The lifetimes and properties of Little Red Dots in the AMBRA simulation

Patrick LaChance, Yihao Zhou, Aklant Kumar Bhowmick, Rupert A. C. Croft, Tiziana Di Matteo, Laura Blecha, Fabio Pacucci, Paul Torrey, Simeon Bird

First listed 2026-09-24 | Last updated 2026-09-22

Abstract

We identify and analyze the little red dots (LRDs) in the AMBRA cosmological hydrodynamic simulation, by producing mock observations of the galaxies and active galactic nuclei (AGN) between redshifts 5 and 8. We produce these mock observations with both a standard AGN emission model, and a ``gas-enshrouded'' model, and find that the presence of a gas-enshrouded AGN is instrumental to the reproduction of dim LRDs (F444W magnitude $>$ 26.0). We find a steeper decrease in LRD density between $z=5$ and $z=8$ within AMBRA than seen in observations, resulting in a relative underproduction of LRDs beyond $z\sim6$ in AMBRA. With the addition of unresolved AGN variability, the decline with redshift flattens, and the LRD redshift evolution in AMBRA becomes broadly consistent with other theoretical datasets, and closer to that seen in observations. We find that the LRDs in AMBRA are pre-existing black hole--galaxy systems that are undergoing an LRD phase, selected primarily by the brightness of the AGN relative to its host. While the exact duration of these LRD phases is not possible to determine due to the unresolved nature of the AGN environment, we use the available time-series data to place limits on the lifetimes of the LRDs in AMBRA. We find that the vast majority of LRDs in AMBRA have lifetimes between $\sim3$ and $\sim 300$ Myr with LRD lifetime increasing with both black hole and host galaxy mass. The lower mass LRDs are more dependent on housing a gas-enshrouded AGN, and have shorter lifetimes ($\rm \sim30~Myr$). The higher mass LRDs are less sensitive to their AGN environment, but require more compact host galaxies, and have longer lifetimes ($\rm \sim100~Myr$).

Short digest

LaChance et al. build mock JWST photometry for AMBRA black hole–galaxy systems at z=5–8 and apply LRD color and compactness selection under both standard and gas-enshrouded AGN emission models. A gas-enshrouded AGN is essential for reproducing the dim F444W>26 LRDs, while added unresolved AGN variability flattens AMBRA’s otherwise overly steep decline in LRD abundance toward high redshift. The simulated LRDs are not a distinct newborn population but ordinary pre-existing black hole–host systems caught in an AGN-dominated observable phase. Their inferred LRD windows are mostly ~3–300 Myr, rising with black hole and host mass: lower-mass, enshrouded systems persist for roughly 30 Myr, whereas compact higher-mass hosts can remain LRDs for roughly 100 Myr or longer.

Key figures to inspect

• Figure 3. This is the selection-function figure: it shows how the mock AMBRA population occupies the color–magnitude, color–color, and compactness cuts, and directly contrasts the gas-enshrouded and standard AGN prescriptions used to define simulated LRDs.
• Figure 6. This figure carries the population-level comparison, showing AMBRA’s redshift evolution against observational estimates and other theoretical datasets. It makes clear why unresolved luminosity variability is consequential: it flattens the predicted decline toward z~8 and brings the model closer to the observed trend.
• Figure 9. This diagnostic establishes the paper’s physical interpretation of selection by showing the AGN contribution to F444W emission across the AMBRA population. It supports the conclusion that an LRD phase is primarily set by the AGN becoming bright relative to its host rather than by a wholly separate class of objects.
• Figure 11. This is the conclusion-driving lifetime measurement, relating the duration of each LRD window to host stellar mass, black hole mass, and bolometric luminosity. It captures the central mass dependence and the important caveat that ongoing high-mass phases provide lower limits rather than fully measured durations.