COMAP Pathfinder – Season 2 results

IV. A stack on eBOSS/DESI quasars

¹ California Institute of Technology 1200 E. California Blvd. Pasadena CA 91125, USA
² Institute of Theoretical Astrophysics, University of Oslo P.O. Box 1029 Blindern N-0315 Oslo, Norway
³ Department of Astronomy, Cornell University Ithaca NY 14853, USA
⁴ Department of Physics, Southern Methodist University Dallas TX 75275, USA
⁵ Canadian Institute for Theoretical Astrophysics, University of Toronto 60 St. George Street Toronto ON M5S 3H8, Canada
⁶ Department of Physics, University of Toronto 60 St. George Street Toronto ON M5S 1A7, Canada
⁷ David A. Dunlap Department of Astronomy, University of Toronto 50 St. George Street Toronto ON M5S 3H4, Canada
⁸ Department of Physics, University of Miami 1320 Campo Sano Avenue Coral Gables FL 33146, USA
⁹ Jodrell Bank Centre for Astrophysics, Department of Physics & Astronomy, The University of Manchester Oxford Road Manchester M13 9PL, U.K.
¹⁰ Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro Yuseong-gu Daejeon postcode34141 Republic of Korea
¹¹ Department of Physics, Cornell University Ithaca NY 14853, USA
¹² Department of Physics and Astronomy, University of British Columbia Vancouver BC V6T 1Z1, Canada

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 28 October 2025
Accepted: 28 January 2026

Abstract

We present a stack of data from the second season of the CO Mapping Array Project (COMAP) 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. Season 2 of COMAP represents a factor of three increase in map-level sensitivity over the previous early science data release. We do not detect any CO emission in the stack; instead, we find an upper limit of 10.0 × 10¹⁰ K km s⁻¹ pc² at 95% confidence within an ∼18 cMpc 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).