Rupert Croft
Professor
- Pittsburgh PA UNITED STATES
Rupert Croft's research includes the structure of the universe, galaxy formation, and the measurement of cosmological parameters.
Biography
Areas of Expertise
Media Appearances
Milky Way-like galaxies may have existed in the early universe
Phys.org online
2015-08-05
Di Matteo and fellow CMU Physics Professor Rupert Croft have long been at the forefront of simulation cosmology, completing some of the largest simulations ever created. Their current simulation, called BlueTides, is 100 times larger than previous simulations. It was so large that it monopolized all of the National Science Foundation (NSF) supercomputer BlueWater's memory and almost 1 million CPUs in order to complete the simulation.
Media
Social
Education
Imperial College
B.Sc.
Physics
1991
Oxford University
Ph.D.
Astrophysics
1995
Articles
The ASTRID Simulation at z = 0: From Massive Black Holes to Large-scale Structure
The Astrophysical JournalYihao Zhou, Tiziana Di Matteo, Simeon Bird, Rupert Croft, Yueying Ni, Yanhui Yang, Nianyi Chen, Patrick Lachance, Xiaowen Zhang, Fatemeh Hafezianzadeh
2026-03-01
We present the z = 0 results for the cosmological simulation ASTRID. Hosting 2 × 55003 ≈ 0.33 trillion particles in a box of 370 Mpc per side, ASTRID is one of the largest cosmological hydrodynamic simulations evolved to z = 0. ASTRID features a large population of massive black holes (MBHs), covering a wide mass range 4 × 104 ∼ 2 × 1011 M⊙. The adopted dynamical friction model provides a relatively accurate description of MBH dynamics, making ASTRID a powerful tool to study MBH growth and mergers in a cosmological context. ASTRID successfully captures the coevolution of MBHs and their host galaxies, producing MBH–M⋆ and MBH–σ relations in good agreement with observations. Notably, ASTRID generates scatter in these relations that is more consistent with observations than previous simulations, indicating a more realistic MBH diversity. The galaxy stellar mass function at z = 0 is generally consistent with observational constraints. When dust attenuation is applied, the galaxy luminosity function also agrees well with observations, and the bimodality in galaxy colors is reproduced as well. ASTRID hosts a large population of massive galaxy groups and clusters: seven halos have M200c > 1015 M⊙, and 9709 halos have M200c > 1013 M⊙. We quantify the stellar mass content in these halos, and find that the correlations between the stellar and halo mass match well with observational constraints. Finally, we present the z = 0 power spectra of MBH and galaxies, as well as their bias with respect to the matter power spectrum. We find that MBHs with MBH ≥ 108 M⊙ and galaxies with M⋆ ≥ 1010.5 M⊙ serve as good tracers of large-scale structure.
The gravitational-wave background from massive black holes in the ASTRID simulation
The Astrophysical Journal LettersNianyi Chen, Tiziana Di Matteo, Yihao Zhou, Luke Zoltan Kelley, Laura Blecha, Yueying Ni, Simeon Bird, Yanhui Yang, Rupert Croft
2025-09-16
Recent pulsar timing array (PTA) observations detected nanohertz gravitational waves, likely originating from massive black hole binaries (MBHBs). The detected amplitude is unexpectedly higher than inferred from the electromagnetic measurements. We present new gravitational-wave background (GWB) results from the ASTRID simulation. Its large volume and on-the-fly dynamics for massive black holes (MBHs) provide new insights into the MBHB population, offering a more accurate assessment of its contribution to the observed GWB. ASTRID predicts a GWB from MBHBs of hc = 2.8 × 10−15, or ∼45% of the observed amplitude at ∼4 nHz with a slope consistent with f−2/3, and hc = 2.5 × 10−16 with hc ∝ f−1.6 at ∼30 nHz. These predictions remain below current PTA constraints but align with empirical models based on the observed MBH mass functions. By comparison, TNG300 with postprocessed MBH dynamics yields a range between 70% and 90% (20% and 30%) of the observed levels at low (high) frequencies. At low frequencies, ASTRID predicts that the bulk of the GWB originates from MBHBs with masses Mtot = 1–3 × 109 M⊙ peaking at z ≈ 0.3, consistent with TNG300. Notably, both simulations predict significant contributions from minor mergers (q < 0.2) by up to ∼40%. By tracing the full merger trees of local MBHs in ASTRID, we show that they generate gravitational waves at ∼10%–80% of the maximum signal assuming no accretion and recent equal-mass mergers. Finally, we demonstrate the importance of on-the-fly MBH dynamics, the lack of which leads to 3–5 times excessive mass growth by merger, and a boost to the GWB prediction from this overestimated mass function, especially at high frequencies.
The Astrid Simulation: Evolution of Black Holes and Galaxies to z = 0.5 and Different Evolution Pathways for Galaxy Quenching
The Astrophysical JournalYueying Ni, Nianyi Chen, Yihao Zhou, Minjung Park, Yanhui Yang, Tiziana Di Matteo, Simeon Bird, Rupert Croft
2025-09-03
We present new results from the ASTRID simulation from z = 3 to z = 0.5, covering the epoch of cosmic noon. The galaxy stellar mass function, as well as the black hole mass and luminosity functions in ASTRID, exhibit good agreement with recent observational constraints. We study the MBH–M*scaling relation and its connections to active galactic nucleus (AGN) luminosity, galaxy color, and star formation rate, demonstrating that AGN feedback plays a crucial role in the quenching of massive galaxies (M* > 1010.5 M⊙). Although AGN feedback ultimately suppresses star formation through quenching, AGN-host galaxies can still exhibit statistically higher star formation rates than inactive ones, reflecting a positive correlation driven by their shared dependence on a common cold gas reservoir. The fraction of quiescent galaxies in ASTRID increases with both galaxy mass and redshift evolution, aligning well with observational trends. We find that different quenching mechanisms can leave distinct morphological imprints on quenched galaxies. Massive, compact quiescent galaxies typically experience shorter quenching timescales, have younger central regions, and host overmassive black holes. This is usually due to a compaction-like quenching mechanism that funnels gas into the galactic center, leading to starbursts and triggering AGN kinetic feedback. In contrast, quiescent galaxies with more diffuse morphologies generally experience “inside-out” quenching, which is characterized by older central regions compared to the outskirts. These galaxies typically experience longer quenching timescales due to quenching processes operating on a larger halo scale, which gradually deplete the galactic star-forming gas. Data of the ASTRID simulation down to z = 0.5 is available at https://astrid.psc.edu.
High-redshift supermassive black hole mergers in simulations with dynamical friction modelling
Monthly Notices of the Royal Astronomical Society2024
In the near future, projects like Laser Interferometer Space Antenna (LISA) and pulsar timing arrays are expected to detect gravitational waves from mergers between supermassive black holes, and it is crucial to precisely model the underlying merger populations now to maximize what we can learn from this new data. Here, we characterize expected high-redshift (z > 2) black hole mergers using the very large volume Astrid cosmological simulation, which uses a range of seed masses to probe down to low-mass black holes (BHs), and directly incorporates dynamical friction so as to accurately model the dynamical processes that bring black holes to the galaxy centre where binary formation and coalescence will occur.
AI-assisted super-resolution cosmological simulations III: time evolution
Monthly Notices of the Royal Astronomical Society2024
In this work, we extend our recently developed super-resolution (SR) model for cosmological simulations to produce fully time-consistent evolving representations of the particle phase-space distribution. We employ a style-based constrained generative adversarial network (StyleGAN), where the changing cosmic time is an input style parameter to the network. The matter power spectrum and halo mass function agree well with results from high-resolution N-body simulations over the full trained redshift range (10 ≤ z ≤ 0). Furthermore, we assess the temporal consistency of our SR model by constructing halo merger trees.
Modeling quasar proximity zones in a realistic cosmological environment with a self-consistent light curve
Monthly Notices of the Royal Astronomical Society2024
We study quasar proximity zones in a simulation that includes a self-consistent quasar formation model and realistic intergalactic medium (IGM) environments. The quasar host halo is 1013 M⊙ at z = 6, more massive than typical halos studied in previous work. Between 6 < z < 7.5, the quasar luminosity varies rapidly, with a mean magnitude of MUV, mean = −24.8 and the fluctuation reaching up to two orders of magnitude. Using this light curve to post-process the dense environment around the quasar, we find that the proximity zone size (Rp) ranges between 0.5 and 5 pMpc. We show that the light curve variability causes a similar degree of scatter in Rp as does the density fluctuation, both of which result in a standard deviation of ∼0.3 pMpc.
PRIYA: a new suite of Lyman-α forest simulations for cosmology
Journal of Cosmology and Astroparticle Physics2023
We present the PRIYA suite of cosmological simulations, based on the code and hydrodynamic model of the ASTRID simulation, and designed for cosmological analyses of the Lyman-α forest. Our simulation suite spans a 9-dimensional parameter space, including 4 cosmological parameters and 5 astrophysical/thermal parameters. We have run 48 low fidelity simulations with 1536 3 particles in a 120 Mpc/h box and 3 high fidelity simulations with 3072 3 particles in a 120 Mpc/h box. All our simulations include a full physics model for galaxy formation, including supernova and AGN feedback, and thus also contain a realistic population of DLAs. We advance on earlier simulations suites by larger particle loads, by incorporating new physical models for patchy hydrogen and helium reionization, and by self-consistently incorporating a model for AGN feedback.
The CAMELS project: Expanding the galaxy formation model space with new ASTRID and 28-parameter TNG and SIMBA suites
The Astrophysical Journal2023
We present CAMELS-ASTRID, the third suite of hydrodynamical simulations in the Cosmology and Astrophysics with MachinE Learning (CAMELS) project, along with new simulation sets that extend the model parameter space based on the previous frameworks of CAMELS-TNG and CAMELS-SIMBA, to provide broader training sets and testing grounds for machine-learning algorithms designed for cosmological studies. CAMELS-ASTRID employs the galaxy formation model following the ASTRID simulation and contains 2124 hydrodynamic simulation runs that vary three cosmological parameters (Ω m, σ 8, Ω b) and four parameters controlling stellar and active galactic nucleus (AGN) feedback.