Not-so-little Red Dots: Two Massive and Dusty Starbursts at z ∼ 5–7 Pushing the Limits of Star Formation Discovered by JWST in the COSMOS-Web Survey

We present the properties of two candidate massive (M_~10¹¹M_) and dusty (Aᵥ>2.5 mag) galaxies at $z=5-7$ in the first 0.28 deg² of the COSMOS-Web survey. One object is spectroscopically confirmed at zspec=5.051, while the other has a robust zₚₕₒₜ=6.7±0.3. Thanks to their extremely red colors (F277W-F444W~1.7 mag), these galaxies satisfy the nominal color-selection for the widely-studied ``little red dot" (LRD) population with the exception of their spatially-resolved morphologies. The morphology of our targets allows us to conclude that their red continuum is dominated by highly obscured stellar emission and not by reddened nuclear activity. Using a variety of SED-fitting tools and star formation histories, we estimate the stellar masses to be log(M₎=11.32+0.07-0.15 M_ and log(M₎=11.2+0.1-0.2 M_, respectively, with a red continuum emission dominated by a recent episode of star formation. We then compare their number density to the halo mass function to infer stellar baryon fractions of ε_~0.25 and ε_~0.5. Both are significantly higher than what is commonly observed in lower-z galaxies or more dust-obscured galaxies at similar redshifts. With very bright ultra-high-z Lyman-Break Galaxies and some non-AGN dominated LRDs, such ``extended" LRDs represent another population that may require very efficient star formation at early times.