Francesca Annibali

We present a study of dwarf galaxy candidates in the deepest optical imaging obtained to date of the field surrounding the nearby galaxies NGC 5068 (5.2 Mpc) and NGC 5084/NGC 5087 (25–30 Mpc). The latter two are located in the peripheries of the Virgo Supercluster. This field, covering ∼2.6 deg ² , was observed as part of the multi-band, wide-field, and very deep data from VST Survey of Mass Assembly and Structural Hierarchy (VST-SMASH), a distance-limited programme ( D < 11 Mpc) that reaches g - and r -band surface brightness depths of µ ∼ 30 mag arcsec ⁻² within a box of 10×10 arcsec ² at 1 σ . Using a two-step visual inspection procedure, we identified 47 dwarf galaxy candidates and performed a surface photometry study of the sample, along with a fitting procedure with 1D Sérsic model on their profiles. Only four galaxies had previously been reported in the literature, augmenting the number of dwarfs discovered in these regions by one order of magnitude. The colours (median g − r = 0.57 mag and r − i = 0.24 mag) and structural properties of the dwarf candidates are consistent with the literature, as are their scaling relations with effective radius, Sérsic index ( n < 2), and absolute magnitude. We also investigated their central colour gradients, which exhibit significant scatter. We discuss them within the broader context of galaxy formation and also analyse the spatial distribution of these dwarf candidates relative to potential host galaxies. Considering the projected separation, we identified reasonable associations with NGC 5084, NGC 5087, and NGC 5068 as likely hosts for a significant fraction of the sample. Several candidates are at physically credible distances from NGC 5068, despite what their offset size–luminosity relation alone might suggest. Future spectroscopic and deeper imaging follow-ups are required to determine distances and velocities, enabling a robust association with hosts, studies of satellite distributions and counts, and comparisons with cosmological expectations for planes in the context of satellites and dark matter models.

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.

Star clusters are valuable indicators of galaxy evolution, offering insights into the buildup of stellar populations across cosmic time. Understanding the intrinsic star cluster populations of dwarf galaxies is particularly important given these systems’ role in the hierarchical growth of larger systems. We use data from Euclid ’s Early Release Observation programme to study star clusters in two star-forming dwarf irregular galaxies in the Local Group, NGC 6822 and IC 10 [ M _⋆ ~ (1–4) × 10 ⁸ M _⊙ ]. With Euclid , star clusters are resolved into individual stars across the main bodies and haloes of both galaxies. Through visual inspection of the I _E images, we uncover 30 new star cluster candidates in NGC 6822 and 16 in IC 10, ranging from compact to diffuse extended clusters. We compile and re-evaluate previously identified literature candidates, resulting in final combined catalogues of 52 (NGC 6822) and 71 (IC 10) cluster candidates with confidence-based classifications. We present homogeneous photometry in I _E , Y _E , J _E , and H _E , and in archival UBVRI data, alongside size measurements and properties derived from the spectral energy distribution fitting code BAGPIPES . Through synthetic cluster injection, we conclude our sample is ~50% complete to M ≲ 10 ³ M _⊙ for ages ≲100 Myr, and to M ≲ 2 × 10 ⁴ M _⊙ for ages of ~10 Gyr. We find that IC 10 has more young clusters than NGC 6822, and its young clusters extend to higher masses, consistent with its starburst nature. We find several old massive (≳10 ⁵ M _⊙ ) clusters in both dwarfs, including an exceptional cluster in NGC 6822’s outskirts with a mass of 1.3 × 10 ⁶ M _⊙ , nearly twice as massive as any other old cluster in either galaxy. In NGC 6822, we also identify a previously undetected, old, and extended cluster ( R _h = 12.4 ± 0.11 pc). Using well-defined criteria, we identify 11 candidate GCs in NGC 6822 and nine in IC 10. Both galaxies have high specific frequencies ( S _N ) for their luminosities but remain consistent with the known GC scaling relationships in the low-luminosity regime.

KK 153 is a star-forming dwarf galaxy that has been recently proposed as a new member of the sparsely populated class of gas-rich ultra faint dwarfs, lying in the outskirts of the Local Group. We used the Large Binocular Telescope under sub-arcsec seeing conditions to resolve for the first time the outer regions of KK 153 into individual stars, reaching the red giant branch. The magnitude of the red giant branch tip was used to measure a distance of D=3.06 (+0.17/-0.14) Mpc, much more accurate and precise than the estimate previously available in the literature, based on the baryonic Tully-Fisher relation (D=2.0 (+1.7/-0.8) Mpc). The new distance places KK 153 clearly beyond the boundaries of the Local Group, and, together with a new measure of the integrated magnitude, implies a stellar mass of M_*=2.4 \pm 0.2 X 10^6 M_{\sun}. The dwarf populates the extreme low-mass tail of the M_* distribution of gas-rich galaxies but it is significantly more massive than the faintest local gas-rich dwarfs, Leo T and Leo P. In analogy with similar systems, the star formation history of KK 153 may have been impacted by the re-ionisation of the Universe while keeping a sufficient gas reservoir to form new stars several Gyr later.

We present JWST/MIRI spectra from the Medium-resolution Spectrometer of I Zw 18, a nearby dwarf galaxy with a metallicity of ∼3% Solar. Here, we investigate warm molecular hydrogen, H ₂ , observed in spectra extracted in ∼120 pc apertures centered on eleven regions of interest. We detect seven H ₂ rotational lines, some of which are among the weakest ever measured. The H ₂ population diagrams are fit with local-thermodynamic-equilibrium models and models of photodissociation regions. We also fit the ortho-/para-H ₂ ratios (OPRs); in three of the six regions for which it was possible to fit the OPR, we find values significantly greater than 3, the maximum value for local thermodynamic equilibrium. To our knowledge, although predicted theoretically, this is the first time that OPR significantly >3 has been measured in interstellar gas. We find that an OPR tends to increase with decreasing H ₂ column density, consistent with the expected effects of self-shielding in advancing photodissociation fronts. The population diagrams are consistent with H nucleon densities of ∼10 ⁵ cm ⁻³ , and an interstellar radiation field scaling factor, G ₀ , of ∼10 ³ . This warm, dense H ₂ gas coexists with the same highly ionized gas that emits [O IV] and [Ne V]. Emission from T ≳ 50 K dust is detected, including an as-yet-unidentified dust emission feature near 14 μ m; possible identification of Al ₂ O ₃ is discussed. The continuum emission from several regions requires that a considerable fraction of the refractory elements be incorporated in dust. Despite stacking spectra in the SE where H ₂ is found, no significant emission from polycyclic aromatic hydrocarbons is detected.

We present JWST/MIRI spectra from the Medium-Resolution Spectrometer of I Zw 18, a nearby dwarf galaxy with a metallicity of ∼3% solar. Its proximity enables a detailed study of highly ionized gas that can be interpreted in the context of newly discovered high-redshift dwarf galaxies. We derive aperture spectra centered on 11 regions of interest; the spectra show very low extinction, A _V ≲ 0.1, consistent with optical determinations. The gas is highly ionized; we have detected 10 fine-structure lines, including [O iv] 25.9 μm with an ionization potential (IP) of ∼55 eV, and [Ne v] 14.3 μm with an IP of ∼97 eV. The ionization state of I Zw 18 falls at the extreme upper end of all of the line ratios we analyzed, but not coincident with galaxies containing an accreting massive black hole (active galactic nucleus). Comparison of the line ratios with state-of-the-art photoionization and shock models suggests that the high-ionization state in I Zw 18 is not due to shocks. Rather, it can be attributed to metal-poor stellar populations with a self-consistent contribution of X-ray binaries or ultra-luminous X-ray sources. It could also be partially due to a small number of hot low-metallicity Wolf−Rayet stars ionizing the gas; a small fraction (a few percent) of the ionization could come from an intermediate-mass black hole. Our spectroscopy also revealed four 14 μm continuum sources, ≳30–100 pc in diameter, three of which were not previously identified. Their properties are consistent with H ii regions ionized by young star clusters.