Marina Rejkuba

We introduce and describe the Globular cluster Observations for Luminosity-Dispersion distances (GOLD) project, which has the principal aim of determining independent distances based on the globular cluster (GC) absolute magnitude-velocity dispersion ( M _V − σ ) relation calibrated with Milky Way GCs with Gaia distances. The project comprises observations of GCs in 19 early- and late-type galaxies taken with FLAMES/GIRAFFE on ESO’s Very Large Telescope. So far, 17 galaxies have been observed. The spectra cover the wavelength range 8483−9000 Å, including the calcium triplet region at a spectral resolution of R ∼ 18 000. The spectra provide line-of-sight velocities with typical uncertainties of Δ v = 1 − 2 km s ⁻¹ . Using primarily velocities to separate extragalactic sources from foreground stars, we present here our initial sample of 1062 GCs and 101 ultra-compact dwarf galaxies (UCDs). For more than 650 of these objects we present the first velocity measurement. We discuss the available photometry and size measurements, and show that our velocities are in good agreement with previous studies. Our velocity catalogues are made publicly available. Beyond the principal aim of obtaining precision distances, additional science to be explored with these data includes host galaxy halo dynamics and clustering analysis, mass-to light ratios of GCs and UCDs in the sample, the stellar initial mass function and black hole occupation fraction of the GCs, and chemical abundances from high-dispersion spectra.

Ultracool dwarfs (UCDs) encompass the lowest-mass stars and brown dwarfs at the stellar–substellar boundary. These intrinsically faint objects are challenging to detect. However, the wide coverage and deep sensitivity of the Euclid survey will increase the number of confirmed and well-characterised UCDs by several orders of magnitude. We carried out a systematic study using Euclid Quick Data Release (Q1) low-resolution near-infrared (NIR) spectra to identify and characterise UCDs. A sample of previously known UCDs in Q1 was compiled to validate the analysis, which was then applied to newly identified UCD candidates in Euclid Deep Field North (EDF-N; 22.9 deg ² , down to J _E ≈ 24.5 mag). We confirm 33 new UCDs in EDF-N, spanning spectral types (SpTs) M7–T1 ( J _E = 17–21 mag) and distances of 50–600 pc. Six of them were also independently identified in other Q1 studies. We followed up on two of these objects with ground-based optical and NIR observations using the Gran Telescopio Canarias, providing an independent validation of the Euclid spectral classification and the ability of Q1 spectra to constrain the stellar parameters of UCDs. We determined the SpTs from Euclid Q1 spectra of 33 newly identified UCDs and 167 previously known UCDs by fitting spectral standards from the SpeX Prism Library Analysis Toolkit. The known UCD sample includes 103 objects in common with the Dark Energy Survey (M7–T0, J _E = 17–22 mag, and d = 50–600 pc), 6 objects from literature compilations of UCD sources (M8–T7, J _E = 18–20 mag, and d = 30–300 pc), and 58 in common with Gaia (M7–L1, J _E = 17–24 mag, and d = 70–300 pc). We derived independent stellar parameter estimates through atmospheric model fitting; the resulting effective temperatures are in good agreement with Euclid SpTs. Combined with complementary ground- and space-based data, these results demonstrate the potential for determining proper motions and identifying companions. Additionally, we investigated Gaia UCDs in Q1 by combining their mean XP spectra with Euclid NIR spectra, extending the wavelength coverage and enabling more comprehensive determinations of stellar parameters, including radii. To support broader classification efforts, we analysed the UCDs in colour–colour space and assessed their separation from quasar loci. We present a publicly available, machine-readable catalogue that includes Euclid observations of known and newly identified sources, with spectral classifications and stellar parameters. This work demonstrates Euclid ’s capability to identify and characterise the faintest stellar populations, establishing a foundation for large-scale UCD studies with forthcoming survey data.

Reticulum is an old, metal-poor, and sparsely populated globular cluster in the outer regions of the Large Magellanic Cloud (LMC) and hosts a rich population of RR Lyrae stars. Being as close as possible to a single stellar population with negligible metallicity spread and low reddening, Reticulum is an ideal laboratory for testing stellar pulsation models and calibrating population II distance indicators. We present homogeneous multiepoch near-infrared (NIR; JHK _s ) observations of RR Lyrae variables in Reticulum obtained with the Flamingos-2 imager on the 8.1 m Gemini South Telescope. Using NIR light-curve templates, we derive accurate intensity-averaged magnitudes and peak-to-peak amplitudes for 32 RR Lyrae stars, including 22 fundamental-mode (RRab), four first-overtone (RRc), and six mixed-mode (RRd) pulsators. The empirical JHK _s period–luminosity (PL) relations of Reticulum RR Lyrae are very tight, exhibiting dispersions (∼0.05 mag) comparable to those observed in Galactic globular clusters. The derived PL slopes are shallower than those reported for Galactic cluster variables. Adopting recent empirical and theoretical period–luminosity–metallicity calibrations based on Galactic globular clusters and pulsation models, we derive a true distance modulus of μ ₀ = 18.472 ± 0.035 mag to Reticulum. This cluster distance is in excellent agreement with the precise geometric distance to the LMC and places Reticulum close to the LMC barycentric distance. The well-characterized RR Lyrae population and a precise distance make Reticulum a potential anchor for calibrating Population II distance ladder.

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.

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 ∼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 α = −1.81 ^+0.16 _−0.16 for the faint end compared to α = −1.41 ^+0.08 _−0.05 in the present day, which is consistent with measurements at high redshift and with cosmological simulations.

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 0.4 R _vir 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 R _e 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 σ 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. 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 ≲ 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.

Globular clusters (GCs) are dense star clusters found in all massive galaxies. Recent work has established that they follow a tight relation between their internal stellar velocity dispersion σ and luminosity, enabling accurate distance measurements. In this work, we aim to apply this GC velocity dispersion (GCVD) distance method to measure the distance to M 104 (NGC 4594, the Sombrero galaxy). We have measured internal stellar velocity dispersions for 85 globular clusters (GCs) and one ultra-compact dwarf galaxy around M 104 using high-resolution multi-object integrated-light spectroscopy with FLAMES/GIRAFFE on the Very Large Telescope. The measured velocity dispersions range from σ = 4 − 30 km s, with a mean uncertainty of Δ σ = 2.5 km s. For a subset of 77 GCs with V -band magnitudes and reliable velocity dispersion measurements above σ > 4 km s, we constructed the M V - σ relation to measure the distance to M 104, finding D = 9.00 ± 0.29 (stat.) ± 0.26 (sys.) Mpc. The GCs follow the Milky Way and M 31 M V − σ relation closely, with the exception of the luminous ultra-compact dwarf SUCD1, which is nearly one magnitude brighter than the mean relation. 29 GCs in the sample have sizes determined from Hubble Space Telescope imaging which allowed us to determine their masses and V -band dynamical mass-to-light ratios (M/L). We find a mean < M / L V > = 2.6 M ⊙ / L ⊙ for the luminous ( M V < − 8 mag) M 104 GCs, which is higher than the Milky Way GCs, but is reminiscent of the brightest GCs in Centaurus A. With the exception of SUCD1, the GCs of M 104 follow the GCVD relation irrespective of their mass-to-light ratio.

The European Space Agency’s Euclid mission will observe approximately 14000 deg² of the extragalactic sky and deliver high-quality imaging of a large number of galaxies. The depth and high spatial resolution of the data will enable a detailed analysis of the stellar population properties of local galaxies through spatially resolved spectral energy distribution (SED) fitting. In this study, we test our pipeline for spatially resolved SED fitting using synthetic images of Euclid, LSST, and GALEX generated from the TNG50 simulation using the SKIRT 3D radiative transfer code. Our pipeline uses functionalities in piXedfit for processing the simulated data cubes and carrying out SED fitting. We apply our pipeline to 25 simulated galaxies at z ∼ 0 to recover their resolved stellar population properties. For each galaxy, we produce three types of data cubes: GALEX + LSST + Euclid, LSST + Euclid, and Euclid-only. We performed the SED fitting tests with two stellar population synthesis (SPS) models in a Bayesian framework. Because the age, metallicity (Z), and dust attenuation estimates are biased when applying only classical formulations of flat priors (even with the combined GALEX + LSST + Euclid data), we examined the effects of additional physically motivated priors in the forms of mass-age and mass-metallicity relations, constructed using a combination of empirical and simulated data. Stellar-mass surface densities can be recovered well using any of the three data cubes, regardless of the SPS model and prior variations. The new priors then significantly improve the measurements of mass-weighted age and Z compared to results obtained without priors, but they may play an excessive role compared to the data in determining the outcome when no ultraviolet (UV) data is available. Compared to varying the spectral extent of the data cube or including and discarding the additional priors, replacing one SPS model family with the other has little effect on the results. The spatially resolved SED fitting method is powerful for mapping the stellar population properties of many galaxies with the current abundance of high-quality imaging data. Our study re-emphasizes the gain added by including multi-wavelength data from ancillary surveys and the roles of priors in Bayesian SED fitting. With the Euclid data alone, we will be able to generate complete and deep stellar mass maps of galaxies in the local Universe (z ≲ 0.1), exploiting the telescope’s wide field, near-infrared sensitivity, and high spatial resolution.

As ancient star clusters, globular clusters (GCs) are regarded as powerful tracers of galaxy evolution and assembly. Due to their brightness and compact sizes, GCs are employed to probe the kinematics and stellar population properties of galaxies, from the central regions out into the halo where the underlying stellar light becomes too faint for spectroscopic studies. In this work, we present a comprehensive study of the GC system of M 104 (NGC 4594, also known as the Sombrero galaxy) based on literature spectroscopic catalogues and newly collected data from Very Large Telescope (VLT) MUSE integral-field spectroscopy combined with multi-object spectroscopy from VLT FLAMES and OSIRIS at the Gran Telescopio de Canarias (GTC). We present a new catalogue of 499 GCs with radial velocity measurements that span from the inner disc region out to ∼70 kpc (24′). In addition to velocities, we measured metallicities from the MUSE, OSIRIS, and FLAMES spectra of 190 GCs. Together with literature values, we collected a sample of 278 metallicities. We found good agreement between the velocity and metallicity measurements of GCs observed with multiple instruments. Studying GC kinematics with a simple model confirms a decreasing velocity dispersion profile and low rotation velocities. The blue GCs appear to be more dispersion-dominated, while the red GCs follow the kinematics of the stars more closely. We find a large scatter of GC metallicities with distance from the centre, and metal-rich GCs are found over all radii. We discuss how the GC metallicity distribution with a broad metal-poor component likely reflects the complex assembly history of M 104.