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 ($M_{\star }\sim {10}^{11}{M}_{\odot }$) and dusty ($A_{\mathrm{v}}>2.5$ mag) galaxies at $z=5-7$ in the first 0.28 deg${}^2$ of the COSMOS-Web survey. One object is spectroscopically confirmed at $z_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}}=5.051$, while the other has a robust $z_{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}}=6.7\pm 0.3$. Thanks to their extremely red colors ($F277W-F444W\sim 1.7$ 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 $\log(M_\star)=11.32^{+0.07}_{-0.15}$ $M_\odot$ and $\log(M_\star)=11.2^{+0.1}_{-0.2}$ $M_\odot$, 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 $ε_\star\sim0.25$ and $ε_\star\sim0.5$. 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.