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

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 (AGNs). 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 disc, 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 per cent ($\sim 10^{-4}\rm Mpc^{-3}$) at $z\sim 4$. Our LRDs are central galaxies spanning $M_\star =10^8{\!-\!}10^{10.5}\rm M_\odot$. A population of galaxies with $M_\star \lt 10^9\rm M_\odot$ appear as LRDs due to older stellar populations. At higher masses, LRDs dominate the halo and stellar mass functions ($M_{\rm vir} \gt 10^{11.5}\rm M_\odot$, $M_\star \gt 10^{9.5}\rm M_\odot$), 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 per cent hosting MBHs) and LRD AGNs are disc-dominated, with LRD AGNs showing larger bulges formed mainly via disc instabilities.