Abstract

The remarkable sensitivity and long-wavelength coverage of JWST has revolutionized our view of the early universe, and revealed an abundant population of compact and extremely red objects—so called “little red dots.” These were initially identified as candidate massive galaxies at z>7, but subsequent spectroscopy revealed broad emission lines, signatures of active galactic nuclei (AGN), reddened by dust. The remarkable abundance of these reddened broad-line AGN, and their lack of other AGN signatures, has raised many questions about the nature of this population and its role in early galaxy evolution. For my second year research talk, I will present my work identifying and characterizing these compact, red objects in the COSMOS-Web survey, the largest JWST extragalactic survey conducted in Cycle 1. Thanks to the wide-area coverage of COSMOS-Web, we identify hundreds of candidates, spanning a redshift range from z~5–9, several of which are exceptionally bright (~22-23 AB mag) in the rest-frame NIR. We consider two “edge cases,” i.e. two extreme physical scenarios for the nature of this population: 1) it is primarily composed of reddened Type I AGN, with the optical emission dominated by the accretion disk, or 2) it is primarily composed of compact/nuclear starbursts, with the optical emission dominated by young stellar populations. Under the former scenario, we derive bolometric luminosities ~10^(45-47) erg/s, spanning the gap between faint AGN in the literature and bright quasars discovered in ground-based surveys. At a given bolometric luminosity, the volume density of these AGN is ~100 times higher than expected from the population of UV-bright quasars. By contrast, under the scenario that these objects represent nuclear starbursts, we derive stellar masses consistent with LCDM cosmology, and with stellar mass densities within expectations for maximally dense stellar systems. We discuss the implications of either scenario for the formation of the most massive objects in the early universe.