The Properties of Little Red Dot Galaxies in the ASTRID Simulation

Patrick LaChance, Rupert A. C. Croft, Tiziana Di Matteo, Yihao Zhou, Fabio Pacucci, Yueying Ni, Nianyi Chen, Simeon Bird

First listed 2025-05-26 | Last updated 2026-01-16

Abstract

We present simulated counterparts of the ``Little Red Dot'' (LRD) galaxies observed with JWST, using the large cosmological hydrodynamic simulation, ASTRID. We create mock observations of the galaxies ($5 \leq z \leq 8$) in ASTRID, and find seventeen which fit the color and size criteria of LRDs. These LRDs are galaxies with high stellar masses ($\rm log(M_*/M_{\odot}) \geq 9.7$), and massive black holes ($\rm log(M_{BH}/M_{\odot}) \geq 6.8$). The host galaxies are dense, with stellar half mass radii ($\rm 325\,pc \leq r_{{\rm half},*} \leq 620\,pc$), and dust attenuation in the F444W band above 1.25. Their star formation has been recently quenched. They host relatively bright AGN that are dust-obscured and contribute significantly to the rest-frame optical red slope and have relatively low luminosity in the rest-frame ultraviolet, where the host galaxy's stars are more dominant. These LRDs are in an evolutionary phase of miniquenching that is the result of AGN feedback from their massive black holes. The LRDs in ASTRID are bright with F444W magnitudes of $23.5-25.5$. The less massive and fainter galaxies in ASTRID lack the dust concentration necessary to produce the red slope of an LRD, though this could be an effect of limited resolution. Most of the highest Eddington black holes are not LRDs due to insufficient dust attenuation from their host galaxies, which are also experiencing relatively high star formation rates. This results in their spectra being too flat, despite their highly accreting black holes.

Short digest

Using the ASTRID cosmological simulation, the authors generate mock NIRCam-like views at z=5–8 and identify 17 systems that meet little red dot color-plus-size cuts. These ASTRID LRDs are massive, compact, and dusty—log(M*/Msun)≥9.7, log(MBH/Msun)≥6.8, r_half,*=325–620 pc, with F444W attenuation >1.25—and are bright at F444W=23.5–25.5. Their recent quenching and a dust-obscured AGN produce a steep rest-optical red slope while UV light is star-dominated, pointing to an AGN-feedback-driven miniquenching phase. Lower-mass/fainter galaxies lack sufficient central dust, and many highest-Eddington BH hosts are not LRDs because high SFRs and low attenuation yield flatter spectra; the paucity of low-mass LRDs may partly reflect limited resolution.

Key figures to inspect

• Figure 1: Inspect the RGB cutouts (F444W/F277W/F150W) and on-stamp annotations (z, pixel size, F444W mag, F277W–F444W) to see how compact morphologies and very red F277W–F444W colors arise in dense hosts embedded in the cosmic web.
• Figure 2: Decomposed SED shows stars vs AGN pre/post dust; verify that dust strongly suppresses the AGN blue/UV while driving the optical red slope, illustrating why stars dominate the UV yet the total spectrum is very red at longer wavelengths.
• Figure 3: Color–magnitude and color–color selection; check how the 17 ASTRID LRDs sit within the F277W–F444W and F150W–F200W cuts, and contrast with green “no-break” LRDs that fail the second criterion.
• Figure 4: MBH–M* plane with F277W–F444W coloring; confirm that ASTRID LRDs cluster at high M* and MBH and compare to observed LRD/AGN loci, noting that the most Eddington-limited hosts are not the reddest due to higher SFR and lower attenuation.