Galaxy formation and evolution

Abstract:

Abstract Strong gravitational lenses (SGLs) are rare systems whose discovery currently relies primarily on supervised machine learning methods trained on large simulated datasets. We present the first application of Astronomaly:PROTEGE to SGL discovery, demonstrating that a human-in-the-loop active learning framework can efficiently identify lenses in large imaging surveys without the need for simulated training data. We consider a sample of 3.7 million bright galaxies from the Kilo-Degree Survey (KiDS) DR4. Feature representations are extracted using a convolutional neural network pre-trained on the ImageNet dataset and subsequently fine-tuned on KiDS data using the self-supervised Bootstrap Your Own Latent (BYOL) framework. Within the embedding of these representations, the active learning loop of Astronomaly iteratively selects the most informative systems for expert inspection. A total of 3,000 objects are inspected across multiple rounds, yielding 34 high-quality (grade A/B) SGL candidates. On the basis that these systems occupy similar regions in the learned feature space, we expand this sample through nearest-neighbour similarity analysis. Including the active learning discoveries, we identify a total of 140 grade A/B candidates and more than 1,000 additional lower-confidence systems (grade C). Among the A/B candidates, 81 are newly identified, while ∼22% of previously known grade A/B KiDS lenses are recovered. These results demonstrate strong potential for next-generation surveys such as Euclid, Roman, and Rubin’s Legacy Survey of Space and Time. With approximately 60% of the high-quality candidates newly reported, this approach complements supervised methods by reducing reliance on simulations and enabling the discovery of a diverse population of SGLs.

Abstract:

We present 50–100 pc-resolution JWST/MIRI and NIRCam measurements of mid-infrared (mid-IR) color variations in the diffuse interstellar medium (ISM) of 71 nearby star-forming galaxies from the PHANGS-JWST survey. Mid-IR emission traces the dust column density, intensity (U) and hardness of the interstellar radiation field, and the physical state (charge, size) and abundance of polycyclic aromatic hydrocarbons (PAHs). Mid-IR colors that trace PAH band ratios remain fairly constant in the diffuse ISM of star-forming disks. However, they show stark variations in extreme environments: highly star-forming central molecular zones (CMZs) and star formation deserts/quiescent bulges. In CMZs, PAH-to-continuum (3.3/21, 7.7/21, and 11.3/21 μm) and the 10/21 μm continuum colors are 0.2–0.4 dex lower than in normal disks. We attribute this to higher U based on the far-infrared dust colors and the high 21 μm/ΣMol, which we suggest to be a good tracer of U outside star-forming regions. Meanwhile, star formation deserts show low 7.7 μm PAH emission, resulting in low 7.7/21 μm and 7.7/11.3 μm, while all other mid-IR colors remain typical. This suggests the presence of more neutral PAHs in star formation deserts, where low 7.7 μm likely reflects ISM conditions similar to early-type and elliptical galaxies. All environments form part of a continuous trend in 7.7/11.3 μm versus specific star formation rate.

Abstract:

Context . Compact galaxies represent a key population for understanding galaxy evolution, as their high stellar densities are closely linked to early formation and/or stripping processes. However, the relative role of internal structure and environment in shaping their origin and evolution remains unclear. Aims . We aim to investigate how the abundance and quenching of compact galaxies depend on environment, and to determine whether their formation is governed by a single evolutionary pathway or multiple channels. Methods . We analyse a large sample of galaxies in the nearby Universe ( z < 0.05) with stellar masses in the range 8.2 < log( M ⋆ /M ⊙ ) < 10.5, comparing compact and control populations as a function of local overdensity by measuring the fraction of galaxies above a given overdensity ( δ ) threshold. Results . We find that the environmental dependence of compact galaxies is non-monotonic, with two characteristic overdensities, the transition overdensity ( δ t ) and the critical overdensity ( δ c ), defining three distinct regimes. Below δ t , compact galaxies are relatively more abundant than the control population, while between δ t and δ c their relative abundance decreases significantly. Above δ c , compact galaxies become increasingly overabundant again, although this behaviour strongly depends on stellar mass. Low-mass (8.2 < log( M/M ⊙ ) < 9.0) compact galaxies exhibit the strongest deficit at intermediate overdensities. In contrast, high-mass compact galaxies (9.0 < log( M/M ⊙ ) < 10.5) dominate the excess population above δ c . In addition, we find that the fraction of red galaxies increases with local overdensity in control and compact samples. However, the red galaxy fraction is systematically higher in compact galaxies than in the control sample, indicating that both environment and internal structure (compactness) play a role in galaxy quenching. The relative importance of these effects strongly depends on stellar mass, with environmental quenching being more prominent in low-mass compact galaxies, while internal structural properties may dominate in the high-mass compact population. Conclusions . Our results support a unified evolutionary framework in which compact galaxies do not arise from a single evolutionary channel, but instead reflect multiple pathways whose relative importance depends strongly on stellar mass and environment. Low-mass compact galaxies appear to be closely linked to environmentally driven transformation and quenching processes acting in groups, filaments, and cluster infall regions. In contrast, high-mass compact galaxies are found across all environments, but become increasingly overabundant relative to the control population in the densest regions. Their uniformly high red galaxy fractions and stellar surface mass densities indicate that they survive efficiently in high-density environments as structurally dense systems, similar to compact relic galaxies.