Research Overview

“Somewhere, something incredible is waiting to be known.” ― Carl E. Sagan

Galaxies come in a variety of shapes, sizes, and colors — from blue, star-forming spirals to red, quenched ellipticals — and reside in a wide range of environments, from low-density voids to densely packed clusters. Just as the environments in which we grow up influence our life outcomes, galaxies are also significantly affected by their surroundings. Observations show that overdense regions of the cosmos — such as galaxy groups and clusters — can impact a galaxy's morphology, gas content, and even its ability to form stars, as shown in the right panel of the figure below. However, the physical mechanisms behind these observations are still poorly understood. My research, situated at the intersection of observations and simulations, is fundamentally focused on understanding the astrophysical processes that drive these environmental trends, with the goal of answering when, why, and how these differences arise.

The Star-Forming Main Sequence and Environmental Quenching
SFR vs Mstar
Caption: The left panel shows the star formation rate versus stellar mass for a subset of galaxies from the Sloan Digital Sky Survey (SDSS) DR7 at z < 0.1. The white line indicates the star-forming main sequence (SFMS), with shaded bands representing the 1σ and 2σ scatter. Galaxies populate the SFMS as well as regions above and below it, with their appearance — both in shape and color — correlating with their position relative to the sequence. The right panel shows that star-forming galaxies are less common in denser environments, which is an indication that dense cosmic environments are efficient at suppressing (or quenching) star formation. My research has focused on isolating the physical mechanisms that give rise to these environmental trends over a vast range of epochs, galaxy stellar masses, and cosmic environments.

Previous & Ongoing Research

Environmental Quenching of Dwarf Galaxies Beyond the Local GroupPrevious

dwarf satellite quenching

In Baxter et al. (2021), we combine supervised machine learning and statistical background subtraction to classify the star formation activity of dwarf galaxies around galaxy groups identified in the Sloan Digital Sky Survey. The figure above shows our main result, which is the inferred satellite quenched fraction, spanning three decades in stellar mass, for galaxies in groups at z<0.1. The key prediction is that the quenched fraction should turn over around 10^9 solar masses, indicating that the quenching becomes more efficient for low-mass dwarf galaxies.

Environmental Quenching in Massive Galaxy Clusters at z~1 Previous

satellite quenching timescales

In Baxter et al. (2022, 2023), we combine satellite quenched fractions from 14 galaxy clusters from the GOGREEN and GCLASS surveys with infall histories from the TNG-300 cosmological simulation to constrain how, when, and on what timescales these galaxies were quenched. The figure above shows one of our main results: the satellite quenching timescales for massive galaxies at z~1 are consistent with the expected total cold gas (atomic+molecular hydrogen) depletion timescale at this epoch, suggesting that "starvation" — the halting of star formation due to the lack of fresh cold gas — plays a key role in quenching massive galaxies in clusters at z>1.

Cosmic Evolution of Environmental Quenching in Groups Since z~0.8Previous

redshift evolution of satellite quenching timescale

In Baxter et al. (2025a) we present results from a multi-cycle Keck/DEIMOS survey targeting faint satellite candidates near spectroscopically confirmed galaxy groups at z~0.8. From this data we obtain the first spectroscopic measurement of the satellite quiescent fraction down to a stellar mass complete limit of ~10^9.5 solar masses at this redshift. Combing these measurements with the environmental quenching model developed in Baxter et al. (2022,2023), we find that the more than doubling of the satellite quenching timescale in groups since z~1, as shown in the figure above, can be explained by a decline in the quenching efficiency via starvation with decreasing redshift.

Quantifying the Impact of Incompleteness in Identifying and Interpreting Galaxy ProtoclustersOngoing

peak recovery fraction

In Baxter et al. (2025b), we use the TNG-Cluster simulation to quantify how stellar mass and star formation rate incompleteness affects the identification and interpretation of galaxy protoclusters. As shown in the plot above, we find that at z > 2, observationally limited samples recover the true region of highest density within the protocluster only 35–40% of the time within 1 proper Mpc (or 2–2.5 arcminutes). This high failure rate has important implications for density-based inferences in observed protoclusters. For example, we find that the highest-density regions are not always sites of accelerated mass growth or enhanced star formation — contrary to what is often inferred from observational studies.

First Author Peer-Reviewed Publications

Select Co-Author Peer-Reviewed Publications

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