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

TL;DR

This work tests whether overmassive black holes in the BRAHMA cosmological simulations, when surrounded by a dense gas shroud around the AGN (gas-enshrouded AGN), can reproduce the properties and abundances of JWST-detected little red dots (LRDs). By comparing BRAHMA to the ASTRID simulation and applying a mock-observation pipeline with JWST-like color and compactness criteria, the authors show that BRAHMA's overmassive BHs coupled with dense gas can generate LRD-like SEDs and number densities similar to observations, while ASTRID underproduces LRDs due to smaller BHs. The results indicate that overmassive BHs and dense, enshrouding gas are essential for AGN-dominated LRDs to match JWST data, with Balmer-break features driving the red optical colors and minimal dust attenuation limiting infrared re-emission. These findings place important constraints on high-redshift BH seeding and growth scenarios and motivate broader-volume studies and more detailed AGN radiative-transfer modeling to refine the LRD population predictions.

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.

Figures (7)

• Figure 1: Example of the AGN Spectra used as input for, and output by, CLOUDY. We are placing this example object at $\rm z=6$ to provide relevant comparisons with observational bands. The top panel shows a broad view of the spectrum, spanning the infrared and X-ray regions. The lower panel zooms in on the JWST NIRCam wavelength range. The black line is the incident AGN spectrum in CLOUDY, which is processed through the surrounding gas cloud. We have chosen a temperature of $\rm T=10^{5.0} K$ for the example AGN. The green, and purple lines show different transmitted spectra based on the properties of the gas cloud. The green line is the cloud properties used in LaChance_2025, and the purple spectrum is produced by the "gas-enshrouded" model we use in this work. The dashed and dotted grey lines are the incident AGN spectra produced by AGN with temperatures $\rm T=10^{4.5} K$, and $\rm T=10^{5.5} K$ respectively.
• Figure 2: Color-Color and Color-Magnitude plots for the colors and magnitudes used in the "little red dot" criteria. The top panel shows sources F200W-F356W color vs. F444W magnitude, and F115W-F150W color. These are the primary factors in determining if a source meets the Red1 LRD criterion. The bottom panel shows sources F277W-F444W color vs. F444W magnitude, and F150W-F200W color. These are used in the Red2 LRD criterion. In order for a source to be considered a Red1 LRD it must be above the horizontal dashed line, and to the left of the vertical dashed line (falling within the highlighted regions) in both plots in the upper panel. The same is true for the Red2 criterion, and the plots in the bottom panel. The green 2D histogram shows the population of sources in BRAHMA. The magenta contours contain $1\sigma$ and $3\sigma$ of sources in the ASTRID simulation. The LRDs identified in BRAHMA are marked with red circles, and the LRDs found in ASTRID are shown with open magenta circles. Both panels include all of the sources in the snapshots we analyzed ($\rm z=5-8$).
• Figure 3: Mock JWST observations of three of the "Little Red Dots" found in the BRAHMA simulation. The images on the left are produced with the F444W, F277W, and F150W filters as the red, green, and blue channels, respectively. The spectra on the right show the total source spectrum in grey, the stellar component in green, and the AGN component in blue. The dashed lines indicate the spectra before dust attenuation is applied.
• Figure 4: The $\rm M_{BH}-M_{\ast}$ for the black holes and their host galaxies in the BRAHMA and ASTRID simulations. The 2D histogram shows the overall BRAHMA population. It is colored by the mean "Rest-visible Color" in each cell, where "Rest-visible Color" is F200W-F356W for z=5,6 sources, and F277W-F444W for z=7,8 sources. The magenta contours contain $1\sigma$ and $3\sigma$ of sources in the ASTRID simulation. The LRDs identified in BRAHMA are marked with red circles, and the LRDs found in ASTRID are shown with open magenta circles. The purple square dataset is comprised of AGN fits from Maiolino_2024_JADESHarikane_2023Ubler_2023Stone_2024Furtak_2024Yue_2024_eiger compiled in Pacucci_Loeb_2024. The green star dataset was produced in Ananna_2024 from the X-ray observations of some LRDs. We also include the local relation per Reines_2015 and an inferred $\rm z\sim 4-7$ relation from Pacucci_2023
• Figure 5: $\rm L_{bol}-M_{BH}$ and $\rm L_{bol}-M_{\ast}$ for the black holes and their host galaxies in the BRAHMA and ASTRID simulations. The 2D histogram shows the overall BRAHMA population. It is colored by the mean "Rest-visible Color" in each cell, where "Rest-visible Color" is F200W-F356W for z=5,6 sources, and F277W-F444W for z=7,8 sources. The magenta contours contain $1\sigma$ and $3\sigma$ of sources in the ASTRID simulation. The dashed grey line in the left panel indicates the Eddington luminosity as a function of black hole mass.