We present a stack of data from the second season of the CO Mapping Array Project (COMAP) Pathfinder on the positions of quasars from eBOSS and DESI. COMAP is a Line Intensity Mapping (LIM) experiment targeting dense molecular gas via CO(1--0) emission at z∼3. COMAP's Season 2 represents a 3× increase in map-level sensitivity over the previous Early Science data release. We do not detect any CO emission in the stack, instead finding an upper limit of 10.0×1010Kkms−1pc2 at 95\% confidence within an ∼18cMpc box. We compare this upper limit to models of the CO emission stacked on quasars and find a tentative (∼3σ) tension between the limit and the brightest stack models after accounting for a suite of additional sources of experimental attenuation and uncertainty, including quasar velocity uncertainty, pipeline signal loss, cosmic variance, and interloper emission in the LIM data. The COMAP-eBOSS/DESI stack is primarily a measurement of the CO luminosity in the quasars' wider environment and is therefore potentially subject to environmental effects such as feedback. With our current simple models of the galaxy-halo connection, we are thus unable to confidently rule out any models of cosmic CO with the stack alone. Conversely, the stack's sensitivity to these large-scale environmental effects has the potential to make it a powerful tool for galaxy formation science, once we are able to constrain the average CO luminosity via the auto power spectrum (a key goal of COMAP).
@article{arxiv.2510.23568,
title = {COMAP Pathfinder -- Season 2 results IV. A stack on eBOSS/DESI quasars},
author = {D. A. Dunne and K. A. Cleary and J. G. S. Lunde and D. T. Chung and P. C. Breysse and N. O. Stutzer and J. R. Bond and H. K. Eriksen and J. O. Gundersen and G. A. Hoerning and J. Kim and E. M. Mansfield and S. R. Mason and N. Murray and T. J. Rennie and D. Tolgay and S. Valentine and I. K. Wehus and COMAP Collaboration},
journal= {arXiv preprint arXiv:2510.23568},
year = {2026}
}