James Webb Space Telescope Feed Post

Harvard ADS: 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

Paper abstract: We present the properties of two candidate massive (M_*~10^{11}M_\odot) and dusty (A_{\rm 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_{\rm spec}=5.051, while the other has a robust z_{\rm phot}=6.7\pm0.3. 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_\odot and \log(M_*)=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 \epsilon_*~0.25 and \epsilon_*~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.