Galaxy formation and evolution

Abstract:

Context. Ultra-compact massive galaxies (UCMGs) often exhibit elevated stellar-to-dynamical mass ratios when dynamical masses are estimated using standard virial prescriptions. This discrepancy has been interpreted as evidence for structural non-homology driven primarily by their compactness. Aims. This study investigates how the stellar-to-dynamical mass ratio depends on compactness (C), internal kinematics (σ*), stellar population properties (mass-weighted age, metallicity, and [Mg/Fe]), and star formation histories. The analysis is based on a homogeneous catalogue of 482 UCMGs from the INSPIRE and E-INSPIRE surveys, extending to significantly smaller sizes than previously analysed samples. Methods. I first derive the compactness–mass relation assuming a constant virial coefficient (K = 5). I then correct stellar masses for initial mass function (IMF) variations and recompute stellar-to-dynamical mass ratios using an empirical prescription in which the virial coefficient varies as a function of radius and stellar mass. Finally, I test whether the relation is modulated by stellar kinematics and population properties, including the degree of relicness (DoR), which quantifies the extremeness of the star formation history. Results. A statistically significant anti-correlation between compactness and the IMF-corrected stellar-to-dynamical mass ratio is recovered when a constant virial coefficient is adopted, even within the relatively narrow range of C spanned by nearby UCMGs. The relation substantially flattens when a structure-dependent K is adopted, in agreement with previous literature. Beyond this one-dimensional behaviour, the data define a structural–dynamical manifold in the (logC,Clog σ*) space. Velocity dispersion sets the dominant axis of variation, and the corresponding plane accounts for ∼62% of the variance in stellar-to-dynamical mass ratio. Including stellar age increases the explained variance to ∼63%, revealing a secondary evolutionary modulation. In contrast, DoR, metallicity, and [Mg/Fe] do not retain independent explanatory power once stellar age is included. Conclusions. The stellar-to-dynamical mass ratio in UCMGs is governed primarily by the depth of the gravitational potential, traced by stellar velocity dispersion, rather than by compactness alone. At fixed size, systems with higher σ* exhibit systematically lower stellar-to-dynamical mass ratio, indicating that dynamical structure regulates the apparent mass imbalance in the ultra-compact regime. Compactness largely reflects this dynamical scaling, while stellar age introduces a coherent secondary modulation linking the structural manifold to the evolutionary state of the galaxy. Non-homology in UCMGs therefore encodes coupled dynamical and assembly processes rather than purely geometric compactness.

Abstract:

We investigate the role of baryonic physics in shaping the population, structure, and internal dynamics of galactic subhalos using the Mochima suite of cosmological zoom-in simulations. A refined method is developed to identify bound subhalo material by isolating the local gravitational potential and applying multi-criteria phase-space selection. This approach enables a robust characterisation of subhalo properties across five baryonic runs with varying prescriptions for star formation, and supernova and protostellar feedback, as well as a dark matter-only baseline. At the population level, we find that the concentration of the central massive host halo, modulated by baryonic physics in the central disc galaxy, is a key predictor of subhalo survival. Subhalos with more massive stellar components exhibit deeper internal potentials and enhanced resilience to tidal disruption. At the structural level, we identify a broad diversity in inner dark matter profiles, consistent with observations of dwarf galaxies. We show that this diversity correlates with both star formation history and environmental interaction. In particular, galaxies that form most of their stars early tend to retain steep cusps, while those with extended or recent star formation exhibit oscillating inner slopes shaped by bursty feedback and tidal perturbations. These findings suggest that the so-called “diversity problem” may reflect the complex interplay between feedback history and gravitational environment, rather than a breakdown of cold dark matter predictions.

Abstract:

We present a far-ultraviolet (FUV) analysis of the star-forming complexes (SFCs) in the nearby spiral galaxy NGC 2090 based on observations from the Ultraviolet Imaging Telescope, and compare the FUV emission with that from the optical and infrared bands. NGC 2090 exhibits prominent star formation in its extended outer disk, with FUV emission traced out to ∼30 kpc, far beyond the truncation of the old stellar disk at ∼5 kpc. It is classified as an extended UV (XUV) disk galaxy. We identified and characterized the SFCs both within and beyond the optical radius (R 25 ), estimating their physical sizes and star formation rates (SFRs). The outer-disk SFCs are generally smaller in area and show a narrower distribution of SFR surface density (Σ SFR ) compared to the inner-disk SFCs. We investigated the properties of the inner disk using mid-infrared data from the James Webb Space Telescope and find that the polycyclic aromatic hydrocarbon emission is strongly correlated with regions of active star formation. The specific SFR increases with radius, consistent with a scenario of inside-out disk growth. The observed number of SFCs and their H α -to-FUV flux ratios in the outer disk of NGC 2090 indicate ongoing massive star formation and are consistent with a top-heavy initial mass function (IMF), implying that the upper end of the IMF is not truncated in the low-density, metal-poor outskirts. These results suggest that XUV disks can host significant massive star formation despite their low stellar densities and metallicities.

Abstract:

We conduct a search for strong gravitational lenses in the Dark Energy Survey (DES) Year 6 imaging data. We implement a pre-trained Vision Transformer (ViT) for our machine learning (ML) architecture and adopt interactive machine learning to construct a training sample with multiple classes to address common types of false positives. Our ML model reduces ∼236 million DES cutout images to 22,564 targets of interest, including ∼85% of previously reported galaxy–galaxy lens candidates discovered in DES. These targets were visually inspected by citizen scientists, who ruled out ∼90% as false positives. Of the remaining 2618 candidates, 149 were expert-classified as “definite” lenses and 516 as “probable” lenses, for a total of 665 systems, with 147 of these candidates being newly identified. Additionally, we trained a second ViT to find double-source plane lens systems, finding at least one double-source system. Our main ViT excels at identifying galaxy–galaxy lenses, consistently assigning high scores to candidates with high expert assessments. The top 800 ViT-scored images include ∼100 of our “definite” lens candidates. This selection is an order of magnitude higher in purity than previous convolutional neural-network-based lens searches and demonstrates the feasibility of applying our methodology for discovering large samples of lenses in future surveys.

Abstract:

The ultraviolet (UV) continuum slope of galaxies, , is a powerful diagnostic of the metallicity and ages of stars, nebular gas properties, dust content, and the escape of Lyman continuum (LyC) photons. In this study, we present measurements for 395 spectroscopically confirmed galaxies at selected primarily from JADES, using high-quality JWST (James Webb Space Telescope) NIRSpec/PRISM spectra. We find a median , finding a mild increase in blueness of with increasing redshift and fainter UV magnitudes. Interestingly, we find evidence for reddening of the average at , deviating from the trend observed at . Using stacked spectra in bins of redshift and , we derive trends between and dust attenuation, metallicity, ionization parameter, and stellar age indicators, finding a lack of dust attenuation to be the dominant driver of bluer -values. We further report five galaxies with , which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder -values at may require rapid build-up of dust reservoirs in the very early Universe or a significant contribution from the nebular continuum emission to the observed UV spectra, with the nebular continuum fraction depending on the gas temperatures and densities. We show that in the absence of dust, nebular emission at cm can reproduce the range of red that we see in our sample. Higher gas densities can also redden the nebular continuum emission, potentially explaining the observed -values.