Flashlights: More than A Dozen High-Significance Microlensing Events of Extremely Magnified Stars in Galaxies at Redshifts z=0.7-1.5

Patrick L. Kelly, Wenlei Chen, Amruth Alfred, Thomas J. Broadhurst, Jose M. Diego, Najmeh Emami, Alexei V. Filippenko, Allison Keen, Sung Kei Li, Jeremy Lim, Ashish K. Meena, Masamune Oguri, Claudia Scarlata, Tommaso Treu, Hayley Williams, Liliya L. R. Williams, Rui Zhou, Adi Zitrin, Ryan J. Foley, Saurabh W. Jha, Nick Kaiser, Vihang Mehta, Steven Rieck, Laura Salo, Nathan Smith, Daniel R. Weisz

Submitted on 4 November 2022


Once only accessible in nearby galaxies, we can now study individual stars across much of the observable universe aided by galaxy-cluster gravitational lenses. When a star, compact object, or multiple such objects in the foreground galaxy-cluster lens become aligned, they can magnify a background individual star, and the timescale of a magnification peak can limit its size to tens of AU. The number and frequency of microlensing events therefore opens a window into the population of stars and compact objects, as well as high-redshift stars. To assemble the first statistical sample of stars in order to constrain the initial mass function (IMF) of massive stars at redshift z=0.7-1.5, the abundance of primordial black holes in galaxy-cluster dark matter, and the IMF of the stars making up the intracluster light, we are carrying out a 192-orbit program with the Hubble Space Telescope called "Flashlights," which is now two-thirds complete owing to scheduling challenges. We use the ultrawide F200LP and F350LP long-pass WFC3 UVIS filters and conduct two 16-orbit visits separated by one year. Having an identical roll angle during both visits, while difficult to schedule, yields extremely clean subtraction. Here we report the discovery of more than a dozen bright microlensing events, including multiple examples in the famous "Dragon Arc" discovered in the 1980s, as well as the "Spocks" and "Warhol" arcs that have hosted already known supergiants. The ultradeep observer-frame ultraviolet-through-optical imaging is sensitive to hot stars, which will complement deep James Webb Space Telescope infrared imaging. We are also acquiring Large Binocular Telescope LUCI and Keck-I MOSFIRE near-infrared spectra of the highly magnified arcs to constrain their recent star-formation histories.


Subjects: Astrophysics - Cosmology and Nongalactic Astrophysics; Astrophysics - Solar and Stellar Astrophysics


The survey will observe each of six HFF galaxy clusters at two epochs. During each \numorbit-orbit visit, imaging alternates between orbits with integrations in WFC3 F200LP and WFC3 F350LP. The full throughputs of ACS WFC CLEAR and WFC3 F200LP as well as the SEDs of two stars at $z=1$ are plotted here. The differences between the two filters provides constraints on the SED (including the reddening) of each star.