Galaxy Formation and Evolution
Research in Crystal Martin's group focuses on the astrophysics of galaxy formation and evolution, with emphasis on feedback, star formation, and the baryon cycle across cosmic time.
Background
Galaxies are central to our understanding of the universe. They trace the geometry of space-time and give birth to stars and planets. Galaxy formation theory accurately describes the gravitational amplification of structure in the matter distribution over cosmic time. Unfortunately, most of the (dark) mass density in the universe is not normal baryonic material, so the theory does not directly predict the observable feature of galaxies: starlight. Research in Crystal Martin's group focuses on the astrophysics of galaxy formation and evolution, trying to understand in detail why the star formation rate varies widely among galaxies. Central to this work is the idea of feedback from supernova explosions, which inject energy, momentum, and heavy elements into the surrounding interstellar gas.
Galaxy Protoclusters as Drivers of Cosmic Reionization
Martin et al. (2026, ApJ 997:102) presents JWST/NIRSpec and Keck/LRIS spectroscopy of nine Ly-alpha emitters in the z~7 protocluster LAGER-z7OD1 to map reionization in 3D. Using Ly-alpha transmission and velocity offsets along multiple sight lines, the study identifies three ionized subregions where bubbles are near overlap or already beginning to percolate. Conservative line-of-sight estimates place bubble radii at roughly 0.07 to 0.69 physical Mpc, while corrections for interstellar attenuation imply larger effective ionized regions and in some cases fully overlapping bubbles. The analysis finds that five of the nine galaxies plausibly provide enough ionizing photons to power their local environments, with clumpy morphologies, bursty recent star formation, moderate-to-high O32, and relatively small Ly-alpha velocity offsets all consistent with efficient LyC leakage. Overall, the paper argues that overdense environments can accelerate reionization and that protocluster structure strongly shapes Ly-alpha transmission at z~7.
Resolving Mechanical and Radiative Feedback in J1044+0353 (Martin et al. 2024)
Martin, Peng, and Li (2024, ApJ 966:190) present KCWI integral-field spectroscopy of the local reionization-era analog J1044+0353. The work maps gas kinematics and ionization structure across the galaxy, finding a density-bounded ionization cone that provides an anisotropic pathway for Lyman-continuum escape. The analysis separates broad and very-broad nebular line components and interprets them as distinct feedback phases tied to superbubble breakthrough and blowout. Together with resolved HeII and [OIII] emission, the results argue that recent star-formation history and clustered feedback strongly regulate how ionizing photons escape compact starbursts.
Kinematics of Circumgalactic OVI Gas (Ho et al. 2026)
Ho et al. (2026, ApJ 998:261) analyze 18 galaxy-quasar pairs at z~0.2 to compare circumgalactic OVI kinematics with galaxy-disk rotation along major-axis sight lines. While individual OVI components do not show a tight one-to-one rotation correlation, the bulk OVI absorption in most sight lines is not counterrotating. OVI components that can be matched to low-ion components are more likely to corotate and tend to occur at smaller impact parameters, consistent with multiphase gas near extended disk planes in the inner CGM. The unmatched OVI components likely trace gas at larger radii where turbulence weakens rotational coherence.
Physical Origins of Outflowing Cold Clouds (Peng et al. 2025)
Peng et al. (2025, ApJ 981:171) use Keck/ESI spectroscopy of 14 local star-forming dwarf galaxies to examine the origin of cold outflow components. They distinguish broad and very-broad emission-line components and show that their scaling and energetics favor different physical channels: broad components are consistent with expanding superbubble shells, while very-broad components are more consistent with galactic-wind material. Pure direct cooling from hot winds underpredicts observed luminosities, and models with turbulent radiative mixing plus stellar photoionization better match the data. The paper also supports anisotropic LyC leakage through low-density channels created by feedback.
Feedback from Young, Metal-Poor Stars: From Local Analogs to the Reionization Era
Which galaxies reionized the universe, how they did it, and why the highest redshift galaxies are so luminous remain fundamental, unanswered questions about the history of the universe. The environment of galaxies must have been very different then than today. Yet low mass galaxies today have gas-phase metallicities similar to those in the reionization era; and, in extremely rare cases, are caught forming stars very efficiently, likely due to galaxy interactions. Studying the physical processes at play in these local analogues teaches us how to correctly interpret the extreme emission-line spectra of reionization-era galaxies and generates empirical scaling relations that can be employed in cosmological simulations.
Probing the Dynamic and Metal Content of Galactic Winds Through Absorption and Emission Lines
Recent advancements in modeling multiphase winds open the possibility of making direct comparisons between theoretical predictions and observations. However, developing this interface requires a deeper understanding of how the structure of outflows shapes the observed absorption and emission line profiles, both in integrated spectra and in maps obtained via integral field spectroscopy. Combining new high-resolution emission-line spectra of starburst galaxies with high S/N ratio spectroscopy of their ultraviolet absorption lines, we can understand the physical origins of the various line components through comparison to a series of increasingly realistic physical models.