Galaxy formation and evolution

Abstract:

Abstract Stellar metallicity gradients (∇[ Z / H ]) provide a fossil record of the assembly history of galaxies. We present an analysis of ∇[ Z / H ] for 90 nearby low-mass galaxies using Very Large Array (VLT)/MUSE integral field unit (IFU) spectroscopy, spanning stellar masses from 10 6.5 to 10 10 M ⊙ (median ∼10 8.5 M ⊙ ) and significantly extending the mass coverage of existing IFU surveys into the classical dwarf regime. Our primary finding is a robust negative correlation between ∇[ Z / H ] and light-weighted stellar age (∣ r ∣ ≳ 0.7) measured out to ∼2× effective radius: older dwarf galaxies have steeper (more negative) gradients. This holds regardless of stellar mass, structural compactness, or large-scale environment (group/field) and is strongest in the intermediate-mass regime ( 8.2 ≲ log M ⋆ / M ⊙ ≲ 9.0 ). The slope of the age–∇[ Z / H ] relation is close to that in the FIRE-2 simulations, indicating that stellar radial migration driven by feedback-induced potential fluctuations may be fundamental in dwarf evolution. But this apparent consistency is likely coincidental given the simulations’ overly efficient feedback and chemical mixing. On the other hand, the H i deficiency parameter, an indicator of past environmental stripping, shows a moderate yet highly significant correlation with ∇[ Z / H ], second only to stellar age in strength: galaxies with higher H i deficiency tend to have more negative gradients, strongly indicating that environment-driven outside-in quenching and the ensuing gradual truncation of metal enrichment reshape the stellar metallicity distribution. Our analysis suggests that the chemical evolution of dwarf galaxies likely arises from a synergy of feedback-driven dynamical heating and external environmental processing, though only the latter has robust observational support.

Abstract:

Abstract Radio emission during X-ray binary outbursts is dominated by synchrotron radiation from relativistic jets, but is usually studied through total-intensity diagnostics such as flux density, spectra, variability, and proper motion. Radio spectropolarimetry provides a complementary probe of the magneto-ionic plasma through Faraday rotation and depolarisation. When the Faraday rotating material is local to the source, these effects can constrain the jet plasma composition and mass content, but this approach is rarely applied to transient jetted sources. We present MeerKAT L-band spectropolarimetry of the black hole X-ray binary Swift J1727 during its 2023 outburst, focusing on the brightest radio flaring interval, when relativistic jets were being launched intermittently. Using multiple spectropolarimetric techniques, we identify transient Faraday-complex structure coincident with the major radio flares. The close temporal association with the flaring activity, together with the stability of the foreground Faraday screen, favours an origin local to the jet rather than in the ISM or in a separate local screen external to the emitting plasma. Since internal Faraday rotation is suppressed in a pure electron–positron plasma, the data favour a dominant electron–proton component. Interpreting the characteristic Faraday thickness as internal rotation, and anchoring the magnetic-field and size scales with synchrotron self-absorption arguments, we infer a characteristic Faraday-rotating mass of order Mrot ~ 1021 g, corresponding to only a small fraction, frot ~ 10−3, of the accreted mass available during the flare. These results show that time-domain spectropolarimetry can turn transient Faraday complexity into a diagnostic of jet composition, mass loading, and plasma evolution in X-ray binary outbursts, and potentially other transient jetted sources.