About

Professor Mike Boylan-Kolchin is a theoretical astrophysicist who works on galaxy formation theory and its interface with cosmology. He works on a variety of questions related to dark matter, cosmological structure formation, and galaxy formation physics across cosmic time. His work combines numerical simulations, analytic models, and observations. His research encompasses cosmology and galaxy formation, including the formation, evolution, and structure of galaxies and dark matter halos, the nature of dark matter, near-field cosmology, high-redshift galaxy formation, globular cluster formation and evolution, and cosmological parameters and tensions. He is associated with the Weinberg Institute, Cosmic Frontier Center, and Texas Center for Cosmology and Astroparticle Physics.

Research topics

• Astrophysics
• Physics
• Astronomy
• Computer Science
• Optics
• Linguistics

Selected publications

• Rise of the Forsaken Relics: Connecting Present-day Stellar Streams and Phase-mixed Galaxies to the Epoch of Reionization

  The Astrophysical Journal · 2026-02-27

  preprintOpen accessSenior author

  Abstract The “near–far” approach to studying reionization leverages the star formation histories of the Milky Way (MW) or Local Group (LG) galaxies, derived from resolved photometry, to infer the low-mass/faint end of the stellar mass functions (SMFs) or the ultraviolet luminosity functions (UVLFs) of high-redshift galaxies ( z ≳ 6), beyond the current James Webb Space Telescope detection limits ( M UV ≳ −15). Previous works considered only intact low-mass galaxies in the MW and LG, neglecting disrupted galaxies such as stellar streams and phase-mixed objects. Using the FIRE-2 simulations, we show that these disrupted galaxies contribute up to ∼50% of the total stellar-mass budget of the proto-MW/LG at z = 6−9. Including all the progenitors of these disrupted galaxies improves the normalization of the recovered SMFs/UVLFs by factors of ∼2–3 and reduces the halo-to-halo variation in the slope by ∼20%–40%. This enables robust constraints down to at least the resolution limit of the simulations, near M ⋆ ∼ 10 5 M ⊙ or M UV ∼ −10 at z ≳ 6. We also show that “fossil-record” reconstructions—which assume each present-day system descends from a single reionization-era progenitor—are sensitive to the stellar-mass/UV-magnitude thresholds, which introduces bias in the inferred slopes at the low-mass/faint end. Additionally, we demonstrate that neglecting disrupted systems underestimates the contribution of galaxies with M UV ≲ −15 to the reionization-era UV luminosity density. Finally, we estimate that a significant fraction (∼50%) of streams with M ⋆ ≳ 10 6 M ⊙ at z = 0 should be detectable from upcoming Rubin Observatory and Roman Space Telescope observations.

  PublisherDOI

• Rise of the Forsaken Relics: Connecting Present-day Stellar Streams and Phase-mixed Galaxies to the Epoch of Reionization

  The Astrophysical Journal · 2026-02-27

  articleOpen accessSenior author

  Abstract The “near–far” approach to studying reionization leverages the star formation histories of the Milky Way (MW) or Local Group (LG) galaxies, derived from resolved photometry, to infer the low-mass/faint end of the stellar mass functions (SMFs) or the ultraviolet luminosity functions (UVLFs) of high-redshift galaxies ( z ≳ 6), beyond the current James Webb Space Telescope detection limits ( M UV ≳ −15). Previous works considered only intact low-mass galaxies in the MW and LG, neglecting disrupted galaxies such as stellar streams and phase-mixed objects. Using the FIRE-2 simulations, we show that these disrupted galaxies contribute up to ∼50% of the total stellar-mass budget of the proto-MW/LG at z = 6−9. Including all the progenitors of these disrupted galaxies improves the normalization of the recovered SMFs/UVLFs by factors of ∼2–3 and reduces the halo-to-halo variation in the slope by ∼20%–40%. This enables robust constraints down to at least the resolution limit of the simulations, near M ⋆ ∼ 10 5 M ⊙ or M UV ∼ −10 at z ≳ 6. We also show that “fossil-record” reconstructions—which assume each present-day system descends from a single reionization-era progenitor—are sensitive to the stellar-mass/UV-magnitude thresholds, which introduces bias in the inferred slopes at the low-mass/faint end. Additionally, we demonstrate that neglecting disrupted systems underestimates the contribution of galaxies with M UV ≲ −15 to the reionization-era UV luminosity density. Finally, we estimate that a significant fraction (∼50%) of streams with M ⋆ ≳ 10 6 M ⊙ at z = 0 should be detectable from upcoming Rubin Observatory and Roman Space Telescope observations.

  PublisherDOI

• Pickles on FIRE: The 3D Shape Evolution of Simulated Milky Way-Mass Galaxies

  Monthly Notices of the Royal Astronomical Society · 2026-05-21 · 1 citations

  articleOpen access

  Abstract We use reduced-mass eigentensors to quantify the 3D ellipsoidal shape evolution of thirteen Milky Way-mass galaxies simulated using zoom simulations with FIRE-2 physics; all but one form disks at z = 0. We find that all of our Milky Way progenitors go through phases when they are elongated. They often oscillate between spheroidal and elongated shapes in the early Universe over billion-year timescales, with ~25 − 45% of the population having elongated luminosity-weighted shapes at any given time at z = 0.5 − 8.5. In contrast, all stellar populations in our z = 0 Milky Way analogs are symmetric about their minor axes at z = 0, even though the old and intermediate-age stellar populations were often arranged in the shape of elongated pickles or triaxial spheroids at the time they formed meaning these populations changed shape significantly over time. During their transient elongated phases, our galaxies have anisotropic velocity dispersion ellipsoids directed along their spatial major axis; however, their shapes do not correlate with their dark matter fraction nor with the shapes and orientations of their underlying dark matter halos. We find that when treated as a population, the fraction of our galaxy progenitors that are elongated at z > 0.5 is roughly consistent with what is observed for systems of the same mass and redshift. Our results suggest that observed elongated galaxies seen in the early Universe with JWST and HST are not stable structures, but rather transitory phases that are nevertheless statistically common. Some of these observed objects may evolve into Milky Way-like galaxies at z = 0.

  PublisherDOI

• Confronting the Diversity Problem: The Limits of Galaxy Rotation Curves as a Tool to Understand Dark Matter Profiles

  The Astrophysical Journal · 2026-03-18

  articleOpen access

  Abstract While galaxy rotation curves (RCs) provide one of the most powerful methods for measuring dark matter profiles in the inner regions of rotation-supported galaxies, at the dwarf scale there are factors that can complicate this analysis. Given the expectation of a universal profile in dark-matter-only simulations, the diversity of the observed RCs has become an often-discussed issue in Lambda cold dark matter cosmology on galactic scales. We analyze a suite of Feedback in Realistic Environments simulations of 10 10 –10 12 M ⊙ halos with standard cold dark matter and compare the true circular velocity to RC reconstructions. We find that, for galaxies with well-ordered gaseous disks, the measured RC may deviate from the true circular velocity by at most ∼10% within the radius of the disk. However, nonequilibrium behaviors, noncircular motions, and nonthermal and nonkinetic stresses may cause much larger discrepancies, of ∼50% or more. Most RC reconstructions underestimate the true circular velocity, while some reconstructions transiently overestimate it in the central few kiloparsecs, due to dynamical phenomena. We further demonstrate that the features that contribute to these failures are not always visibly obvious in H I observations. If such dwarf galaxies are included in galaxy catalogs, they may give rise to the appearance of “artificial” RC diversity that does not reflect the true variation in underlying dark matter profiles.

  PublisherDOI

• <tt>Bloodhound</tt> unleashed: particle-based substructure tracking for cosmological simulations

  Monthly Notices of the Royal Astronomical Society · 2026-02-13

  articleOpen access

  ABSTRACT Modern studies of galaxy formation rely heavily on numerical simulations, which in turn require tools to identify and track self-bound structures in stars and dark matter. In this paper, we present Bloodhound, a new halo tracking algorithm optimized to track and characterize substructure in cosmological simulations, a regime that is crucial for studies of the nature of dark matter but where standard methods often have difficulties. Using simulations of Milky Way-mass haloes, we demonstrate that Bloodhound extends subhalo tracking by $3{\text{-}}4\, \mathrm{Gyr}$ on average, and significantly longer for subhaloes with small pericentres, relative to the widely used Rockstar $+$ consistent-trees halo tracking pipeline. We also show that Bloodhound provides continuous tracking, mitigating an issue for the standard technique where subhaloes can be lost and then found again – but assigned to a new merger tree – after several snapshots. This improved tracking leads to a substantially larger number of surviving subhaloes in the inner regions of dark matter haloes, which has several implications for studies of the Milky Way’s satellite galaxy system and its use for constraining properties of dark matter. For example, within the radius where current surveys are complete to ultra-faint galaxies ($D_{\rm MW} \lesssim 50$ kpc), Bloodhound finds more than twice as many subhaloes above the atomic cooling scale relative to the standard tracking method. Our results underscore the importance of robust subhalo tracking techniques in advancing our understanding of galaxy formation and cosmological models.

  PublisherDOI

• Dwarf Galaxy Number Counts within 25 Mpc: Predictions from Local Group Analogs in TNG50

  The Astrophysical Journal · 2026-05-20

  articleOpen access

  Abstract The modern generation of wide-field galaxy surveys, such as LSST, Euclid, and Roman, will enable studies of dwarf galaxies (10 6 ≤ M * / M ⊙ ≤ 10 9 ) beyond the Local Group (LG) in unprecedented detail. Improved theoretical understanding of this population is necessary to guide these observations, since predictions in this regime are generally limited to specific environments like the LG. We present predictions for the population of dwarf galaxies from the TNG50 run of the IllustrisTNG suite of cosmological hydrodynamical simulations, focusing on the environments within 1 &lt; D /Mpc &lt; 25 of LG analogs at z = 0. In the simulated sample, there are ∼1000 and ∼12,000 dwarf galaxies within 10 and 25 Mpc, respectively. We compare our results with the 50 Mpc Galaxy Catalog and estimate that current observations are highly incomplete at low masses: for 10 6 ≤ M * / M ⊙ ≤ 10 7 (−13 ≲ M r ≲ −10), we find completeness fractions of ∼23% within 10 Mpc and ∼4% within 25 Mpc. The simulated galaxies below the completeness limits of the observations exist in a range of environments, with notable populations of field dwarfs at all distances and satellites around centrals with masses 10 8 ≲ M * / M ⊙ ≲ 10 11 within 10–25 Mpc. We find that there are ∼8 times more quiescent dwarf galaxies in the TNG50 sample than are currently cataloged. Our results suggest that upcoming observations should uncover a substantial population of dwarf galaxies, and that ≳15% of these will be red, currently quenched galaxies in the field.

  PublisherDOI

• Bloodhound Unleashed: Particle-based Substructure Tracking for Cosmological Simulations

  Monthly Notices of the Royal Astronomical Society · 2026-02-13

  articleOpen access

  Abstract Modern studies of galaxy formation rely heavily on numerical simulations, which in turn require tools to identify and track self-bound structures in stars and dark matter. In this paper, we present Bloodhound, a new halo tracking algorithm optimized to track and characterize substructure in cosmological simulations, a regime that is crucial for studies of the nature of dark matter but where standard methods often have difficulties. Using simulations of Milky Way-mass haloes, we demonstrate that Bloodhound extends subhalo tracking by 3–4 Gyr on average, and significantly longer for subhaloes with small pericentres, relative to the widely used ROCKSTAR + consistent-trees halo tracking pipeline. We also show that Bloodhound provides continuous tracking, mitigating an issue for the standard technique where subhaloes can be lost and then found again — but assigned to a new merger tree — after several snapshots. This improved tracking leads to a substantially larger number of surviving subhaloes in the inner regions of dark matter haloes, which has several implications for studies of the Milky Way’s satellite galaxy system and its use for constraining properties of dark matter. For example, within the radius where current surveys are complete to ultra-faint galaxies (DMW ≲ 50 kpc), Bloodhound finds more than twice as many subhaloes above the atomic cooling scale relative to the standard tracking method. Our results underscore the importance of robust subhalo tracking techniques in advancing our understanding of galaxy formation and cosmological models.

  PublisherDOI

• How do Massive Primordial Black Holes Impact the Formation of the First Stars and Galaxies?

  The Astrophysical Journal · 2025-07-08 · 9 citations

  articleOpen access

  Abstract We investigate the impact of massive primordial black holes (PBHs; m BH ∼ 10 6 M ⊙ ) on the star formation and first galaxy assembly process using high-resolution hydrodynamical simulations from z = 1100 to z ∼ 9. We find that PBH accretion is self-regulated by feedback, suppressing mass growth unless feedback is weak. PBHs accelerate structure formation by seeding dark matter (DM) halos and gravitationally attracting gas, but strong feedback can delay cooling and suppress star formation. In addition, the presence of baryon-DM streaming creates an offset between the PBH location and the peaks induced in gas density, promoting earlier and more efficient star formation compared to standard ΛCDM. By z ∼ 10, PBH-seeded galaxies form dense star clusters, with PBH-to-stellar mass ratios comparable to observed high- z active galactic nuclei like UHZ-1. Our results support PBHs as viable supermassive black hole (SMBH) seeds but do not exclude alternative scenarios. We emphasize that PBH-seeding provides a natural explanation for some of the newly discovered overmassive SMBHs at high redshift, in particular those with extreme ratios of BH-to-dynamical (virial) mass that challenge standard formation channels. Future studies with ultra-deep JWST surveys, the Roman Space Telescope, and radio surveys with facilities such as the Square Kilometre Array and Hydrogen Epoch of Reionization Array will be critical in distinguishing PBH-driven SMBH growth from other pathways.

  PublisherDOI

• The Hubble Space Telescope Survey of M31 Satellite Galaxies IV. Survey Overview and Lifetime Star Formation Histories

  ArXiv.org · 2025-01-22

  preprintOpen access

  From $&gt;1000$ orbits of HST imaging, we present deep homogeneous resolved star color-magnitude diagrams that reach the oldest main sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies ($-6 \ge M_V \ge -17$) associated with the M31 halo, and for 10 additional fields in M31, M33, and the Giant Stellar Stream. From our SFHs we find: i) the median stellar age and quenching epoch of M31 satellites correlate with galaxy luminosity and galactocentric distance. Satellite luminosity and present-day distance from M31 predict the satellite quenching epoch to within $1.8$ Gyr at all epochs. This tight relationship highlights the fundamental connection between satellite halo mass, environmental history, and star formation duration. ii) There is no difference between the median SFH of galaxies on and off the great plane of Andromeda satellites. iii) $\sim50$\% of our M31 satellites show prominent ancient star formation ($&gt;12$ Gyr ago) followed by delayed quenching ($8-10$ Gyr ago), which is not commonly observed among the MW satellites. iv) A comparison with TNG50 and FIRE-2 simulated satellite dwarfs around M31-like hosts show that some of these trends (dependence of SFH on satellite luminosity) are reproduced in the simulations while others (dependence of SFH on galactocentric distance, presence of the delayed-quenching population) are weaker or absent. We provide all photometric catalogs and SFHs as High-Level Science Products on MAST.

  PublisherOA PDFDOI

• Central densities of dark matter haloes in <scp>fire-2</scp> simulations of low-mass galaxies with cold dark matter and self-interacting dark matter

  Monthly Notices of the Royal Astronomical Society · 2025-09-13 · 4 citations

  articleOpen access

  ABSTRACT We investigate the central density structure of dark matter haloes in cold dark matter (CDM) and self-interacting dark matter (SIDM) models using simulations that are part of the Feedback In Realistic Environments (fire) project. For simulated haloes of dwarf galaxy scale ($M_{\rm halo}(z=0)\approx 10^{10}\, \mathrm{ M}_\odot$), we study the central structure in both dissipationless simulations and simulations with full fire-2 galaxy formation physics. As has been demonstrated extensively in recent years, both baryonic feedback and self-interactions can convert central cusps into cores, with the former process doing so in a manner that depends sensitively on stellar mass at fixed $M_{\rm halo}$. Whether the two processes (baryonic feedback and self-interactions) are distinguishable, however, remains an open question. Here we demonstrate that, compared to feedback-induced cores, SIDM-induced cores transition more quickly from the central region of constant density to the falling density at larger radial scales. This result holds true even when including identical galaxy formation modelling in SIDM simulations as is used in CDM simulations, since self-interactions dominate over galaxy formation physics in establishing the central structure of SIDM haloes in this mass regime. The change in density profile slope as a function of radius therefore holds the potential to discriminate between self-interactions and galaxy formation physics as the driver of core formation in dwarf galaxies.

  PublisherOA PDFDOI

Recent grants

• Collaborative Research: Starless Dark Matter Halos as a Definitive Test of Dark Matter Models

  NSF · $363k · 2019–2025

• Collaborative Research: Hydrodynamic Simulations of the Local Group

  NSF · $248k · 2015–2019

• CAREER: The Faint Frontier of Galaxy Formation

  NSF · $709k · 2018–2024

• Collaborative Research: Constraining Fuzzy Dark Matter with Cosmological Simulations

  NSF · $285k · 2021–2025

Frequent coauthors

• James S. Bullock

  123 shared

• Andrew Wetzel

  117 shared

• Philip F. Hopkins

  California Institute of Technology

  70 shared

• Claude‐André Faucher‐Giguère

  65 shared

• Daniel R. Weisz

  University of California, Berkeley

  60 shared

• Dušan Kereš

  43 shared

• Christopher C. Hayward

  Flatiron Institute

  41 shared

• Shea Garrison-Kimmel

  California Institute of Technology

  41 shared

Education

• Ph.D., Physics

  University of California Berkeley

  2006