linked to the structural properties of the host halo. Predictions are compared with observations from the VST Early-type GAlaxy Survey (VEGAS) and Fornax Deep Survey (FDS). Results. The SH mass correlates well with the BCG and ICL masses, with tighter scatter in the SH–ICL relation. The transition radius peaks at 30−40 kpc nearly independent of redshift in the model predictions, but can reach ∼400 kpc in the most massive halos, after z = 0.5. SHs and ICL show nearly identical color distributions at all epochs, both reddening toward z = 0. At z = 2, SHs and the ICL are ∼0.4 dex more metal–poor than BCGs, but the gap shrinks to ∼0.1 dex by the present time. Observed colors are consistent with model predictions, while observed metallicities are lower, suggesting a larger contribution from disrupted dwarfs. Conclusions. SHs emerge as transition regions between BCGs and the ICL, dynamically and chemically coupled to both. Their properties depend on halo concentration, ICL formation efficiency, and the progenitor mass spectrum. Upcoming wide–field photometric and spectroscopic surveys (e.g. LSST, WEAVE, 4MOST) will provide crucial tests by mapping structure, metallicity, and kinematics in large galaxy samples.

Context. This paper presents new results from the ESO Large Programme Looking into the faintEst WIth MUSE (LEWIS). The LEWIS sample consists of low-surface-brightness galaxies (LSBs) and ultra-diffuse galaxies (UDGs) located inside 0.4Rvir of the Hydra I cluster. Integral-field spectroscopy is acquired for 24 galaxies with the MUSE spectrograph mounted on the Very Large Telescope (VLT). Aims. Our main objectives are to analyse possible correlations between the environment and the integrated stellar population properties of our targets, based on which we infer clues about their formation. Methods. For each galaxy in the sample, we extracted the 1D stacked spectrum in an aperture of one effective radius Re and adopted previously published stellar kinematics to derive the age, metallicity, and [Mg/Fe] through a full spectral fitting technique. Results. We find that the analysed LEWIS sample has a mean metallicity of ⟨[M/H]⟩= − 0.9 ± 0.2 dex and a mean age of 10 ± 2 Gyr, comparable to previous results of UDGs in other clusters. According to their position in the projected phase space, galaxies can be classified into two groups: very early infaller galaxies, which on average have slightly higher metallicities (⟨[M/H]⟩early = −0.8 ± 0.1 dex), and late infaller galaxies, with slightly lower values (⟨[M/H]⟩late = −1.0 ± 0.1 dex). According to their properties, late-infallers tend to be rotation-supported systems. Conversely, two types of galaxies are found in the early-infall region. Roughly half have metallicities consistent with the dwarf galaxy mass–metallicity relation. The other half show higher metallicities (with ⟨[M/H]⟩≥ − 1.0 dex) and are located outside the 1σ scatter of the mass-metallicity relation. The two subgroups of early-infallers also display different timescales for stellar mass assembly. Metal-rich galaxies reached 50% of their stellar mass in less than 1 Gyr and show a prolonged and almost constant star formation over more than 12 Gyr. The other galaxies exhibit a star formation history similar to that found for galaxies in the late-infall region. Both early and late infallers show solar-like α abundances. Conclusions. From the analysis of stellar population properties presented in this work and of stellar kinematics previously obtained from LEWIS, we identified different classes of UDGs within the Hydra I cluster – as shown by metallicities, quenching timescales, and kinematics – which suggest different formation mechanisms. Almost all of the UDGs and LSBs in this cluster are consistent with the puffed-up dwarf formation scenario, having dwarf-like metallicities and being consistent with the mass-metallicity relation for dwarfs. In the innermost regions of the cluster, where more metal-rich UDGs are found, tidal effects or the environment might have influenced their formation and evolution.

Context. This paper presents a detailed analysis of a gas-rich star-forming ultra-diffuse galaxy (UDG) as part of the ESO Large Programme Looking into the faintEst WIth MUSE (LEWIS). Among the UDGs in the LEWIS sample, UDG 6 is the only galaxy that hosts a significant amount of ionised gas with evidence of emission lines, suggesting recent star-forming activity. Aims. The main goal of this work is to constrain the formation history of this UDG by comparing its properties with the main formation scenarios proposed for this extreme class of galaxies. Methods. We adopted integral field spectroscopy from MUSE to derive the morphology and the structural properties of the stellar and gas components of UDG 6. We applied spectral fitting and Voronoi tessellation algorithms to the MUSE data-cube to derive the kinematics and properties of the gas and stellar component, explore gas dynamics, and characterise emission lines. Moreover, we derived the properties of the globular cluster (GC) populations by applying a multi-band spectrophotometric analysis. Results. We confirmed that UDG 6 is a member of the Hydra I cluster and that it has a systemic velocity of V-sys,V- * similar to 3580 km s(-1). It is characterised by a regular and elongated shape and contains a significant dust content (A(V) = 1.2 +/- 0.5 mag), a metal-poor (12 + log(10)(O/H) = 7.7 +/- 0.2 dex) ionised gas fraction (f(gas) = 0.24 +/- 0.08), and an underlying old-to-intermediate (greater than or similar to 3 Gyr) stellar component. Evidence of local and clumpy star-forming activity has been revealed through the analysis of emission line ratios, and an arc-like tidal feature was discovered from unsharp masking analysis. The number counts of GCs in UDG 6 is N-GC = 0.2 +/- 5.4. Conclusions. Similar to the other UDGs in LEWIS, UDG 6 might originate from a "puffed-up dwarf" whose stellar content has been stretched out to larger radii, passively evolving into a more diffuse galaxy. Since UDG 6 is located in a dynamically active region of the cluster characterised by tidal features and stripping phenomena, we suggest that the environmental processes have played a role in shaping its properties. A tidal interaction with a nearby galaxy might have triggered recent star-formation activity without dramatically altering the coherent gas rotation in UDG 6.

The ultraluminous infrared galaxy (ULIRG) IRAS 00183-7111(z = 0.328) is one of the three ULIRGs that are currently known to host an active galactic nucleus (AGN) with small-scale radio jets. We present a detailed study of the link between galaxy merger , AGN ignition, radio jet expansion and kpc-scale molecular outflow in IRAS 00183-7111, using high-resolution Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of the (CO)-C-12(1-0) and (CO)-C-12(3-2) lines and very deep i-band VLT Survey Telescope (VST) imaging. The latter allows us to put constraints on the assembly history of the system, suggesting that it formed through a major merger between two gas-rich spirals, likely characterized by a prograde encounter and no older than approximate to 2Gyr. The recent merger channelled about (1.5 +/- 0.3) x 10(10) M-circle dot of molecular gas in the central regions of the remnant, as traced by the CO detections. The spatial correlation between the CO distribution and the radio core suggests that this gas likely contributed to the ignition of the AGN and thus to the launch of the radio jets. Furthermore, by comparing the relative strength of the two CO transitions, we find extreme gas excitation (i.e. T-ex >> 50 K) around the radio lobes, supporting the case for a jet-ISM interaction. A qualitative study of the CO kinematics also demonstrates that, despite the overall disturbed dynamical state with no clear signs of regular rotation, at least one non-rotational kinematic component can be identified and likely associated to an outflow with v(out) approximate to 439 km s(-1) and M-out(center dot) approximate to 609 M-circle dot yr(-1) .

Context. This paper presents new results from the ESO Large Programme Looking into the faintEst WIth MUSE (LEWIS). The LEWIS sample consists of low-surface-brightness galaxies (LSBs) and ultra-diffuse galaxies (UDGs) located inside 0.4Rvir of the Hydra I cluster. Integral-field spectroscopy is acquired for 24 galaxies with the MUSE spectrograph mounted on the Very Large Telescope (VLT). Aims. Our main objectives are to analyse possible correlations between the environment and the integrated stellar population properties of our targets, based on which we infer clues about their formation. Methods. For each galaxy in the sample, we extracted the 1D stacked spectrum in an aperture of one effective radius Re and adopted previously published stellar kinematics to derive the age, metallicity, and [Mg/Fe] through a full spectral fitting technique. Results. We find that the analysed LEWIS sample has a mean metallicity of <[M/H]>= - 0.9 +/- 0.2 dex and a mean age of 10 +/- 2 Gyr, comparable to previous results of UDGs in other clusters. According to their position in the projected phase space, galaxies can be classified into two groups: very early infaller galaxies, which on average have slightly higher metallicities (<[M/H]>(early) = -0.8 +/- 0.1 dex), and late infaller galaxies, with slightly lower values (<[M/H]>(late) = -1.0 +/- 0.1 dex). According to their properties, late-infallers tend to be rotation-supported systems. Conversely, two types of galaxies are found in the early-infall region. Roughly half have metallicities consistent with the dwarf galaxy mass-metallicity relation. The other half show higher metallicities (with <[M/H]>>= - 1.0 dex) and are located outside the 1 sigma scatter of the mass-metallicity relation. The two subgroups of early-infallers also display different timescales for stellar mass assembly. Metal-rich galaxies reached 50% of their stellar mass in less than 1 Gyr and show a prolonged and almost constant star formation over more than 12 Gyr. The other galaxies exhibit a star formation history similar to that found for galaxies in the late-infall region. Both early and late infallers show solar-like alpha abundances.Conclusions. From the analysis of stellar population properties presented in this work and of stellar kinematics previously obtained from LEWIS, we identified different classes of UDGs within the Hydra I cluster - as shown by metallicities, quenching timescales, and kinematics - which suggest different formation mechanisms. Almost all of the UDGs and LSBs in this cluster are consistent with the puffed-up dwarf formation scenario, having dwarf-like metallicities and being consistent with the mass-metallicity relation for dwarfs. In the innermost regions of the cluster, where more metal-rich UDGs are found, tidal effects or the environment might have influenced their formation and evolution.

In galaxy clusters, the hierarchical model of galaxy assembly predicts the formation of stellar substructures and intracluster light (ICL), a diffuse component consisting of stars that are not gravitationally bound to any single galaxy but instead follow the global gravitational potential of the cluster. These features encode the details of the cluster's assembly history. However, observations are challenging due to their faint surface brightness, so independent tracers such as intracluster planetary nebulae and globular clusters (GCs) can provide valuable insight. For this work, we used deep VLT/FORS V- and I-band imaging to study the GC population in the Hydra I galaxy cluster, a rich environment of galaxies that is located at a distance of 45.7 Mpc. Our photometric sample of GC candidates was constructed from the VI colour-magnitude diagram, where point sources with a similar colour as confirmed GCs were selected. Dividing our GC sample in two colour groups, we show a striking difference between the two populations: while red GCs tend to be clustered around Hydra's massive galaxies (mainly NGC 3311 and NGC 3309), blue GCs are more extended and spatially coincide with the peak of the cluster's X-ray emitting gas. The GCs around the central galaxies also have different spatial distributions according to their stellar population properties. Young metal-rich GCs are more extended and may be associated with ram-pressure tails, whereas old metal-poor GCs are more concentrated and could be related to disrupted dwarfs. The red, old, and metal-rich GCs are likely associated with the central massive galaxies. Comparing the GC number density profiles to the surface brightness profile of NGC 3311, we find that the red GCs closely follow the galaxy's light, while the blue population significantly deviates from it and traces the global gravitational potential of the cluster. This result is also evidenced by the specific frequency of blue GCs, which is similar to 5 times larger in the ICL-dominated outskirts when compared to the inner parts of the cluster and to the red population. Finally, we introduce a novel method to constrain the evolution of the galaxy luminosity function of the cluster from GC specific frequencies and colour distributions. This method results in a past Schechter slope of alpha = -1.81(-0.16)(+0.16) for the faint end compared to alpha = -1.41(-0.05)(+0.08) in the present day, which is consistent with measurements at high redshift and with cosmological simulations.

In galaxy clusters, hierarchical assembly predicts the formation of stellar substructures and intracluster light (ICL), a diffuse stellar component tracing the global cluster potential. Because these features are extremely faint, alternative tracers such as globular clusters (GCs) provide a powerful tool to study cluster assembly. We use deep VLT/FORS V- and I-band imaging to investigate the GC population in the nearby Hydra I galaxy cluster (∼ 45.7 Mpc). GC candidates were selected from the VI colour-magnitude diagram and divided into blue and red subpopulations. We find a clear spatial dichotomy: red GCs are concentrated around the massive central galaxies NGC 3311 and NGC 3309, while blue GCs are more extended and offset from the centre, coinciding with a secondary peak of X-ray-emitting gas. In the central regions, GC spatial distributions further depend on stellar population properties: young metal-rich GCs are more extended and may be linked to ram-pressure stripping, whereas old metal-poor GCs are more centrally concentrated, possibly originating from disrupted dwarf galaxies. Comparing the GC number density profiles to the surface brightness profile of NGC 3311, we find that the red GCs closely follow the galaxy light, while the blue population significantly deviates from it and traces the global gravitational potential of the cluster. This is also reflected in the specific frequency of blue GCs, which is approximately ∼ 5× higher in the ICL-dominated outskirts than in the inner regions dominated by red GCs. Finally, we present a novel method to constrain the evolution of the galaxy luminosity function of the cluster using GC specific frequencies and colour distributions, yielding a past faint-end slope of α=-1.81^+0.16_-0.16 compared to α=-1.41^+0.08_-0.05 today, consistent with high-redshift observations and cosmological simulations.

Ultra-Diffuse Galaxies (UDGs) challenge current galaxy formation models within the ΛCDM framework, particularly regarding their dark matter content and formation histories. Globular Clusters (GCs) serve as excellent tracers for probing the halos of these faint systems; however, current spectroscopic facilities struggle to reach the depth required to fully characterize GC populations beyond the local neighborhood. In this work, we present the scientific potential of NEXUS@SHARP, a near-infrared multi-slit spectrograph proposed on the upcoming Extremely Large Telescope (ELT). We demonstrate that NEXUS is uniquely positioned to overcome current observational limitations by combining high spatial resolution with efficient multiplexing. We show that NEXUS can efficiently help to characterize the GC population in UDGs across diverse environments – out to 60 to 70 Mpc – with a few hours of ELT time. These capabilities will allow for precise measurements of radial velocities and stellar populations, enabling robust dynamical mass estimates and distinguishing between competing UDG formation scenario.

Aims. We investigate the formation and evolution of stellar halos (SHs) around bright central galaxies (BCGs), focusing on their scaling relations, colors, and metallicities across cosmic time, and we compare model predictions with ultra-deep imaging data. Methods. We used the semianalytic model FEGA25, applied to merger trees from high-resolution dark matter simulations, including an updated treatment of intracluster light (ICL) formation. SHs are defined as the stellar component within a physically motivated transition radius, linked to the structural properties of the host halo. Predictions are compared with observations from the VST Early-type GAlaxy Survey (VEGAS) and Fornax Deep Survey (FDS). Results. The SH mass correlates well with the BCG and ICL masses, with tighter scatter in the SH-ICL relation. The transition radius peaks at 30-40 kpc nearly independent of redshift in the model predictions, but can reach similar to 400 kpc in the most massive halos, after z = 0.5. SHs and ICL show nearly identical color distributions at all epochs, both reddening toward z = 0. At z = 2, SHs and the ICL are similar to 0.4 dex more metal-poor than BCGs, but the gap shrinks to similar to 0.1 dex by the present time. Observed colors are consistent with model predictions, while observed metallicities are lower, suggesting a larger contribution from disrupted dwarfs. Conclusions. SHs emerge as transition regions between BCGs and the ICL, dynamically and chemically coupled to both. Their properties depend on halo concentration, ICL formation efficiency, and the progenitor mass spectrum. Upcoming wide-field photometric and spectroscopic surveys (e.g. LSST, WEAVE, 4MOST) will provide crucial tests by mapping structure, metallicity, and kinematics in large galaxy samples.

Context. As some of the oldest stellar systems in the Universe, globular clusters (GCs) are key fossil tracers of galaxy formation and interaction histories. This paper is part of the LEWIS project, which provides the first homogeneous MUSE integral-field spectroscopic survey of a complete sample of ultra-diffuse galaxies (UDGs) in the Hydra I cluster. Aims. We use MUSE spectroscopy and new VIRCAM H-band imaging data to study the GC populations and dark matter content in four dwarf galaxies from the LEWIS sample, which were found to host several GC candidates based on previous photometric studies. Methods. We retrieved line-of-sight velocities (LOSVs) for all the sources in the observed MUSE fields and classified them based on their spectral features and LOSVs. Because the spectroscopic measurements are limited to relatively bright sources (m(H) less than or similar to 23.5 AB mag), we developed a multi-band photometric procedure to identify additional GC candidates that are too faint for spectroscopic confirmation. GC candidates were selected based on a combination of photometric properties (colors, magnitudes) and morphometric criteria (shape and size). The same selection criteria were applied to empty fields to estimate a statistical background correction for the number of identified GC candidates. Additionally, H-band observations were used to constrain the stellar masses of the studied galaxies. Results. Based on the spectroscopic classification, we confirm one GC in UDG 3, two in UDG 7, and four in UDG 11, while UDG 9 has no spectroscopically confirmed bright GCs. We identify four intracluster GCs in the vicinity of UDG 3 and UDG 11, and one ultra-compact dwarf (UCD) with a radial velocity only -85 +/- 10 km/s different from that of UDG 7 and thus possibly bound to it. Considering the completeness correction and accounting for possible contamination by unresolved background galaxies, from the photometry we estimate that the number of GCs ranges between 0 and similar to 40 for the investigated UDGs. Their specific frequencies suggest that three out of four UDGs are either GC rich, similar to those in the Coma cluster, or belong to an intermediate population, as seen in the Perseus cluster. Dark matter content estimates, inferred from GC counts and stellar mass, indicate that these galaxies are dark matter dominated, with dynamical-to-stellar mass ratios ranging from similar to 10 - 1000.

Context. This paper presents a detailed analysis of a gas-rich star-forming ultra-diffuse galaxy (UDG) as part of the ESO Large Programme Looking into the faintEst WIth MUSE (LEWIS). Among the UDGs in the LEWIS sample, UDG 6 is the only galaxy that hosts a significant amount of ionised gas with evidence of emission lines, suggesting recent star-forming activity. Aims. The main goal of this work is to constrain the formation history of this UDG by comparing its properties with the main formation scenarios proposed for this extreme class of galaxies. Methods. We adopted integral field spectroscopy from MUSE to derive the morphology and the structural properties of the stellar and gas components of UDG 6. We applied spectral fitting and Voronoi tessellation algorithms to the MUSE data-cube to derive the kinematics and properties of the gas and stellar component, explore gas dynamics, and characterise emission lines. Moreover, we derived the properties of the globular cluster (GC) populations by applying a multi-band spectrophotometric analysis. Results. We confirmed that UDG 6 is a member of the Hydra I cluster and that it has a systemic velocity of V sys, * ∼ 3580 km s −1 . It is characterised by a regular and elongated shape and contains a significant dust content ( A V = 1.2 ± 0.5 mag), a metal-poor (12 + log 10 (O/H) = 7.7 ± 0.2 dex) ionised gas fraction ( f gas = 0.24 ± 0.08), and an underlying old-to-intermediate (≳3 Gyr) stellar component. Evidence of local and clumpy star-forming activity has been revealed through the analysis of emission line ratios, and an arc-like tidal feature was discovered from unsharp masking analysis. The number counts of GCs in UDG 6 is N GC = 0.2 ± 5.4. Conclusions. Similar to the other UDGs in LEWIS, UDG 6 might originate from a “puffed-up dwarf” whose stellar content has been stretched out to larger radii, passively evolving into a more diffuse galaxy. Since UDG 6 is located in a dynamically active region of the cluster characterised by tidal features and stripping phenomena, we suggest that the environmental processes have played a role in shaping its properties. A tidal interaction with a nearby galaxy might have triggered recent star-formation activity without dramatically altering the coherent gas rotation in UDG 6.

Context. As some of the oldest stellar systems in the Universe, globular clusters (GCs) are key fossil tracers of galaxy formation and interaction histories. This paper is part of the LEWIS project, which provides the first homogeneous MUSE integral-field spectroscopic survey of a complete sample of ultra-diffuse galaxies (UDGs) in the Hydra I cluster. Aims. We use MUSE spectroscopy and new VIRCAM H-band imaging data to study the GC populations and dark matter content in four dwarf galaxies from the LEWIS sample, which were found to host several GC candidates based on previous photometric studies. Methods. We retrieved line-of-sight velocities (LOSVs) for all the sources in the observed MUSE fields and classified them based on their spectral features and LOSVs. Because the spectroscopic measurements are limited to relatively bright sources (mH ≲ 23.5 AB mag), we developed a multi-band photometric procedure to identify additional GC candidates that are too faint for spectroscopic confirmation. GC candidates were selected based on a combination of photometric properties (colors, magnitudes) and morphometric criteria (shape and size). The same selection criteria were applied to empty fields to estimate a statistical background correction for the number of identified GC candidates. Additionally, H-band observations were used to constrain the stellar masses of the studied galaxies. Results. Based on the spectroscopic classification, we confirm one GC in UDG 3, two in UDG 7, and four in UDG 11, while UDG 9 has no spectroscopically confirmed bright GCs. We identify four intracluster GCs in the vicinity of UDG 3 and UDG 11, and one ultra-compact dwarf (UCD) with a radial velocity only −85 ± 10 km/s different from that of UDG 7 and thus possibly bound to it. Considering the completeness correction and accounting for possible contamination by unresolved background galaxies, from the photometry we estimate that the number of GCs ranges between 0 and ∼40 for the investigated UDGs. Their specific frequencies suggest that three out of four UDGs are either GC rich, similar to those in the Coma cluster, or belong to an intermediate population, as seen in the Perseus cluster. Dark matter content estimates, inferred from GC counts and stellar mass, indicate that these galaxies are dark matter dominated, with dynamical-to-stellar mass ratios ranging from ∼10 − 1000.

As old stellar systems, globular clusters (GCs) are key fossil tracers of galaxy formation and interaction histories. This paper is part of the LEWIS project, an integral-field spectroscopic survey of ultra-diffuse galaxies (UDGs) in the Hydra I cluster. We use MUSE spectroscopy and new VIRCAM H-band imaging data to study the GC populations and dark matter content in four dwarf galaxies. We retrieved line-of-sight velocities for all sources in the observed MUSE fields. Since the spectroscopic measurements are limited to relatively bright sources, we developed a multi-band photometric procedure to identify additional GC candidates too faint for spectroscopic confirmation. GC candidates were selected using a combination of photometric properties and morphometric criteria. Additionally, the H-band observations were used to constrain the stellar masses of the studied galaxies. Based on the spectroscopic classification, we confirm one GC in UDG3, two in UDG7, and four in UDG11, while UDG9 has no spectroscopically confirmed bright GCs. We identify four intra-cluster GCs in the vicinity of UDG3 and UDG11, and one ultra-compact dwarf with a radial velocity only Δv = -85 ± 10km s^-1 relative to UDG7, suggesting it may be bound to it. Considering completeness corrections and accounting for possible contamination, from photometry we estimate that the number of GCs ranges between 0 and ∼40 for the investigated UDGs. Their specific frequencies suggest that three out of four UDGs are either GC-rich, similar to those in the Coma cluster, or belong to an intermediate population as seen in the Perseus cluster. Dark matter content estimates, inferred from GC counts and stellar mass, indicate that these galaxies are dark-matter dominated, with dynamical-to-stellar mass ratios of M_dyn / M_⋆∼ 10-1000.

Context. This paper focuses on a class of galaxies characterised by an extremely low surface brightness: ultra-diffuse galaxies (UDGs). We used new integral-field (IF) spectroscopic data, obtained with the ESO Large Programme Looking into the faintEst WIth MUSE (LEWIS). It provides the first homogeneous IF spectroscopic survey performed by MUSE at the Very Large Telescope of a complete sample of UDGs and low-surface-brightness galaxies within a virial radius of 0.4 in the Hydra I cluster, according to the UDG abundance-halo mass relation. Aims. Our main goals are addressing the possible formation channels for this class of objects and investigating possible correlations of their observational properties, including the stacked (1D) and spatially resolved (2D) stellar kinematics. In particular, we derive the stellar velocity dispersion from the stacked spectrum integrated within the effective radius (σeff) and measure the velocity map of the galaxies in LEWIS. These quantities are used to estimate their dynamical mass (Mdyn). Methods. We extracted the 1D stacked spectrum inside the effective radius (Reff), which guarantees a high signal-to-noise ratio, to obtain an unbiased measure of σeff. To derive the spatially resolved stellar kinematics, we first applied the Voronoi tessellation algorithm to bin the spaxels in the datacube, and then we derived the stellar kinematics in each bin, following the same prescription as adopted for the 1D case. We extracted the velocity profiles along the galaxy major and minor axes and measured the semi-amplitude (ΔV) of the velocity curve. Results. We found that 7 out of 18 UDGs in LEWIS show a mild rotation (ΔV ∼ 25 − 40 km s−1), 5 lack evidence of any rotation, and the remaining 6 UDGs are unconstrained cases. This is the first large census of velocity profiles for UDGs. The UDGs in LEWIS are characterised by low values of σeff (≤30 km s−1) on average, which is comparable with available values from the literature. Two objects show higher values of σeff (∼30 − 40 km s−1). These higher values might reasonably be due to the fast rotation observed in these galaxies, which affects the values of σeff. In the Faber-Jackson relation plane, we found a group of UDGs consistent with the relation within the error bars. Outliers of the Faber-Jackson relation are objects with a non-negligible rotation component. The UDGs and LSB galaxies in the LEWIS sample have a larger dark matter (DM) content on average than dwarf galaxies (Mdyn/LV, eff ∼ 10 − 100 M⊙/L⊙) with a similar total luminosity. We do not find clear correlations between the derived structural properties and the local environment. Conclusions. By mapping the stellar kinematics for a homogenous sample of UDGs in a cluster environment, we found a significant rotation for many galaxies. Therefore, two classes of UDGs are found in the Hydra I cluster based on the stellar kinematics: rotating and non-rotating systems. This result, combined with the DM content and the upcoming analysis of the star formation history and globular cluster population, can help us to distinguish between the several formation scenarios proposed for UDGs.

Context. This paper focuses on a class of galaxies characterised by an extremely low surface brightness: ultra-diffuse galaxies (UDGs). We used new integral-field (IF) spectroscopic data, obtained with the ESO Large Programme Looking into the faintEst WIth MUSE (LEWIS). It provides the first homogeneous IF spectroscopic survey performed by MUSE at the Very Large Telescope of a complete sample of UDGs and low-surface-brightness galaxies within a virial radius of 0.4 in the Hydra I cluster, according to the UDG abundance-halo mass relation. Aims. Our main goals are addressing the possible formation channels for this class of objects and investigating possible correlations of their observational properties, including the stacked (1D) and spatially resolved (2D) stellar kinematics. In particular, we derive the stellar velocity dispersion from the stacked spectrum integrated within the effective radius (sigma(eff)) and measure the velocity map of the galaxies in LEWIS. These quantities are used to estimate their dynamical mass (M-dyn). Methods. We extracted the 1D stacked spectrum inside the effective radius (R-eff), which guarantees a high signal-to-noise ratio, to obtain an unbiased measure of sigma(eff). To derive the spatially resolved stellar kinematics, we first applied the Voronoi tessellation algorithm to bin the spaxels in the datacube, and then we derived the stellar kinematics in each bin, following the same prescription as adopted for the 1D case. We extracted the velocity profiles along the galaxy major and minor axes and measured the semi-amplitude (Delta V) of the velocity curve. Results. We found that 7 out of 18 UDGs in LEWIS show a mild rotation (Delta V similar to 25 - 40 km s(-1)), 5 lack evidence of any rotation, and the remaining 6 UDGs are unconstrained cases. This is the first large census of velocity profiles for UDGs. The UDGs in LEWIS are characterised by low values of sigma(eff) (<= 30 km s(-1)) on average, which is comparable with available values from the literature. Two objects show higher values of sigma eff (similar to 30 - 40 km s(-1)). These higher values might reasonably be due to the fast rotation observed in these galaxies, which affects the values of sigma eff. In the Faber-Jackson relation plane, we found a group of UDGs consistent with the relation within the error bars. Outliers of the Faber-Jackson relation are objects with a non-negligible rotation component. The UDGs and LSB galaxies in the LEWIS sample have a larger dark matter (DM) content on average than dwarf galaxies (M-dyn/L-V,L- eff similar to 10 - 100 M-circle dot/L-circle dot) with a similar total luminosity. We do not find clear correlations between the derived structural properties and the local environment. Conclusions. By mapping the stellar kinematics for a homogenous sample of UDGs in a cluster environment, we found a significant rotation for many galaxies. Therefore, two classes of UDGs are found in the Hydra I cluster based on the stellar kinematics: rotating and non-rotating systems. This result, combined with the DM content and the upcoming analysis of the star formation history and globular cluster population, can help us to distinguish between the several formation scenarios proposed for UDGs.

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015-2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14,000 deg^2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

The WHT Enhanced Area Velocity Explorer (WEAVE) is a new, massively multiplexing spectrograph. This new instrument will be exploited to obtain high S/N spectra of $\sim$25000 galaxies at intermediate redshifts for the WEAVE Stellar Population Survey (WEAVE-StePS). We test machine learning methods for retrieving the key physical parameters of galaxies from WEAVE-StePS-like spectra using both photometric and spectroscopic information at various S/Ns and redshifts. We simulated $\sim$105000 galaxy spectra assuming SFH with an exponentially declining star formation rate, covering a wide range of ages, stellar metallicities, sSFRs, and dust extinctions. We then evaluated the ability of the random forest and KNN algorithms to correctly predict such parameters assuming no measurement errors. We checked how much the predictive ability deteriorates for different S/Ns and redshifts, finding that both algorithms still accurately estimate the ages and metallicities with low bias. The dispersion varies from 0.08-0.16 dex for ages and 0.11-0.25 dex for metallicity, depending on the redshift and S/N. For dust attenuation, we find a similarly low bias and dispersion. For the sSFR, we find a very good constraining power for star-forming galaxies, log sSFR$\gtrsim$ -11, where the bias is $\sim$ 0.01 dex and the dispersion is $\sim$ 0.10 dex. For more quiescent galaxies, with log sSFR$\lesssim$ -11, we find a higher bias, 0.61-0.86 dex, and a higher dispersion, $\sim$ 0.4 dex, for different S/Ns and redshifts. Generally, we find that the RF outperforms the KNN. Finally, the retrieved sSFR was used to successfully classify galaxies as part of the blue cloud, green valley, or red sequence. We demonstrate that machine learning algorithms can accurately estimate the physical parameters of simulated galaxies even at relatively low S/N=10 per angstrom spectra with available ancillary photometric information.

Context. Globular clusters (GCs) are among the oldest stellar systems in the universe. As such, GC populations are valuable fossil tracers of galaxy formation and interaction history. This paper is part of the VEGAS-SSS series, which focuses on studying the properties of small stellar systems (SSSs) in and around bright galaxies. Aims. We used the multiband wide-field images obtained with the VST to study the properties of the GC population in an interacting pair of galaxies. Methods. We derived ugri photometry over 1.5 x 1.5 sq. degrees centered on the galaxy group composed of two elliptical galaxies: NGC 3640 and its fainter companion NGC 3641. We studied the GC system properties from both the ugri and gri matched catalogs. GC candidates were identified based on a combination of photometric properties (colors and magnitudes) and morphometric criteria (concentration index, elongation, FWHM, etc.), using sources with well-defined classifications from spectroscopic or imaging data available in the literature and numerical simulations as references. The selection criteria were also applied to empty fields to determine a statistical background correction for the number of identified GC candidates. Results. The 2D density maps of GCs appear to align with the diffuse light patches resulting from the merging events of the galaxies. The highest density peak of GCs is observed to be on NGC 3641 rather than NGC 3640, despite the latter being the more massive galaxy. The azimuthal averaged radial density profiles in both galaxies reveal that the GC population extends beyond the galaxy light profile and this indicates the likely presence of an intra-group GC component. A color bimodality in (u - r) and (g - i) is observed for NGC 3641, whereas NGC 3640 shows a broad unimodal distribution. Analysis of the GC luminosity function indicates that both galaxies are roughly located at the same distance (similar to 27 Mpc). We provide an estimate of the total number of GCs, and determine the specific frequency for NGC 3640, S-N = 2.0 +/- 0.6, which aligns with expectations, while for NGC 3641 we find a large S-N = 4.5 +/- 1.6.

We studied the effects of cluster environments on galactic structures by using the TNG50 cosmological simulation and observed galaxies in the Fornax cluster. We focused on galaxies with stellar masses of 10 8 − 12 M ⊙ at z = 0 that reside in Fornax-like clusters with total masses of M 200c = 10 13.4 − 14.3 M ⊙ . We characterized the stellar structures by decomposing each galaxy into a dynamically cold disk and a hot non-disk component, and studied the evolution of both the stellar and gaseous constituents. In TNG50, we find that the cold (i.e., star-forming) gas is quickly removed when a galaxy falls into a Fornax-mass cluster. About 42%, 73%, and 87% of the galaxies have lost 80% of their star-forming gas at 1, 2, and 4 billion years after infall, respectively, with the remaining gas concentrating in the inner regions of the galaxy. The radius of the star-forming gaseous disk decreases to half its original size at 1, 2, and 4 billion years after infall for 7%, 27%, and 66% of the galaxies, respectively. As a result, star formation (SF) in the extended dynamically cold disk sharply decreases, even though a low level of SF persists at the center for a few additional gigayears. This leads to a tight correlation between the average stellar age in the dynamically cold disk and the infall time of galaxies. Furthermore, the luminosity fraction of the dynamically cold disk in ancient infallers (i.e., with an infall time ≳8 Gyr ago) is only about one-third of that in recent infallers (infall time ≲4 Gyr ago), controlling for galaxy stellar mass. This quantitatively agrees with what is observed in early-type galaxies in the Fornax cluster. Gas removal stops the possible growth of the disk, with gas removed earlier in galaxies that fell in earlier, and hence the cold-disk fraction is correlated with the infall time. The stellar disk can be significantly disrupted by tidal forces after infall, through a long-term process that enhances the difference among cluster galaxies with different infall times.

Context. Due to their relatively low stellar mass content and diffuse nature, the evolution of dwarf galaxies can be strongly affected by their environment. Analyzing the properties of the dwarf galaxies over a wide range of luminosities, sizes, morphological types, and environments, we can obtain insights about their evolution. At ∼50 Mpc, the Hydra I cluster of galaxies is among the closest cluster in the z ≃ 0 Universe, and an ideal environment to study dwarf galaxy properties in a cluster environment. Aims. We exploit deep imaging data of the Hydra I cluster to construct a new photometric catalog of dwarf galaxies in the cluster core, which is then used to derive properties of the Hydra I cluster dwarf galaxy population as well as to compare it with other clusters. Moreover, we investigate the dependency of dwarf galaxy properties on their surrounding environment. Methods. The new wide-field g- and r-band images of the Hydra I cluster obtained with the OmegaCAM camera on the VLT Survey Telescope (VST) in the context of the VST Early-type GAlaxy Survey (VEGAS) were used to study the dwarf galaxy population in the Hydra I cluster core down to r-band magnitude Mr = −11.5 mag. We used an automatic detection tool to identify dwarf galaxies from a ∼1 deg2 field centered on the Hydra I core, covering almost half of the cluster virial radius. The photometric pipeline was used to estimate the principal photometric parameters for all targets. Scaling relations and visual inspection were used to assess the cluster membership and construct a new dwarf galaxy catalog. Finally, based on the new catalog, we studied the structural (Sérsic index n, effective radius Re, and axis ratio) and photometric (colors and surface brightness) properties of the dwarf galaxies, also investigating how they vary as a function of clustercentric distance. Results. The new Hydra I dwarf catalog contains 317 galaxies with a luminosity between −18.5 < Mr < −11.5 mag, a semi-major axis larger than ∼200 pc (a = 0.84″), of which 202 are new detections, and previously unknown dwarf galaxies in the Hydra I central region. We estimate that our detection efficiency reaches 50% at the limiting magnitude Mr = −11.5 mag, and at the mean effective surface brightness μ̄e,r = 26.5 mag arcsec−2. We present the standard scaling relations for dwarf galaxies, which are color-magnitude, size-luminosity, and Sérsic n-magnitude relations, and compare them with other nearby clusters. We find that there are no observational differences for dwarfs scaling relations in clusters of different sizes. We study the spatial distribution of galaxies, finding evidence for the presence of substructures within half the virial radius. We also find that mid- and high-luminosity dwarfs (Mr < −14.5 mag) become, on average, redder toward the cluster center, and that they have a mild increase in Re with increasing clustercentric distance, similar to what is observed for the Fornax cluster. No clear clustercentric trends are reported for surface brightness and Sérsic index. Considering galaxies in the same magnitude bins, we find that for high and mid-luminosity dwarfs (Mr < −13.5 mag), the g − r color is redder for the brighter surface brightness and higher Sérsic n index objects. This finding is consistent with the effects of harassment and/or partial gas stripping.