Astrophysical Applications of Gravitational Lensing: Nature of Dark Matter and Galaxies in the early Universe

Dark Matter (DM) dominates the mass of the Universe, such that visible galaxies assemble around DM halos, and clusters of galaxies are immersed in and held together by DM. What, however, is DM? When did gas in DM halos turn into stars, giving birth to visible galaxies, and what were the physical properties of galaxies in the early Universe? These outstanding questions intersect when we utilize gravitational lensing to probe the physical properties of DM, as well as to magnify and hence make possible the detection and study of distant galaxies.

Over the past 5 years, we have trained a growing pool of talented students at HKU — starting early during their undergraduate studies before undertaking undergraduate research projects, and then progressing onto graduate studies — in various astrophysical applications of gravita-tional lensing, a number of which we pioneered. Our work makes use of the best data from the Hubble Space Telescope that employ massive galaxies or galaxy clusters as cosmic lenses.

(I) Structure of DM . In fulfillment of the 2016 RGC/GRF proposal, we have made lens models — which largely capture the structure of DM — for all six galaxy clusters in the Hubble Frontier Fields (HFF) program, and also derived lens models for clusters of interest in the Cluster Lens-ing And Supernova survey with Hubble (CLASH) program. Key measures for the fidelity of our lens models are their ability to reproduce the sky positions, distorted morphologies, and red-shifts of multiply-lensed images (confirming the internal consistency of the model), as well as the relative brightnesses of counter-images (which are not used as constraints on, and therefore constitute predictions of, the model); and, in the case of the first multiply-lensed supernova, to correctly predict its reappearance in both sky position and time. We plan to extend our work to the Reionization Cluster Lensing Survey (RELICS) with Hubble program.

(II) Galaxies in the early Universe. Even with lensing, identifying star-forming galaxies in the early Universe is made difficult by their similar spectral energy distributions (SEDS) with passive elliptical galaxies at intermediate redshifts. In lieu of spectroscopic redshifts (often im-practical given object faintness), redshifts derived purely through lensing geometry (geometric redshift) can resolve ambiguities in and potentially provide more accurate redshifts than those derived from SEDS (photometric redshifts). We plan to continue our pioneering work in this area by determining geometric redshifts for distant galaxies in the RELICS program.

(III) Weighing supermassive black holes (SMBHS). Do galaxies and their SMBHS co-evolve? Addressing this question requires knowledge of SMBH masses spanning cosmic history. We have demonstrated how gravitational lensing can be used to directly measure SMBH masses in distant galaxies, and plan to expand upon this work as suitable new examples arise from the RELICS and other surveys.

(IV) Observational tests of DM. We showed that the space density of distant lensed galax-ies in the HFF program does not appear to rise monotonically towards lower luminosities, in better agreement with the predictions of Wavelike DM (IVDM) rather than Cold DM (CDM), and determine a boson mass similar to that inferred to reproduce the cores of DM-dominated spheroidal galaxies in the Local Group. We also found that the number density of luminous galaxies suddenly ramps up towards more recent epochs close to the midpoint epoch of reion-ization, suggesting that luminous galaxies were responsible for reionizing the Universe. We plan to expand upon this work by using a larger sample of distant galaxies discovered in the RELICS program.

(V) Granulation of Wavelike DM. A unique property of wavelike DM is its very strong and fine granulation in density resulting from the quantum interference of its constituent ultralight bosons. We describe our pioneering work to theoretically predict the resulting perturbations in lensing magnification on small spatial scales, and the observational experiments we are carrying out to test this prediction through lensing by massive galaxies and galaxy clusters.