Galaxy formation and evolution

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.

Abstract:

Context . Collisional ring galaxies are a rare class of interacting systems, comprising only ∼0.01% of galaxies in the local Universe. Their formation is typically attributed to a head-on collision of a massive galaxy with a compact satellite (intruder), triggering density waves propagating outwards that produce the characteristic ring morphology. Here, we report the discovery and detailed analysis of GS18660, the most distant ring galaxy known to date, at z = 3.076, based on JWST data obtained within of the GA-NIFS programme. Aims . This work aims to characterise the physical and dynamical properties of GS18660 and shed light on the formation of its ring. Specifically, we analysed the ionised gas properties, stellar populations, and gas kinematics of the system, and used the observed geometry to constrain the timescale of the collision. Methods . Our analysis is based on NIRSpec integral field spectrograph (IFS) data, including low-resolution ( R ∼ 100) spectroscopy covering 0.6–5.3 μ m (∼0.2–1.3 μ m rest-frame), and high-resolution ( R ∼ 2700) spectroscopy covering 1.66–3.17 μ m (0.4–0.8 μ m rest-frame). Multi-wavelength techniques (e.g. emission line diagnostics, full spectral fitting, and gas dynamics) were applied to derive nebular gas conditions and stellar population properties. Results . Gas kinematic analysis reveals that GS18660 exhibits a rotating disc component with an additional radial expansion velocity of ∼200 km s −1 , consistent with a propagating collisional wave. Nebular line diagnostics indicate an intense star formation (star formation rate SFR ∼100 M ⊙ yr −1 ) along the ring and in the nucleus. Stellar population analysis shows that the most recent star formation episode, occurring within the last ≈50 Myr, predominantly took place in the ring. We also identify a close companion, the intruder galaxy responsible for the collision, moving away with a relative velocity of ∼425 km s −1 . Conclusions . The evidence favours a collisional origin for the ring in GS18660, though the presence of a recently formed bar (and hence a resonance ring) cannot be completely excluded. This discovery provides a unique window to the physics of galaxy interactions and the rapid assembly of stellar structures in the early Universe.