James Webb Space Telescope Feed Post

Harvard ADS: COSMOS-Web: The over-abundance and physical nature of "little red dots"--Implications for early galaxy and SMBH assembly

Paper abstract: JWST has revealed a population of compact and extremely red galaxies at z>4, which likely host active galactic nuclei (AGN). We present a sample of 434 ``little red dots'' (LRDs), selected from the 0.54 deg^2 COSMOS-Web survey. We fit galaxy and AGN SED models to derive redshifts and physical properties; the sample spans z~5-9 after removing brown dwarf contaminants. We consider two extreme physical scenarios: either LRDs are all AGN, and their continuum emission is dominated by the accretion disk, or they are all compact star-forming galaxies, and their continuum is dominated by stars. If LRDs are AGN-dominated, our sample exhibits bolometric luminosities ~10^{45-47} erg\,s^{-1}, spanning the gap between JWST AGN in the literature and bright, rare quasars. We derive a bolometric luminosity function (LF) ~100 times the (UV-selected) quasar LF, implying a non-evolving black hole accretion density of ~10^{-4} M_\odot yr^{-1} Mpc^{-3} from z~2-9. By contrast, if LRDs are dominated by star formation, we derive stellar masses ~10^{8.5-10}\,M_\odot. MIRI/F770W is key to deriving accurate stellar masses; without it, we derive a mass function inconsistent with \LambdaCDM. The median stellar mass profile is broadly consistent with the maximal stellar mass surface densities seen in the nearby universe, though the most massive ~50\% of objects exceed this limit, requiring substantial AGN contribution to the continuum. Nevertheless, stacking all available X-ray, mid-IR, far-IR/sub-mm, and radio data yields non-detections. Whether dominated by dusty AGN, compact star-formation, or both, the high masses/luminosities and remarkable abundance of LRDs implies a dominant mode of early galaxy/SMBH growth.