Tracing galaxy assembly, stellar halos, and dark matter nature from cores to outskirts with VST-SMASH, Euclid, Rubin, and 4MOST
Understanding how galaxies assemble their mass and how dark matter clusters on small scales requires probing the full mass spectrum—from low-mass dwarf galaxies (M★ ≲ 109 M☉) to massive early-type galaxies (M★ ≳ 1011 M☉)—and mapping their spatial structure from their dense central cores out to their low-surface-brightness (LSB) outskirts. The faint outskirts of galaxies, including stellar halos, tidal streams, and diffuse envelopes, preserve an unperturbed fossil record of hierarchical accretion and merger histories. Simultaneously, dwarf galaxies and ultra-diffuse galaxies (UDGs), governed by shallow gravitational potential wells, represent the most sensitive astrophysical laboratories for studying the interplay between stellar feedback, gas dynamics, and dark matter clustering. At these regimes, the historical small-scale challenges to the standard Cold Dark Matter (ΛCDM) paradigm—such as the core-cusp problem, missing satellites, the planes of satellite galaxies, and the structural diversity of dwarfs—become acute, offering a direct window into alternative scenarios such as Warm Dark Matter (WDM) and Self-Interacting Dark Matter (SIDM)
This PhD project aims to decode the assembly history of local galaxies and test the fundamental nature of dark matter through a multi-scale, multi-facility approach. The student will map spatially resolved stellar populations and mass profiles from dense cores to extreme low-surface-brightness outskirts, perform systematic censuses of satellite dwarfs, and investigate spatial alignments and structural scaling relations. The project will initially benchmark these methodologies on deep optical imaging from VST-SMASH, systematically scale them up to large datasets from Euclid and the Vera C. Rubin Observatory’s LSST (complemented by 4MOST spectroscopy), and ultimately confront the empirical findings with state-of-the-art cosmological hydrodynamical simulations.
The project will encompass one or more of the following research directions:
- Deep optical surface photometry in VST-SMASH: Performing ultra-deep surface photometry on multi-band optical imaging of 27 local galaxies (D < 10 Mpc) from the VST-SMASH survey (VST Survey of Mass Assembly and Structural Hierarchy; PI: C. Tortora; Ragusa et al. 2026; Tortora et al. 2024, 2026), observed with OmegaCAM at the VLT Survey Telescope down to surface brightness limits of μr ≳ 29 mag arcsec−2. The student will implement accurate background modeling, sky subtraction, and masking techniques to reliably map the low-surface-brightness regime of local galaxies, uncovering faint stellar halos, extended outer disks, and diffuse tidal structures/merger streams. For one of the galaxies in the sample, IC 5332, the student will also analyze deep narrow-band Hα imaging obtained with VST, mapping the diffuse ionized gas to investigate star formation processes and gas-stripping mechanisms in the outskirts.
- Pixel-level SED fitting and 2D parameter mapping with piXedfit: Applying an automated data cube generation, PSF-matching, and spatial pixel-binning workflow based on pixel-by-pixel spectral energy distribution (SED) fitting using the piXedfit (Abdurro’uf et al. 2025) software suite. By applying Bayesian inference with stellar population models to deep multi-band imaging, the student will produce high-fidelity, spatially resolved 2D maps of stellar mass surface density (Σ★) and star formation rate surface density (ΣSFR), tracing the spatial distribution of stellar populations and localized star formation.
- 1D Mass profiling and structural parameter decomposition: Extracting azimuthal surface brightness and stellar mass density profiles out to large radial distances (R ≳ 3–5 Re). The student will fit single- and multi-component Sérsic models to derive structural parameters from both light and stellar mass density distributions (effective radius Re, Sérsic index n, and bulge-to-total ratio B/T).
- Scaling to Euclid and Rubin/LSST datasets: Scaling up the spatially resolved analysis pipeline (particularly the resolved stellar mass and population modeling with piXedfit) to the large sample of local Universe galaxies observed with the multi-band space imaging of Euclid (VIS optical and NISP Y, J, H near-infrared) and the deep ground-based imaging from the Vera C. Rubin Observatory’s LSST. This combination will leverage Euclid’s exquisite spatial resolution to resolve sub-kiloparsec cores, complemented by the multi-band optical depth of LSST and VST to trace mass profiles out to multiple effective radii across large samples of local galaxies (building upon the baseline Euclid DR1 analysis presented in Tortora et al., in prep.).
- Mapping radial mass density slopes, color gradients, and characteristic mass scales: Measuring radial stellar mass density slopes (∇ log10 Σ★) and optical-to-NIR color gradients across distinct radial zones (in the inner galactic regions and in the outskirts) to investigate the structural transitions occurring at the “gas-richness” threshold (M★ ≈ 1–3 × 109 M☉) and the “golden mass” (M★ ≈ 1–3 × 1010 M☉). This will allow the student to isolate the signatures of turbulent feedback-driven dwarf expansion, secular inside-out disk growth, inside-out quenching, and ex-situ minor merger accretion across different galaxy morphologies.
- Extending the distance baseline with 4MOST and spectroscopic surveys: Exploiting upcoming spectroscopic data from the 4MOST WAVES survey (or DESI in the north) to access redshifts and stellar kinematics for millions of galaxies at z < 0.2. The student will combine these wide-area spectroscopic datasets with spatially resolved imaging across both hemispheres, expanding the baseline of local galaxy evolution studies to broader cosmological volumes.
- Dwarf galaxy census, spatial distribution, and the planes of satellites: Identifying, cataloging, and modeling faint dwarf galaxies and ultra-diffuse galaxies (UDGs) in VST-SMASH, Euclid, and LSST fields (following Marleau et al. 2025). Beyond measuring structural parameters, central densities, and color gradients, the student will investigate the 2D/3D spatial distribution of dwarf satellites around host galaxies to test for coherent planar alignments (the cosmological “planes of satellites” problem). This census will lay the empirical groundwork for spectroscopic observing proposals (e.g., using ESO-VLT) to measure satellite line-of-sight velocities and trace their orbital motions within the host halo potential.
- Tracer populations in galaxy outskirts: Identifying and characterizing Globular Cluster (GC) systems in the low-surface-brightness outskirts of the VST-SMASH local galaxies, exploiting their spatial and photometric properties to reconstruct the assembly history and accretion events of the surrounding stellar halos (an established research direction within the VST-SMASH survey).
- Testing Dark Matter nature and cosmological models with simulations: Comparing observed structural parameters, stellar and dark matter density slopes, dwarf satellite counts, and spatial distributions with state-of-the-art cosmological hydrodynamical simulations (e.g., IllustrisTNG, CAMELS, DREAMS) run under ΛCDM, WDM, and SIDM models (building upon Busillo et al. 2023, 2025; Silvestrini et al. 2026a,b). The student will use the observed dwarf galaxy populations, satellite distributions, and halo properties to constrain cosmological parameters, the nature of dark matter (such as WDM and SIDM) and disentangle baryonic feedback from non-standard dark matter physics. In this context, the project will also focus on key small-scale cosmological challenges, including the structural diversity and central density profiles of dwarf galaxies.
Supervision & Affiliation. The proposed work within SHADOWS will be carried out at the INAF–Osservatorio Astronomico di Capodimonte (Napoli) under the supervision of Crescenzo Tortora (expert in galaxy evolution, dynamics, and gravitational lensing); co-chair of the LSST Strong Lensing Science Collaboration (SLSC); former lead of the “Physical Parameters and Redshift” WP within the Euclid Local Universe SWG, and currently lead of one of the Key Projects of the survey. Tortora is also a core-team member of the WEAVE-StePS and 4MOST-StePS spectroscopic surveys, lead of the Working Group on spatially resolved stellar populations in the WAVES survey (4MOST), and lead of the Working Group “From the Local Universe to the Cosmic Noon” within the science team of MORFEO (the adaptive optics unit for the ESO ELT).
The student will work in close collaboration with researchers and colleagues in the Napoli area, as well as with international teams across the VST-SMASH collaboration, the Euclid Consortium (Local Universe SWG and Galaxy and AGN evolution SWG), Rubin/LSST (Galaxies Science Collaboration), and 4MOST, and experts in galaxy evolution, cosmological simulations, and low-surface-brightness astronomy.
Contact: Crescenzo Tortora (crescenzo.tortora@inaf.it)
Website: https://www.crescenzotortora.com/
