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Arxiv: 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 Published: 8/19/2024 9:00:01 PM Updated: 8/19/2024 9:00:01 PM

Paper abstract: We present the properties of two candidate massive(M_*~10^{11}M_\odot) and dusty (A_{\rm v}>2.5 mag) galaxies atz=5-7 in the first 0.28 deg^2 of the COSMOS-Web survey. One object isspectroscopically confirmed at z_{\rm spec}=5.051, while the other has arobust z_{\rm phot}=6.7\pm0.3. Thanks to their extremely red colors(F277W-F444W~1.7 mag), these galaxies satisfy the nominal color-selectionfor the widely-studied ``little red dot" (LRD) population with the exception oftheir spatially-resolved morphologies. The morphology of our targets allows usto conclude that their red continuum is dominated by highly obscured stellaremission and not by reddened nuclear activity. Using a variety of SED-fittingtools and star formation histories, we estimate the stellar masses to be\log(M_*)=11.32^{+0.07}_{-0.15} M_\odot and\log(M_*)=11.2^{+0.1}_{-0.2} M_\odot, respectively, with a redcontinuum emission dominated by a recent episode of star formation. We thencompare their number density to the halo mass function to infer stellar baryonfractions of \epsilon_*~0.25 and \epsilon_*~0.5. Both aresignificantly higher than what is commonly observed in lower-z galaxies or moredust-obscured galaxies at similar redshifts. With very bright ultra-high-zLyman-Break Galaxies and some non-AGN dominated LRDs, such ``extended" LRDsrepresent another population that may require very efficient star formation atearly times.