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

Abstract

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 $z_{\mathrm{spec}}=5.051$, while the other has a robust $z_{\mathrm{phot}}=6.7\pm 0.3$. 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 $\log (M_{\star })=11.32_{-0.15}^{+0.07}$ $M_{\odot }$ and $\log (M_{\star })=11.2_{-0.2}^{+0.1}$ $M_{\odot }$, 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 ${\epsilon }_{\star }\sim 0.25$ and ${\epsilon }_{\star }\sim 0.5$. 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.

arXiv comment11 pages, 4 figures, 1 table. Accepted for publication in ApJL

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