Casey Lam

4 letter mollusk AT carnegiescience DOT edu*
*Insert appropriate mollusk constructed using my first initial + last name. This is my attempt at foiling spambots.

About me

I am a Harrison/Carnegie Postdoctoral Fellow at the Carnegie Observatories in Pasadena, CA.

I earned my PhD Summer 2023 from the Department of Astronomy at UC Berkeley. My thesis, advised by Professor Jessica Lu, was focused on understanding the Galactic black hole population with gravitational microlensing.

I earned my bachelor's degree in math and physics (and also a pirate's license) from MIT in 2017.

A PDF of my CV (current as of August 2023) can be found here.

Current research projects and interests

I am generally interested in the Galactic population of stellar-mass black holes.

Finding and characterizing black hole + star binaries with radial velocities and astrometry

In collaboration with Josh Simon (Carnegie OBS), Kareem El-Badry (Caltech), Nidia Morrell (LCO), Jessica Lu (UC Berkeley), and Howard Isaacson (UC Berkeley), we are conducting several radial-velocity surveys targeting a variety of stars using Magellan/MIKE (Las Campanas Observatory) and APF/Levy (Lick Observatory).

In collaboration with Kyle Kremer (UCSD) and Mario Mateo (Michigan), we are conducting a radial-velocity survey of several globular clusters using Magellan/IFU-M (Las Campanas Observatory).

With Kyle, Mattia Libralato (INAF), Andrea Bellini (STScI), and Kareem, we have been awarded 10 orbits of HST/WFC3-UVIS to measure the masses of two black holes in NGC 3201 by mapping their astrometric orbital motion.

Black hole microlensing with the Roman Space Telescope

The Nancy Grace Roman Space Telescope is NASA's next flagship mission, scheduled for launch October 2026. One of Roman's mission objectives is to perform a census of cold and free-floating exoplanets via the Galactic Bulge Time Domain Survey with microlensing. This survey is will also be an excellent way to detect a large sample of free-floating black holes. However, the optimal survey design for making black hole detections likely differs from that for making exoplanets detections. I am interested in designing a survey that would enable the detection and characterization of both exoplanets and black holes.

In a white paper I led, we advocate to include the study of isolated black holes as part of the Galactic Bulge Time Domain Survey.

I am also interested in what Roman can do in conjunction with other observatories (e.g. Rubin Observatory's LSST and more).

Summary of 1st author publications

A Fast, Analytic Empirical Model of the Gaia Data Release 3 Astrometric Orbit Catalog Selection Function

Two black hole + star binaries have been discovered in Gaia DR3's catalog of astrometric orbits. However, converting these detections into population constraints is difficult because the selection function of the catalog is not well understood. We use a combination of analytic and empirical prescriptions to construct a function that computes the probability that a binary with a given set of properties would have been published in the Gaia Data Release 3 astrometric orbit catalog.

View the pre-print (submitted to ApJ) on arXiv: ADS link

Also see the companion paper (El-Badry, Lam, et al. 2024, OJAp accepted): ADS link

An isolated black hole detected with astrometric microlensing

By combining ground-based photometry from the OGLE and MOA microlensing surveys with archival HST follow-up observations, we measure the lens masses of five microlensing black hole candidates. In addition, we identify the nature of the lenses (whether brown dwarf, star, white dwarf, neutron star, or black hole). We then constrain the number of black holes in the Milky Way. In particular, we find that one of the candidates (OGLE-2011-BLG-0462/MOA-2011-BLG-191, aka OB110462) is possibly a neutron star or mass-gap black hole; the remaining candidates are not black holes. Finding 0 or 1 black holes in this sample is consistent with 100 million black holes in the Milky Way.

Get the published papers here: Astrophysical Journal Letter, companion Astrophysical Journal Supplement

In a follow-up study, we re-analyze OB110462 with both new and re-reduced data, as well as updated analysis tools, to verify its nature. We find that OB110462 is a 6 solar mass black hole, and that the mass in the previous study had been underestimated. This is due to a bright nearby star that induces a positional bias in the measured astrometry of OB110462. Our previous method of measuring the bias underestimated its true value. With the improved determination of the positional bias, we measure a different astrometric signal than before. Modeling the updated astrometry leads to the updated higher mass for OB110462.

Get the published paper here: Astrophysical Journal

PopSyCLE (Population Synthesis for Compact object Lensing Events)

I was the primary developer for PopSyCLE v1.0.0, a Milky Way microlensing simulation code. PopSyCLE can be used to simulate microlensing surveys in the Milky Way, as well as identifying optimal strategies for selecting black hole candidates for astrometric follow up. In particular, we present an new selection criteria to photometrically identify black hole candidates via their small microlensing parallaxes.

Get the published paper (Astrophysical Journal) here.

The code repository for the simulation can be found on GitHub. PopSyCLE is still being actively developed and we welcome contributions.

Full publication record

My full publication record can be found on ADS, ORCID, or arXiv.

A summary and full list (current as of August 2023) can be found in my CV.

Following in the tradition of other great academic webpages, here is an outdated photo of myself.