0 Assigned ID: 2143 ----- Attempting Submission 1 (Fri Apr 09 19:28:03 GMT 2021) ----- HST Phase I Proposal 2143 successfully submitted. Receipt: # 2143-1 ----- Attempting Submission 2 (Fri Apr 09 23:25:45 GMT 2021) ----- Revised visit schedule to account for a past failed visit. -1582990170 ----- Attempting Submission 1 (Wed Jul 28 23:35:30 GMT 2021) ----- ----- Attempting Submission 2 (Wed Oct 18 04:29:51 GMT 2023) ----- ----- Attempting Submission 3 (Tue Oct 31 19:49:19 GMT 2023) ----- ----- Attempting Submission 4 (Fri Jul 26 19:34:39 GMT 2024) ----- ----- Attempting Submission 5 (Thu Feb 20 18:05:58 GMT 2025) ----- Hunting for Black Holes with Astrometric Microlensing Although there are likely 10^7 - 10^9 stellar mass black holes (BHs) in the Milky Way, only a handful have been detected in the Universe, all in X-ray binaries or BH-BH mergers; no isolated BHs have been definitively detected. A census of isolated BHs will provide important constraints on stellar evolution, the Milky Way BH mass function, supernovae physics, and BH/neutron star formation. Gravitational microlensing is ideal for finding isolated BHs, as properties of the lens can be inferred from changes in the brightness and position of a background star. By combining both the photometric and astrometric microlensing signal, we can directly determine the lens mass. We propose to use HST to measure the astrometric shift of 4 likely BH microlensing candidates. Combined with other ongoing measurements, this will allow us to constrain the number of BHs in the Milky Way to better than 50%, a major improvement over the current orders of magnitude uncertainty. This will also enable the first constraints on the Milky Way BH mass function, binarity, and kick velocities. Four black hole candidates will be selected from photometric microlensing surveys for follow-up astrometric observations with HST WFC3-UVIS. The 4 targets will be non-disruptive ToOs and, once selected, 5 HST measurements will be obtained spread as evenly as possible over Cycle 29. The targets will be changing brightness, thus exposure times and number of exposures will be adjusted prior to each new observation. Further astrometric follow-up will be obtained in Cycles 30 and 31. Each target observation consists of >5 dither positions (to maximize pixel phase coverage and thus astrometric precision) in a single orbit. The exposure times will be chosen in order to not saturate the source star or any bright neighbor stars within a few arcseconds. We will observing using 2k x 2k sub-array mode as the astrometric reference stars are those closest to the target and we would like to minimize the astrometric impacts of CTE. In order to maximize observability and time sampling throughout the year, we have placed no restrictions on roll angle. Prof. J. Lu has expertise in high-precision astrometry both from ground-based adaptive optics and HST observations. Prof. Lu has used high-precision astrometry to study resolved stellar populations throughout the Milky Way, including showing evidence for a top-heavy IMF in the massive, young clusters in the Galactic Center and disk and conducting the first search for astrometric microlensing signals by black holes from the ground. Dr. Will Dawson leads the PALS project, using DECam and Subaru HyperSuprimeCam to search for intermediate mass and primordial black holes. Dr. Dawson has extensive experience with gravitational lensing of galaxies as well. Dr. Nathan Golovich is also a member of the PALS project and has experience developing microlensing modeling codes and fitting thousands of microlensing events. Dr. Peter Nugent is a member of the Zwicky Transient Facility (ZTF) and is an expert in supernovae and other transient phenomena as well as high-performance computing. Nugent and Lu co-advise UC Berkeley graduate student Michael Medford who is searching for photometric microlensing signals in ZTF in order to increase the number of BH candidates. Lu also advises UC Berkeley graduate student Casey Lam, who has built the population synthesis codes. Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 6 false false false false false false SUPPORTED STELLAR CLUSTER BULGE GRAVITATIONAL LENS NO J2000 J2000 I-Mag 18.37 WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F814W /Users/mhuston/Documents/APT/OP-cache/hst16985-v01.xml Timing link suitability includes constraints ([02 After 01 by 30.0 to 180.0 days]) The scheduling data was generated by CASM 33.0. CASM was run in Phase II mode. 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