Back to basics: Little Red Dots as galaxies and dust-obscured AGNs in a synthetic NIRCam sky simulated with L-GalaxiesBH

Diego Herrero-Carrión, Daniele Spinoso, David Izquierdo-Villalba, Tong Su, Silvia Bonoli, Pablo Renard

First listed 2025-11-13 | Last updated 2025-11-13

Abstract

The enigmatic Little Red Dots (LRDs) discovered by the James Webb Space Telescope (JWST) exhibit properties challenging their interpretation as common galaxies or Active Galactic Nuclei (AGN). Understanding their nature is key to placing them within our picture of early galaxy and massive black hole (MBH) evolution. To this aim, we build a realistic comparison between LRD observations with photometric properties of galaxies and AGN simulated by the L-GalaxiesBH model in a NIRCam mock sky. We model stellar continua and emission lines, the MBH emission from accretion disk, infrared radiation from dusty torus, and lines from narrow and broad line regions, accounting for dust attenuation and obscuration. Using realistic photometric cuts, we select a population of LRDs including both AGN and galaxies. The LRD fraction peaks at 40% ($\sim10^{-4}\rm Mpc^{-3}$) at $z\sim4$. Our LRDs are central galaxies spanning $M_*=10^8-10^{10.5}\rm M_\odot$. A population of galaxies with $M_*<10^9\rm M_\odot$ appear as LRDs due to older stellar populations. At higher masses, LRDs dominate the halo and stellar mass functions ($M_{\rm vir} > 10^{11.5}\rm M_\odot$, $M_* > 10^{9.5}\rm M_\odot$), and the interplay between AGN and galaxy emission drives the LRD selection. AGN dominate rest-frame UV-optical emission, while dust obscuration is secondary. LRDs host lighter MBHs ($\sim 10^{6.5}\rm M_\odot$) than non-LRDs ($\sim 10^{7.5}\rm M_\odot$), with fainter emission unable to balance their hosts Balmer breaks. We find no evidence for dominant heavy-seed origin of MBHs. LRD Galaxies (97% hosting MBHs) and LRD AGNs are disk-dominated, with LRD AGNs showing larger bulges formed mainly via disk instabilities.

Short digest

Builds a synthetic NIRCam sky with the L-GalaxiesBH semi-analytic model, adding stellar continua/lines plus MBH accretion, torus IR, and BLR/NLR lines to test photometric Little Red Dot (LRD) selections. Realistic cuts recover LRDs that include both galaxies and dust-obscured AGN; the LRD fraction peaks at 40% (~10^-4 Mpc^-3) near z~4, with central hosts spanning M*=10^8–10^10.5 Msun and low-mass LRDs arising from older stellar populations. At Mvir>10^11.5 Msun (M*>10^9.5 Msun) LRDs dominate the halo and stellar mass functions, with AGN–galaxy interplay setting the selection while dust is secondary and AGN typically control the rest-UV/optical. LRDs host lighter MBHs (~10^6.5 Msun vs ~10^7.5 Msun in non-LRDs), show disk-dominated morphologies with bulges grown via disk instabilities, and do not require heavy-seed MBHs.

Key figures to inspect

• NIRCam color–color selection panel implementing the LRD photometric cuts: verify where simulated galaxies vs. AGN land, and how AGN components drive the blue UV–optical side of the V-shaped SED.
• Redshift evolution of LRD fraction and comoving number density: confirm the 40% peak at z~4 and ~10^-4 Mpc^-3 normalization across redshift bins.
• Stellar and halo mass function breakdown (LRD vs. non-LRD): inspect dominance above Mvir>10^11.5 Msun and M*>10^9.5 Msun, and the contribution split between galaxy-only and AGN-influenced LRDs.
• Representative SED decompositions for LRDs: show AGN vs. host components and dust attenuation, illustrating AGN control of rest-UV/optical and the Balmer-break balance in galaxy-dominated cases.
• MBH and morphology diagnostics: side-by-side MBH mass distributions (LRD ~10^6.5 Msun vs. non-LRD ~10^7.5 Msun) and B/T or disk-instability contributions highlighting disk-dominated LRDs with instability-grown bulges.