About

Volker Bromm is a Professor of Astronomy at the University of Texas, holding the Jane and Roland Blumberg Centennial Professorship in Astronomy and the Josey Centennial Professorship in Astronomy. His research focuses on the formation of the first stars and quasars, high-redshift supernovae and metal enrichment, supermassive black hole formation, gamma-ray bursts, reionization of the intergalactic medium, and present-day star formation, utilizing computational astrophysics. Bromm earned his Ph.D. from Yale University in 2000 and his M.Sc. from the University of Heidelberg in Germany in 1993. He has made significant contributions to understanding the formation of the first stars and galaxies, including the fragmentation of primordial gas, the primordial star-forming cloud, and the contribution of the first stars to the cosmic infrared background. Bromm has also explored the formation mechanisms of brown dwarfs, the expected redshift distribution of gamma-ray bursts, and the formation of the first supermassive black holes. His work is recognized through numerous awards, including the University of Texas System Regents’ Outstanding Teaching Award in 2009, the College of Natural Sciences Teaching Excellence Award in 2008, and the Robert J. Trumpler Award in 2002.

Research topics

• Astronomy
• Astrophysics
• Physics

Selected publications

• Two Exciting High-redshift Galaxy Candidates Turn Out to Be Two Exciting Ultra-cool Brown Dwarfs

  arXiv (Cornell University) · 2026-04-26

  preprintOpen access

  From the onset of observations of JWST we have discovered unexpectedly luminous galaxies at redshifts $z>10$ and as high as $z=14$. With their discovery, the question immediately followed as to where their progenitors are, since such progenitors should be within reach of existing surveys. However, the discovery of several bright candidates at $z>15$ may indicate further discrepancies between pre-JWST model predictions and current observations. Progenitors of the bright $z\sim 14$ galaxies should be visible at redshifts as high as $z\sim 20$--$30$, showing in the data as F356W and F277W dropouts. We identify two such candidates in the Bullet Cluster JWST data; however, subsequent NIRSpec follow-up data show spectra that can be well fit with ultra-cool Y dwarf templates with temperatures $T_{\rm eff} = 350^{+110}_{-80}\,\mbox{K}$ and $T_{\rm eff} = 410^{+110}_{-50}\,\mbox{K}$ and distances of $\sim 500\,\mbox{pc}$. The first is one of the lowest temperature brown dwarfs known spectroscopically. With additional NIRCam imaging taken $\sim 1$ year later, we also detect their proper motions of $(49 \pm 8)\,\mbox{mas/yr}$ and $(24 \pm 3)\,\mbox{mas/yr}$, further indicating that at least some F277W and F356W dropouts are sub-stellar cold Milky Way objects such as brown dwarfs. We find a sky density of 0.14 Y dwarfs per arcmin$^2$ and caution that the probability of detecting such objects may increase significantly in surveys at low galactic latitudes.

  PublisherDOI

• Hunting for the First Explosions at the High-Redshift Frontier

  ArXiv.org · 2026-01-05

  articleOpen access

  The James Webb Space Telescope (JWST) has spectroscopically confirmed galaxies up to $z\sim14$, 300 Myr after the Big Bang, and several candidates have been discovered at $z\sim15-25$, with one candidate as high as $z\sim30$, only 100 Myr after the Big Bang. Such objects are unexpected, since theoretical studies have not predicted the existence of detectable galaxies at $z\sim30$. While any $z\sim30$ candidates may be contaminants at lower redshifts, we explore whether such extreme redshift sources could be consistent with hyper-energetic transient events linked to the formation of the first, metal-free, stars. Specifically, we consider pair-instability supernovae (PISNe), a predicted class of extreme thermonuclear explosions that leave no remnant behind. Using cosmological simulations, we investigate an overdense cosmic region, where star formation and subsequent PISNe occur at $z\sim30-40$, even within standard cosmology. Assessing the likelihood of such a region, the corresponding number of PISNe at $z\gtrsim20$, and their observed flux, we find that JWST has a non-negligible chance to detect a PISN event at extremely high redshifts. If a transient event were confirmed at $z\sim30$, this would provide a direct glimpse into the epoch of first star formation, dramatically extending the empirical reach of astronomy.

  PublisherOA PDF

• Hunting for the First Explosions at the High-Redshift Frontier

  arXiv (Cornell University) · 2026-01-05

  preprintOpen access

  The James Webb Space Telescope (JWST) has spectroscopically confirmed galaxies up to $z\sim14$, 300 Myr after the Big Bang, and several candidates have been discovered at $z\sim15-25$, with one candidate as high as $z\sim30$, only 100 Myr after the Big Bang. Such objects are unexpected, since theoretical studies have not predicted the existence of detectable galaxies at $z\sim30$. While any $z\sim30$ candidates may be contaminants at lower redshifts, we explore whether such extreme redshift sources could be consistent with hyper-energetic transient events linked to the formation of the first, metal-free, stars. Specifically, we consider pair-instability supernovae (PISNe), a predicted class of extreme thermonuclear explosions that leave no remnant behind. Using cosmological simulations, we investigate an overdense cosmic region, where star formation and subsequent PISNe occur at $z\sim30-40$, even within standard cosmology. Assessing the likelihood of such a region, the corresponding number of PISNe at $z\gtrsim20$, and their observed flux, we find that JWST has a non-negligible chance to detect a PISN event at extremely high redshifts. If a transient event were confirmed at $z\sim30$, this would provide a direct glimpse into the epoch of first star formation, dramatically extending the empirical reach of astronomy.

  PublisherDOI

• Primordial Black Holes as Seeds for Extremely Overmassive Active Galactic Nuclei Observed by JWST

  The Astrophysical Journal Letters · 2026-03-16 · 3 citations

  articleOpen access

  Abstract The James Webb Space Telescope (JWST) has recently identified A2744–QSO1 as a compact, metal-poor, black hole (BH)–dominated galaxy at z ≃ 7. This system exhibits an extreme black-hole-to-stellar mass ratio and unusually low metallicity, posing significant challenges to BH seeding models. Motivated by these discoveries, we perform high-resolution cosmological simulations with a massive primordial black hole (PBH; M BH = 5 × 10 7 M ⊙ ) seed, incorporating for the first time a fully coupled treatment of PBH accretion, BH feedback, and Population III/II star formation and stellar feedback. Although PBHs accelerate structure formation through the seed effect, the associated strong thermal feedback from the accretion delays the onset of star formation to z ≲ 10, producing short, bursty episodes throughout the subsequent evolution. PBH-driven outflows expel enriched gas from the nucleus, while sustained inflows from the intergalactic medium continuously replenish pristine material. This feedback-regulated cycle naturally yields low accretion rates ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mover accent="true"> <mml:mi>m</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> <mml:mi>BH</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mover accent="true"> <mml:mi>m</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> <mml:mi>edd</mml:mi> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> <mml:mo>%</mml:mo> <mml:mo>−</mml:mo> <mml:mn>10</mml:mn> <mml:mo>%</mml:mo> </mml:math> ), subsolar metallicities ( Z / Z ⊙ ≲ 10 −2 ), and extreme M BH / M ⋆ ratios during both the initial star-forming phase and the subsequent quenching phases, in excellent agreement with JWST observations. Our results demonstrate that massive PBHs offer a viable pathway for forming the most extreme high-redshift systems, providing a physically motivated explanation for the extraordinary properties of A2744–QSO1, as a subclass of the broader population of JWST-discovered “little red dots.”

  PublisherDOI

• Hunting for the First Explosions at the High-redshift Frontier

  The Astrophysical Journal · 2026-04-01 · 1 citations

  articleOpen access

  Abstract The James Webb Space Telescope (JWST) has spectroscopically confirmed galaxies up to z ∼ 14, 300 Myr after the Big Bang, and several candidates have been discovered at z ∼ 15–25, with one candidate as high as z ∼ 30, only 100 Myr after the Big Bang. Such objects are unexpected, since theoretical studies have not predicted the existence of detectable galaxies at z ∼ 30. While any z ∼ 30 candidates may be contaminants at lower redshifts, we explore whether such extreme redshift sources could be consistent with hyperenergetic transient events linked to the formation of the first, metal-free, stars. Specifically, we consider pair-instability supernovae (PISNe), a predicted class of extreme thermonuclear explosions that leave no remnant behind. Using cosmological simulations, we investigate an overdense cosmic region, where star formation and subsequent PISNe occur at z ∼ 30–40, even within standard cosmology. Assessing the likelihood of such a region, the corresponding number of PISNe at z ≳ 20, and their observed flux, we find that JWST has a nonnegligible chance of detecting a PISN event at extremely high redshifts. If a transient event were confirmed at z ∼ 30, this would provide a direct glimpse into the epoch of first star formation, dramatically extending the empirical reach of astronomy.

  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 Emerging Black Hole Mass Function in the High-redshift Universe

  The Astrophysical Journal · 2025-07-17 · 10 citations

  articleOpen accessSenior author

  Abstract Observations with the James Webb Space Telescope (JWST) have identified an abundant population of supermassive black holes (SMBHs) already in place during the first few hundred million years of cosmic history. Most of them appear overmassive relative to the stellar mass in their host systems, challenging models of early black hole seeding and growth. Multiple pathways exist to explain their formation, including heavy seeds formed from direct collapse/supermassive stars or sustained super-Eddington accretion onto light stellar remnant seeds. We use the semianalytical code Ancient Stars and Local Observables by Tracing Halos to predict the emerging SMBH mass function under physically motivated models for both light- and heavy-seed formation, to be compared with upcoming ultradeep JWST surveys. We find that both pathways can reproduce observations at z ∼ 5–6, but have distinct features at higher redshifts of z ∼ 10. Specifically, JWST observations have the potential to constrain the fraction of efficiently accreting (super-Eddington) SMBHs, as well as the existence and prevalence of heavy seeds, in particular through ultradeep observations of blank fields and/or gravitational lensing surveys. Such observations will provide key insights to understand the process of SMBH formation and evolution during the emergence of the first galaxies. We further emphasize the great promise of possible SMBH detections at z ≳ 15 with future JWST observations to break the degeneracy between light- and heavy-seed models.

  PublisherDOI

• CAPERS-LRD-z9: A Gas Enshrouded Little Red Dot Hosting a Broad-line AGN at z=9.288

  ArXiv.org · 2025-05-07 · 1 citations

  preprintOpen access

  We present CAPERS-LRD-z9, a little red dot (LRD) which we confirm to be a $z=9.288$ broad-line AGN (BLAGN). First identified as a high-redshift LRD candidate from PRIMER NIRCam photometry, follow-up NIRSpec/PRISM spectroscopy of CAPERS-LRD-z9 from the CANDELS-Area Prism Epoch of Reionization Survey (CAPERS) has revealed a broad $3500$ km s$^{-1}$ H$β$ emission line and narrow [O III]$λ\lambda4959,5007$ lines, indicative of a BLAGN. Based on the broad H$β$ line, we compute a canonical black-hole mass of $\log(M_{\textrm{BH}}/M_{\odot})=7.58\pm0.15$, although full consideration of systematic uncertainties yields a conservative range of $6.65&lt;\log(M_{\textrm{BH}}/M_{\odot})&lt;8.50$. These observations suggest that either a massive black hole seed, or a lighter stellar remnant seed undergoing periods of super-Eddington accretion, is necessary to grow such a massive black hole in $\lesssim500$ Myr of cosmic time. CAPERS-LRD-z9 exhibits a strong Balmer break, consistent with a central AGN surrounded by dense ($\sim 10^{10}\textrm{ cm}^{-3}$) neutral gas. We model CAPERS-LRD-z9 using CLOUDY to fit the emission red-ward of the Balmer break with a dense gas-enshrouded AGN, and bagpipes to fit the rest-ultraviolet emission as a host-galaxy stellar population. This upper limit on the stellar mass of the host galaxy ($&lt;10^9\,{\rm M_\odot}$) implies that the black-hole to stellar mass ratio may be extremely large, possibly $&gt;5\%$ (although systematic uncertainties on the black-hole mass prevent strong conclusions). However, the shape of the UV continuum differs from typical high-redshift star-forming galaxies, indicating that this UV emission may also be of AGN origin, and hence the true stellar mass of the host may be still lower.

  PublisherOA PDFDOI

• Effects of chemically homogeneous evolution of the first stars on the 21-cm signal and reionization

  Monthly Notices of the Royal Astronomical Society · 2025-07-22 · 4 citations

  articleOpen access

  ABSTRACT The first generation of stars, known as Population III (Pop III), played a crucial role in the early Universe through their unique formation environment and metal-free composition. These stars can undergo chemically homogeneous evolution (CHE) due to fast rotation, becoming more compact and hotter/bluer than their (commonly assumed) non-rotating counterparts. In this study, we investigate the impact of Pop III CHE on the 21-cm signal and cosmic reionization under various assumptions on Pop III star formation, such as their formation efficiency, initial mass function, and transition to metal-enriched star formation. We combine stellar spectra computed by detailed atmosphere models with seminumerical simulations of Cosmic Dawn and the epoch of reionization ($z\sim 6-30$). The key effect of CHE arises from the boosted ionizing power of Pop III stars, which reduces the Pop III stellar mass density required to reproduce the observed Thomson scattering optical depth by a factor of $\sim 2$. Meanwhile, the maximum 21-cm global absorption signal is shallower by up to $\sim 15$ mK (11 per cent), partly due to the reduced Lyman-band emission from CHE, and the large-scale ($k\sim 0.2\ \rm cMpc^{-1}$) power drops by a factor of a few at $z\gtrsim 25$. In general, the effects of CHE can be comparable to those of Pop III star formation parameters, showing an interesting interplay with distinct features in different epochs. These results highlight the importance of metal-free/poor stellar evolution in understanding the early Universe and suggest that future studies should consider joint constraints on the physics of star/galaxy formation and stellar evolution.

  PublisherOA PDFDOI

• A Novel Formation Channel for Supermassive Black Hole Binaries in the Early Universe via Primordial Black Holes

  ArXiv.org · 2025-08-01

  preprintOpen accessSenior author

  We present a novel formation channel for supermassive black hole (SMBH) binaries in the early Universe, driven by primordial black holes (PBHs). Using high-resolution hydrodynamical simulations, we explore the role of massive PBHs ($m_{BH} \sim 10^6 M_\odot$) in catalyzing the formation of direct-collapse black holes (DCBHs), providing a natural in situ pathway for binary SMBH formation. PBHs enhance local overdensities, accelerate structure formation, and exert thermal feedback on the surrounding medium via accretion. Lyman-Werner (LW) radiation from accreting PBHs suppresses H$2$ cooling, shifting the dominant gas coolant to atomic hydrogen. When combined with significant baryon-dark matter streaming velocities ($v_{bχ} \gtrsim 0.8 σ_{bχ}$, where $σ_{bχ}$ is the root-mean-square streaming velocity), these effects facilitate the formation of dense, gravitationally unstable, atomically cooling gas clouds in the PBH's wake. These clouds exhibit sustained high inflow rates ($\dot{M}_{infall} \gtrsim 0.01 - 0.1 M_\odot yr^{-1}$), providing ideal conditions for DCBH formation from rapidly growing supermassive stars of $\sim 10^5 M_\odot$ at redshifts $z \sim 20 - 10$. The resulting systems form SMBH binaries with initial mass ratios $q \sim O(0.1)$ and separations of $\sim 10$ pc. Such PBH-DCBH binaries provide testable predictions for JWST and ALMA, potentially explaining select high-$z$ sources such as the Little Red Dots (LRDs), and represent gravitational-wave sources for future missions like LISA and TianQin-bridging early-Universe black hole physics, multi-messenger astronomy, and dark matter theory.

  PublisherOA PDFDOI

Recent grants

• Transport and Mixing of Metals during the First Billion Years

  NSF · $489k · 2010–2014

• Early Coevolution of Massive Black Holes and Stellar Systems in the First Galaxies

  NSF · $398k · 2007–2011

Frequent coauthors

• Naoki Yoshida

  The University of Tokyo

  114 shared

• Thomas H. Greif

  89 shared

• Ralf S. Klessen

  80 shared

• Simon C. O. Glover

  68 shared

• Daniel J. Whalen

  59 shared

• Abraham Loeb

  43 shared

• Boyuan Liu

  42 shared

• Daniel J. Whalen

  38 shared

Awards & honors

• The University of Texas System Regents’ Outstanding Teaching…
• College of Natural Sciences Teaching Excellence Award (2008)
• Board of Visitors Teaching Excellence Award (2007)
• Space Telescop Science Institute Postdoctoral Prize Fellowsh…
• Northeastern Association of Graduate Schools Dissertation Aw…