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COSMOS-Web: The over-abundance and physical nature of "little red dots"--Implications for early galaxy and SMBH assembly
Hollis B. Akins, Caitlin M. Casey, Erini Lambrides, Natalie Allen, Irham T. Andika, Malte Brinch, Jaclyn B. Champagne, Olivia Cooper, Xuheng Ding, Nicole E. Drakos, Andreas Faisst, Steven L. Finkelstein, Maximilien Franco, Seiji Fujimoto, Fabrizio Gentile, Steven Gillman, Ghassem Gozaliasl, Santosh Harish, Christopher C. Hayward, Michaela Hirschmann, Olivier Ilbert, Jeyhan S. Kartaltepe, Dale D. Kocevski, Anton M. Koekemoer, Vasily Kokorev, Daizhong Liu, Arianna S. Long, Henry Joy McCracken, Jed McKinney, Masafusa Onoue, Louise Paquereau, Alvio Renzini, Jason Rhodes, Brant E. Robertson, Marko Shuntov, John D. Silverman, Takumi S. Tanaka, Sune Toft, Benny Trakhtenbrot, Francesco Valentino, Jorge Zavala
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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 deg² 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 ∼10⁴⁵⁻⁴⁷ 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_⊙ 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_⊙. MIRI/F770W is key to deriving accurate stellar masses; without it, we derive a mass function inconsistent with ΛCDM. 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.
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