From ASTRID to BRAHMA – The role of overmassive black holes in little red dots in cosmological simulations
Year: 2026
Type: article
Abstract: We investigate the origin of Little Red Dots (LRDs) using a realization of the BRAHMA cosmological hydrodynamic simulation that produces overmassive black holes at z ∼ 4 - 8 . We generate mock JWST observations by modeling galaxy spectral energy distributions (including dust attenuation) together with AGN emission using cloudy-based templates, and test whether such systems reproduce the observed LRD population. We consider both a standard low-redshift AGN emission model and a gas-enshrouded AGN emission model, in which the AGN is embedded within unresolved dense gas that produces a strong Balmer break, and find that LRDs emerge in significantly larger numbers with the latter model. Assuming that all AGN reside in this gas-enshrouded phase, we recover LRD number densities ( Φ ∼ 2 × 10 − 4 M p c − 3 ), close to the observed abundance. This implies an effective gas-enshrouded duty cycle approaching unity, which is not predicted by current theoretical models. Current JWST constraints on the high-z black hole mass function suggest that this tension may arise from a deficit of low-mass () LRDs in BRAHMA. Applying the same gas-enshrouded model to the ASTRID simulation, which lacks overmassive black holes, we find that the LRD abundance remains below observations. Overall, while the overmassive black holes in this BRAHMA realization substantially enhance the production of LRD-like systems, reproducing the observed LRD abundance requires them to reside in dense gas environments with unrealistically high duty cycles. We therefore conclude that overmassive black holes are unlikely to be the sole origin of the observed JWST LRD population, and may instead account for only a subset of these systems.
Source: The Open Journal of Astrophysics
DOI: 10.33232/001c.173389
Authors: Patrick LaChance, Aklant Kumar Bhowmick, Rupert A. C. Croft, Tiziana Di Matteo, Yihao Zhou, Fabio Pacucci, Laura Blecha, Paul Torrey, Yueying Ni, Nianyi Chen, Simeon Bird
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: diamond
Awards: Unraveling the seeds of supermassive black holes using a multi-scale suite of cosmological simulations, G2689075568, Characterizing the Early Co-evolution of Galaxies, Black Holes, and Gravitational-Wave Sources, G3096611332, INTENSITY MAPPING (IM) IS THE USE OF ONE OR MORE EMISSION LINES TO TRACE OUT THE STRUCTURE OF THE UNIVERSE WITHOUT NEEDING TO RESOLVE INDIVIDUAL OBJECTS (SUCH AS GALAXIES OR GAS CLOUDS). IT IS ONE OF THE MOST PROMISING WAYS TO RADICALLY EXTEND THE SKY SURVEY REVOLUTION IN COSMOLOGY. BY MAKING SPECTRA OF THE ENTIRE SKY RATHER THAN THE ONE PART IN ONE MILLION CAPTURED BY CURRENT FIBER SPECTROGRAPHS ONE WOULD BE SENSITIVE TO ALL STRUCTURE. THERE ARE POTENTIALLY HUGE DISCOVERIES TO BE MADE IN THE VAST MAJORITY OF THE SKY THAT IS CURRENTLY SPECTRALLY UNMAPPED AND ALSO GREAT GAINS IN SIGNAL TO NOISE OF COSMOLOGICAL CLUSTERING MEASUREMENTS. INTENSITY MAPPING WITH THE 21CM RADIO LINE HAS BEEN EXPLORED THEORETICALLY BY MANY AND INSTRUMENTS ARE BEING BUILT PARTICULARLY TARGETING THE EPOCH OF REIONIZATION. IN THE UV VISIBLE AND INFRARED HOWEVER OTHER LINES HAVE ENORMOUS PROMISE AND WILL BE EXPLOITED BY A RANGE OF FUTURE NASA MISSIONS INCLUDING WFIRST EUCLID AND THE PROPOSED SPHEREX INSTRUMENT A DEDICATED INTENSITY MAPPING SATELLITE.THE FIRST MEASUREMENT OF LARGE-SCALE STRUCTURE OUTSIDE THE RADIO (USING LYMAN-ALPHA EMISSION) WAS RECENTLY MADE BY THE PI AND COLLABORATORS. THE LY-A ABSORPTION LINE ALSO TRACES A CONTINUOUS COSMOLOGICAL FIELD THE LYMAN-ALPHA FOREST AND THE ENORMOUS RECENT INCREASE IN THE NUMBER OF OBSERVED QUASAR SPECTRA HAVE MADE IT POSSIBLE TO INTERPOLATE BETWEEN QUASAR SIGHTLINES TO CREATE THREE DIMENSIONAL MAPS. BEING ABLE TO TRACE THE SAME COSMIC STRUCTURE IN EMISSION AND ABSORPTION OFFERS HUGE ADVANTAGES WHEN WE SEEK TOUNDERSTAND THE PROCESSES INVOLVED. IT WILL HELP US MAKE COMPREHENSIVE MAPS OF THE UNIVERSE'S CONTENTS AND OFFER US THE OPPORTUNITY TO CREATE NEW POWERFUL COSMOLOGICAL TESTS.IN OUR PROPOSED WORK WE WILL EXPLORE THE POSSIBILITIES AFFORDED BY TAKING GRISM AND INTEGRAL FIELD SPECTRA OF LARGE VOLUMES OF THE UNIVERSE USING STATE-OF-THE-ART COSMOLOGICAL HYDRODYNAMIC SIMULATIONS. WE WILL MAKE USE OF ANALYSIS TECHNIQUES DEVELOPED FOR THE LYMAN-ALPHA FOREST AS WELL AS FOREST DATA ITSELF TO TEST THEM. OUR AIM IS TO DEVELOP INTENSITY MAPPING AS A COSMOLOGICAL TOOL AND SHOWHOW IT CAN BE USED TO ANSWER QUESTIONS ABOUT THE CONTENTS OF THE UNIVERSE AND THE FORMATION OF STRUCTURE THAT ARE NOT ACCESSIBLE TO TRADITIONAL TECHNIQUES. THE PROJECT WILL INVOLVE BOTH DIRECT SAMPLING OF COSMIC STRUCTURE AND CROSS-CORRELATIONS OF LINE INTENSITY AND OBJECTS (INCLUDING GALAXIES QUASARS AND ABSORPTION LINES). EMISSION (E.G. H-ALPHA EMISSION) AND ABSORPTION (LY ALPHA FOREST) WILLBE VIEWED AS CONTINUOUS FIELDS. USING LARGE VOLUME COSMOLOGICAL SIMULATIONS COMBINED WITH POPULATION SYNTHESIS TECHNIQUES WE WILL MAKE SIMULATED SPECTRAL DATA SETS. THE TECHNIQUES TO ANALYZE THESE COSMOLOGICAL DATA CUBES WILL BE DEVELOPED. THE EXPECTED OUTCOMESARE THE FOLLOWING:(A) PREDICTIONS FOR THE LARGE-SCALE STRUCTURE OF STRONG EMISSION LINES (INCLUDING HA HB LYA OII OIII) IN THE UNIVERSE USING HYDRODYNAMIC SIMULATIONS INCLUDING THE CONTRIBUTION FROM ALL COMPONENTS FROM QUASARS TO DIFFUSE EMSSION.(B) SIMULATIONS OF REALISTIC EXAMPLES OF THE USE OF IM AS A COSMOLOGICAL PROBE} INCLUDING BARYON OSCILLATIONS AND WEAK GRAVITATIONAL LENSING.(C) TESTS OF TECHNIQUES TO DETECTION AND QUANTIFY THE LOW SURFACE BRIGHTNESS UNIVERSE LEADING TO A COMPLETE CENSUS OF THE COSMIC INTENSITY IN SPECIFIC LINES SUCH AS OII AND HA.(D) DEVELOPMENT OF TECHNIQUES TO EXTRACT REDSHIFTS FOR INDIVIDUAL GALAXIES FROM LOW ANGULAR RESOLUTION IM SPECTROSCOPY.(E) MOCK CATALOGS FOR SPHEREX EUCLID AND WFIRST SPECTROSCOPY OF DIFFUSE EMISSION AS WELL AS FOR THE GALEX GRISM SURVEY AND TESTS OF ANALYSIS TECHNIQUES ON DATA FROM THE LATTER., G4349310131, G4352801196, CDS&E: New Constraints on Extensions to the Cosmological Standard Model with the Lyman-Alpha Forest., Collaborative Research: Discriminating Between Galactic Feedback Models with Next Generation Observations, G824294325, AI Institute: Planning: Physics of the Future, NSF-Simons AI Institute for Cosmic Origins
OpenAlex ID: W4417465429