Denys Wilkinson Building, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH
Dr Shubham Srivastav
The bewildering zoo of white dwarf supernovae
Type Ia supernovae (SNe Ia) originate from exploding white dwarfs in a binary system. Although SNe Ia are used as cosmic probes owing to their standardisable properties, the exact mechanism of the explosion or the nature of the progenitor (eg. single-degenerate involving a white dwarf accreting from a non-degenerate companion, or doubledegenerate involving a white dwarf merger) are not well understood. Recent wide-field optical sky surveys have unearthed a staggering diversity within SNe Ia, revealing several “peculiar” subclasses that do not follow the widthluminosity relation. These include the type Iax subclass, low luminosity and low energy cousins of SNe Ia thought to arise from a partial disruption of a carbon-oxygen white dwarf, possibly leaving a remnant behind. Other peculiar subclasses include the sub-luminous 02es-like and over-luminous 03fg-like SNe Ia. Recent evidence from early and late-time observations suggests 02es-like and 03fg-like SNe Ia may be the result of white dwarf mergers. Another intriguing subclass is the family of calcium-strong transients, these are generally much fainter than typical SNe Ia and tend to occur at large separations from early-type host galaxies. Mergers of low mass carbon-oxygen and helium white dwarfs could be a promising route to explain these enigmatic transients. Gaining a better understanding of the progenitors of these various white dwarf supernova subclasses has ramifications for continuing to effectively use these objects as dark energy probes. The talk will present an overview of the white dwarf supernova zoo and constraints on the progenitor system(s) for the different subclasses from recent observations.
Dr Rohan Varadaraj
Discovery of Bright Galaxies at Redshift 7 in UltraVISTA and Euclid COSMOS
Measuring the demographics of luminous galaxies within the first billion years after the Big Bang is essential for revealing the onset of feedback processes that regulate galaxy growth across cosmic time. However, selections of highredshift galaxies (via spectral energy distribution fitting of photometric data) face the major challenge of contamination by low-redshift interlopers, including dusty galaxies at cosmic noon and brown dwarfs in the Milky Way. In this talk, I will present a robust sample of galaxies at redshift z=7, selected using a powerful combination of ground-based optical+near-infrared data from Subaru and VISTA, together with new data from Euclid. This combination is powerful at eliminating brown dwarf contaminants, providing a clean view of high-redshift galaxy populations. I will then explore Euclid’s ability to remove brown dwarfs where ancillary ground-based data is not available, and discuss how incorporating prior knowledge of the low-redshift Universe can improve the purity of high-redshift galaxy selections.
Dr Imogen Whittam
Evidence for inverse Compton scattering in high redshift Lyman break galaxies
Radio continuum emission provides a unique opportunity to study star-formation unbiased by dust. However, if radio observations are to be used to accurately trace star-formation to high redshifts, it is crucial that the physical processes which affect the radio emission from star-forming galaxies are well understood. While inverse Compton (IC) losses from the cosmic microwave background (CMB) are negligible in the local universe, the rapid increase in the strength of the CMB energy density with redshift [~(1+z)^4] means that this effect quickly becomes important at z >~3. In this talk I will describe recent work investigating IC scattering from high-redshift (3 < z < 5) Lyman break galaxies by stacking observations from the MIGHTEE radio survey. We find that the observed decrease in 1.4-GHz flux density with redshift is consistent with the theoretical predicted effect of energy losses due to IC scattering off the CMB. This is the first compelling observational evidence for inverse Compton scattering from the CMB in high-redshift star-forming galaxies.