From ASTRID to BRAHMA -- The role of overmassive black holes in little red dots in cosmological simulations

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

First listed 2025-12-15 | Last updated 2025-12-18

Abstract

We leverage the overmassive black holes ($\rm M_{BH}/M_{\ast} \approx0.1$) present in a realization of the BRAHMA cosmological hydrodynamic simulation suite to investigate their role in the emission of the unique ``little red dot'' (LRD) objects identified by the James Webb Space Telescope (JWST). We find that these black holes can produce LRD-like observables when their emission is modeled with a dense gas cloud shrouding the active galactic nucleus (AGN). Between redshifts 5 and 8, we find the number density of LRDs in this simulation to be $\rm 2.04 \pm 0.32 \times 10^{-4} \space Mpc^{-3}$, which is broadly consistent with current estimates for the total LRD population from JWST. Their emission in the rest-frame visible spectrum is dominated by their AGN, which induces the red color indicative of LRDs via a very strong Balmer break. Additionally, the elevated mass of the black holes reduces the temperature of their accretion discs. This shifts the peak of the AGN emission towards longer wavelengths, and increases their brightness in the rest-frame visible spectrum relative to lower mass black holes accreting at the same rate. These simulated LRDs have very minimal dust attenuation ($\rm A_V = 0.21 \pm 0.12$), limiting the amount of dust re-emission that would occur in the infrared, making them very likely to fall below the observed detection limits from observatories like the Atacama Large Millimeter Array (ALMA). In contrast to the BRAHMA box, the ASTRID simulation produces systematically smaller black holes and predicts LRD number densities that are more than two orders of magnitude lower than current measurements. We therefore conclude that the presence of black holes that are overmassive relative to their host galaxy, and enshrouded in dense gas, is necessary for AGN-dominated LRD models to reproduce both the observed properties and abundances of JWST LRD populations.

Short digest

Using BRAHMA, the authors test whether overmassive black holes (MBH/M*≈0.1) can explain little red dots by post-processing AGN emission with a dense, surrounding gas cloud. They obtain an LRD number density of 2.04±0.32×10^-4 Mpc^-3 at z=5–8, with rest-visible light dominated by the AGN and a very strong Balmer break. Elevated BH masses cool the accretion discs, shifting the SED peak to longer wavelengths and brightening the rest-visible, while minimal dust (A_V=0.21±0.12) keeps IR re-emission below ALMA limits. ASTRID, with systematically smaller BHs, yields LRD counts over two orders of magnitude lower, pointing to dense-gas–enshrouded, overmassive BHs as necessary to match both properties and abundances.

Key figures to inspect

• Figure 1: Compare incident vs transmitted AGN SEDs through different gas cloud setups to see how the dense-enshrouded model deepens the Balmer break and suppresses X-rays, and how changing AGN temperature shifts the SED peak into the rest-visible.
• Figure 2: Inspect Red1/Red2 color–color and color–magnitude selections to verify where BRAHMA LRDs land relative to the cut lines, and contrast BRAHMA’s distribution with the much sparser ASTRID contours.
• Figure 3: In the mock JWST image+spectra, check that the AGN component dominates the rest-visible continuum and produces the strong Balmer break, while the dashed (pre-dust) vs solid curves show how little dust is required.
• Figure 4: Read the MBH/M*–based panel colored by rest-visible color to see BRAHMA LRDs clustering at MBH/M*~0.1, and compare against literature AGN fits, X-ray–based estimates, and the local relation to underscore the ‘overmassive’ regime.