Anna Ho: "The Landscape of Relativistic Stellar Explosions:
Abstract: For the last half-century, relativistic outflows accompanying the final collapse of massive stars have predominantly been detected via high-energy emission, as long-duration gamma-ray bursts (GRBs). From wide-field optical and radio time-domain surveys, there have been hints of related phenomena at lower energies, such as X-ray flashes. With the Zwicky Transient Facility (ZTF) we are conducting a systematic exploration of the broader landscape of engine-driven explosions, of which traditional GRBs are just one manifestation. The emerging zoo includes relativistic afterglows at cosmological distances, broad-lined Ic (Ic-BL) supernovae with X-ray and radio emission, and fast-luminous transients powered by circumstellar interaction such as AT2018cow. Our work helps set the stage for discovering and characterizing relativistic stellar explosions and pre-explosion mass-loss ("death omens") during the era of the Large Synoptic Survey Telescope (LSST), and millimeter-band facilities like ALMA and the NOrthern Extended Millimeter Array (NOEMA).
Aklant Bhowmick: "Supermassive Black hole fueling in IllustrisTNG: Impact of environment "
The connection between galaxy mergers and AGN activity is a subject of considerable debate. It involves two broad questions: 1) Do mergers trigger AGN activity? 2) What is their relative contribution to driving AGN activity and BH growth, compared to secular processes? I will present recent results on the AGN properties of supermassive black hole (BH) systems (pairs, triples and higher order multiples) in the IllustrisTNG (TNG100) universe. These systems are precursors to gravitationally bound BH binaries which are expected to be detected by LISA. We find enhanced AGN activity (by factors of up to a few) in BH systems on scales less than 0.1 Mpc/h. We also find that the abundance of dual AGNs in TNG100 is consistent with the observed ultra hard X-ray AGN samples. On larger spatial scales (∼ 1 Mpc/h), however, no significant enhancements in AGN activity are associated with BH pairs and multiples, even at high Eddington ratios. The enhancement of AGN activity in rich, small-scale (~0.1 Mpc/h) environments is therefore likely to be driven by galaxy interactions and mergers. Nonetheless, the overall percentage of AGN that live in <0.1 Mpc/h scale multiples is still subdominant. Thus, our results support the existence of a merger-AGN connection, but they also suggest that mergers and interactions play a relatively minor role in fueling the AGN population as a whole.