samuel.mccarty@cfa.harvard.edu
Office A-102
60 Garden Street, MS-10
Cambridge, MA 02138
Second year PhD student in Astronomy & Astrophysics at the Center for Astrophysics | Harvard & Smithsonian
I am an astrophysicist broadly interested in extragalactic astronomy and cosmology (i.e. the big stuff). I've recently earned a B.S.
magna cum laude with honors in Physics and Astronomy from the University of Washington (Go Dawgs!),
and started a PhD in Astronomy & Astrophysics at the Center for Astrophysics | Harvard & Smithsonian in Fall 2025.
At Harvard, I use Fast Radio Bursts to study the diffuse and difficult-to-detect gas that makes up >90% of the ordinary matter in the Universe.
This gas fills the space around and between galaxies, and is crucial for understanding galaxy formation and cosmology.
During my undergrad, I worked in several areas including projects focused on
the intergalactic medium with Matthew McQuinn, the circumgalactic medium with Jessica Werk, and exoplanet biosignatures with Victoria Meadows.
I spent a summer at Caltech learning about radio astronomy and strong gravitational lensing with Liam Connor, which led to my senior thesis.
Liam is now my PhD advisor.
Aside from research, I have been involved in several outreach and mentoring activities and will continue to give back to my
new community in Boston during grad school. My hobbies include hiking and being outside, going to the gym, rock climbing, and spending time with my amazing partner Kate.
Research
Tracing cosmic baryons with Fast Radio Bursts
McCarty, S., Connor, L., & Konietzka, R. (2026a). The CGM with local universe FRBs: evidence of strong AGN feedback in a massive elliptical galaxy.
McCarty, S. et al. (2026b). A new measurement of the FRB DM-galaxy cross correlation and a first joint analysis with the kinematic SZ effect.
Most of the baryonic matter in the universe exists in diffuse, hard to measure gas around and between galaxies.
This is the circumgalactic, intragroup, intracluster and intergalactic medium. We know surprisingly little about this gas, despite the fact that it is
crucial for the process of galaxy formation and evolutuion, ultimately supplying the gas that forms stars and planets. It is further becoming a leading nuisance to
cosmological measurements, and next generation cosmological surveys such as by Rubin may be limited by our understanding of this gas.
Fast radio bursts (FRBs) are a promising new probe this gas. In McCarty 2026a, I used local universe FRBs to
study the circumgalactic medium (CGM) of the Milky Way (MW) and relatively nearby galaxies. The key idea is that nearby FRBs travel through less of the cosmic web. The dispersion measure (DM)
of nearby FRBs will be dominated by the MW and the host galaxy, rather than the intergalactic medium. Because the MW contribution is known (sort of), this allows us to isolate the CGM of the host galaxy. See the pretty diagram below.
What we found is that the halos of the host galaxies in our sample contain much less gas than would be expected from a Universe with only gravity. Therefore baryonic processes like exploding stars and super massive black hole jets are lifting the gas out of the halos of these galaxies. Astrophysicists call this "feedback". One galaxy in particular, a massive elliptical with very little active star formation, has a halo that is almost completely devoid of gas. This type of galaxy is a prime candidate for feedback from an active galactic nucleus (AGN).
While these types of targeted studies are insightful for individual systems, what we would like is a census of many galaxies. In McCarty 2026b, I used a sample of ~100 localized FRBs to measure the cross correlation between DM and galaxies from the Dark Energy Spectroscopic Instrument (DESI) survey. We measure this statistic at the highest signifigance to date and analyze our measurement in the context of cosmological simulations and theoretical models. We again find strong evidence that the gas in galactic halos is expelled by baryonic feedback. This is a pioneering measurement that will be improved with the next generation of FRB surveys, such as the Deep Synoptic Array Chronoscope, which will localize thousands of FRBs per year.
Radio Strong Lensing
McCarty, S., & Connor, L. (2025). Strong gravitational lensing with upcoming wide-field radio surveys.
Strong gravitational lensing has important astrophysical and cosmological applications. Strong lensing science at radio wavelengths has distinct advantages,
but has been limited for decades by small sample size. In McCarty and Connor 2025, I forescasted the number of strong lenses expected to be discovered by the upcoming DSA-2000 and SKA-mid radio telescopes,
finding that they will each discover as many as 105 new lenses. In Wu et al. (in prep), I demonstrated that emerging deep learning radio imaging algorithms
will enable the discovery of lens systems with Einstein radii at or below the instrument PSF limit, potentially increasing the yield of galaxy scale lenses by an
order of magnitude. I have just been awarded 16 hours of VLA time to follow up promising lens candidates we have
identified in the Very Large Array Sky Survey. These radio lenses will provide immediate science gains but also help pave the way for lensing science with the next
generation of radio telescopes.
