The Properties of Little Red Dot Galaxies in the ASTRID Simulation

TL;DR

This work investigates the nature of Little Red Dot (LRD) galaxies observed by JWST by creating mock JWST observations of galaxies in the ASTRID cosmological hydrodynamic simulation across $5 \le z \le 8$. The authors augment a stellar+dust imaging pipeline with AGN emission via CLOUDY and birth-cloud attenuation to identify LRD analogs that satisfy JWST color and size criteria, finding $N_{LRD}=17$ in the simulated volume. The LRDs are massive, compact systems with $\log(M_*/M_{\odot}) \ge 9.75$, $\log(M_{BH}/M_{\odot}) \approx 7.5$, and dust-attenuated Balmer-break dominated spectra where the AGN contributes significantly at long wavelengths but is heavily obscured at shorter wavelengths. They infer a mini-quenching scenario driven by AGN feedback, with high-Eddington BHs typically not producing LRDs due to insufficient dust attenuation, and note that dimmer, lower-mass LRDs are rare in the simulation but can emerge under alternate AGN/dust modeling. The study highlights how dust geometry, Balmer breaks, and SED choices shape LRD demographics and motivates higher-resolution or alternative modeling (e.g., gas-enshrouded AGN) to uncover the full dim LRD population observed by JWST.

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.

Figures (12)

• Figure 1: A slice of the Astrid simulation at redshift $z=5$, and false color images of the Little Red Dot galaxies in Astrid . The slice is $\rm 25\,Mpc/h$ thick and shows the gas density within the simulation, with color indicating the gas temperature. The three overlayed images are LRDs present in this slice of Astrid and their position is indicated by the cross-hair. The false color images use the F444W, F277W, and F150W filter images for the red, green and blue channels, respectively. Each image is 2.0" x 2.0" with pixels of 0.030". The numbers on each image indicate the redshift (top value on the left), $r_{e}$ in pixels (bottom left value), F444W magnitude (top value on the right), F277W-F444W color (bottom right value).
• Figure 2: Example spectrum of a little red dot galaxy in Astrid. The solid blue and green lines show the contributions from the AGN and stars respectively, while the thick grey line shows the total observed spectra. The dashed blue and green lines indicate the stellar and AGN spectra before dust attenuation.
• Figure 3: Color-magnitude, and color-color plots showing the main criteria for LRD identification. The red circles indicate galaxies that fit both criteria, while the green squares are galaxies that fit the F277W-F444W criteria, but not the F150W-F200W criteria. The contours contain $1\sigma$ and $2\sigma$ of galaxies in their subpopulation.. Observational data sets from Perez_Gonzalez_2024Barro_2023; and Labbe_2023 are included as grey stars, squares, and triangles respectively
• Figure 4: Galaxy black hole mass plotted against galaxy stellar mass. The underlying 2D histogram shows the population of all galaxies in Astrid colored by the average F277W-F444W color of galaxies in that pixel, while the red and green points indicate LRDs and no-break LRDs found in Astrid and in JWST observations. The contours contain $1\sigma$ and $2\sigma$ of galaxies in their subpopulation. The purple dataset is comprised of AGN fits from Maiolino_2024_JADESHarikane_2023Ubler_2023Stone_2024Furtak_2024Yue_2024_eiger compiled in Pacucci_Loeb_2024. The blue star dataset was produced in Ananna_2024 from the X-ray observations of some LRDs.
• Figure 5: The top two plots show the ratio of AGN to total luminosity vs the magnitude of the galaxy in the F444W band on the left and F150W band on the right. The bottom two plots show the overall attenuation of stellar and AGN light in the same observation bands. The Astrid population is presented in the background colored by average F277W-F444W color, while the LRDs are indicated with red circles, and the no-break LRDs are shown with green squares. The inset plot on the top left plot zooms in on the region where the LRD and no-break LRDs are found. The contours contain $1\sigma$ and $2\sigma$ of galaxies in their subpopulation.