University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Monday, November 25th 2019
12:15 pm:
Speaker: Patrick Kelly, UMN
Subject:  Flashlights: Many Extremely Magnified Individual Stars as Probes of Dark Matter and Stellar Populations to Redshift z~2

A total of four bright microlensing events (<26.5 mag AB) of individual luminous stars at redshift z=1-2 have recently been discovered in Hubble Space Telescope (HST) imaging of galaxy-cluster gravitational lenses. The frequency of these events appears to be greater than expected, and a sufficiently elevated rate of events would directly imply a component (even 1-2% consisting of compact objects) of dark matter that is not a weakly interacting massive particle. Galaxy-cluster fields also highly magnify the galaxies responsible for the reionization of the universe, yet lens models cannot accurately identify the locations of the greatest magnification along the critical curve. Finally, we lack direct measurements of the luminous stellar population at z=1-2.

I will discuss the new HST program that will obtain two epochs of ultra-deep, unfiltered imaging of the six Hubble Frontier Field galaxy clusters fields. The observations, with a single-visit five-sigma limiting magnitude of 31 AB, can be expected to increase the current sample of microlensing events by a factor of order twenty, and detect a similar number of pairs of stellar images. If only 2% of dark matter consists of primordial black holes, then we will detect five times as many events. The pairs of stellar images will stringently test lens models, probe the abundance of low-mass dark-matter halos, and evaluate predictions for dark matter as ultra-light bosons. The relative rates of events across lensed galaxies will measure variation in their stellar luminosity functions. The proposed imaging will, finally, identify low-redshift contaminants in James Webb Space Telescope Lyman-break galaxy samples,
and detect ultra-faint galaxies.

The weekly calendar is also available via subscription to the physics-announce mailing list, and by RSS feed.