The Square Kilometre Array (SKA)

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:

Abstract The rotation velocities of disc galaxies trace dark matter halo structure, providing direct constraints on the galaxy–halo connection. We construct a Bayesian forward model to connect the dark matter halo population predicted by ΛCDM with an observed sample of disc galaxies (SPARC) through their maximum rotation velocities. Our approach combines a subhalo abundance matching scheme (accounting for assembly bias) with a parameterised halo response to galaxy formation. When assuming no correlation between selection in the SPARC survey and halo properties, reproducing the observed velocities requires strong halo expansion, low abundance matching scatter and a halo proxy that strongly suppresses the stellar masses in satellite haloes. This is in clear tension with independent clustering constraints. Allowing for SPARC-like galaxies to preferentially populate low Vmax haloes at fixed virial mass greatly improves the goodness-of-fit and resolves these tensions: the preferred halo response shifts to mild contraction, the abundance matching scatter constraint relaxes to σSHAM < 0.30 dex at 1σ and the proxy becomes consistent with clustering. However, the inferred selection threshold is extreme, implying that SPARC galaxies occupy the lowest ~15 per cent of the Vmax, halo distribution at fixed Mvir. Moreover, even with selection, the inferred scatter remains in statistical disagreement with the low-mass clustering constraints, which are most representative of the SPARC galaxies in our sample. Our analysis highlights the advantage of augmenting clustering-based constraints on the galaxy–halo connection with kinematics and suggests a possible tension using current data.

Abstract:

We present the discovery of 27 new Galactic transients made by the commensal MeerTRAP single pulse search programme at the MeerKAT telescope. We determine the location of 14 of these discoveries with arcsecond accuracy by imaging the data captured in the dedicated transient buffer. A preliminary estimate of the period was made for 8 sources using the arrival times of several pulses detected by MeerTRAP. The periods of these transients range between 0.78 s and 4 s. For the previously published MeerTRAP source MTP0040 (PSR J1357−6507), we are able to provide the position and period using new pulses detected since it was reported. We also estimate the radio fluences and discuss the burst rates derived from the commensal search detections. The accurate image-domain localizations were used to conduct follow-up observations and timing analysis. Furthermore, some follow-up observations revealed ordinary pulsar-like pulsed behavior for four of the new sources, illustrating the blurred separation between rotating radio transients and canonical pulsars. We also present coherent timing solutions for 4 sources, providing insight into their rotational properties and their place within the Galactic neutron star population.