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

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 fr om narr ow and br oad-line r egions, accounting for dust at tenuation and obscur ation. Using realistic photometric cuts, we select a population of LRDs including both AGN and galaxies. The LRD fraction peaks at 40 per cent ( similar to 10( -4) Mpc( -3) ) at z similar to 4 . Our LRDs are central galaxies spanning M = 10 (8) -10 (10 . 5) M-circle dot . A population of galaxies with M < 10( 9) M-circle dot appear as LRDs due to older stellar populations. At higher masses, LRDs dominate the halo and stellar mass functions ( M (vir) > 10( 11 . 5) M-circle dot , M > 10( 9 . 5) M-circle dot), 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 ( similar to 10( 6 . 5) M-circle dot ) than non-LRDs ( similar to 10 M-7 . 5 (circle dot) ), 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.