The Square Kilometre Array (SKA)

Abstract:

We have conducted a search for z ≃ 7 Lyman-break galaxies over 8.2 deg² of near-infrared imaging from the Visible and Infrared Survey Telescope for Astronomy (VISTA) Deep Extragalactic Observations (VIDEO) survey in the XMM–Newton-Large Scale Structure (XMM-LSS) and the Extended Chandra Deep Field-South (ECDF-S) fields. Candidate galaxies were selected from a full photometric redshift analysis down to a Y + J depth of 25.3 (5σ), utilizing deep auxiliary optical and Spitzer/Infrared Array Camera (IRAC) data to remove brown dwarf and red interloper galaxy contaminants. Our final sample consists of 28 candidate galaxies at 6.5 ≤ z ≤ 7.5 with −23.5 ≤ M_UV ≤ −21.6. We derive stellar masses of 9.1 ≤ log₁₀(M_⋆/M_⊙) ≤ 10.9 for the sample, suggesting that these candidates represent some of the most massive galaxies known at this epoch. We measure the rest-frame ultraviolet (UV) luminosity function (LF) at z ≃ 7, confirming previous findings of a gradual decline in number density at the bright end (M_UV < −22) that is well described by a double power law (DPL). We show that quasar contamination in this magnitude range is expected to be minimal, in contrast to conclusions from recent pure-parallel Hubble studies. Our results are up to a factor of 10 lower than previous determinations from optical-only ground-based studies at M_UV ≲ −23. We find that the inclusion of YJHK_s photometry is vital for removing brown dwarf contaminants, and z ≃ 7 samples based on red optical data alone could be highly contaminated (≳50 per cent). In comparison with other robust z > 5 samples, our results further support little evolution in the very bright end of the rest-frame UV LF from z = 5–10, potentially signalling a lack of mass quenching and/or dust obscuration in the most massive galaxies in the first Gyr.

Abstract:

Gamma-Ray Bursts (GRBs) are brief flashes of gamma-rays that last from a fraction of second up to a few hundreds of seconds, during which a significant amount of isotropic equivalent energy is released (ranging from 10^48 to 10^54 erg). These explosive transients are associated with the catastrophic explosion of an isolated massive star (long-duration GRBs), or with the merger of compact objects (short-duration GRBs). Both scenarios lead to the formation of a highly magnetised neutron star or a spinning, stellar-mass black hole, which are thought to accrete material and launch two relativistic jets, causing the observed gamma-ray emission via magnetic processes or internal shocks. These jets interact with the surrounding material, producing the afterglow emission that extends from gamma-rays to radio waves. To understand GRB formation and evolution, a standard model involving an ultra-relativistic outflow is commonly employed. However, even sophisticated models face degeneracy in the multi-dimensional parameter space. To alleviate or possibly break the degeneracy, broad-band observations across the electromagnetic spectrum are crucial. In particular, the Very Long Baseline Interferometry technique (VLBI) has proven to be a unique asset, providing direct evidence of apparent superluminal expansion (for on-axis GRBs), centroid displacement of the outflow (for slightly off-axis GRBs) and the first confirmation that merger events can launch successful jets. In this Thesis, we employed radio and VLBI observations to characterise and constrain the outflow, the circum-burst medium, and the properties of the progenitors of GRBs. This included studies on individual events (GRB201015A and GRB221009A), which are important to test the predictions of current models, GRB host galaxies (GRB200716C), which are fundamental to constrain the nature of the progenitor through the characterisation of the surrounding environment, and the statistical properties of GRB afterglows, in order to verify the existence of potential GRB sub-populations