COMAP Pathfinder -- Season 2 results IV. A stack on eBOSS/DESI quasars

TL;DR

This work presents a three-dimensional stack of COMAP Season 2 CO(1--0) data centered on $z\sim3$ quasars from eBOSS and DESI, achieving a $3\times$ sensitivity gain over prior data. Using a template-based matched-filter extraction that accounts for pipeline signal loss and a suite of simulated LIM and quasar models, the study derives a $2\sigma$ upper limit of $\langle L'_{\mathrm{CO}}\rangle\leq 10.0\times10^{10}$ K km s$^{-1}$ pc$^{2}$ for the average CO luminosity in the $18\times18\times9$ cMpc$^{3}$ quasar environment, with a combined value of $\langle L'_{\mathrm{CO}}\rangle = 4.15\times10^{10}$ K km s$^{-1}$ pc$^{2}$ and $\sigma=2.94\times10^{10}$ for the measured stack. After applying attenuation due to pointing, redshift uncertainty, model-dependence, and interloper emission, several bright CO models are excluded at $>3\sigma$, notably the Li+2016-Keating+2020 and the fiducial UM+COLDz+COPSS family, while others remain consistent within uncertainties. The result suggests the stack is sensitive to large-scale environmental effects and halo clustering but does not yet pin down the cosmic CO luminosity density; full interpretation awaits constraints from the auto power spectrum and cross-correlations with richer tracers. These findings highlight the potential of LIM stacking around biased tracers to illuminate feedback and environmental processes in galaxy formation, as data from future LIM auto-spectra and multi-tracer analyses become available.

Abstract

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 $z\sim3$. COMAP's Season 2 represents a $3\times$ 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 $10.0\times 10^{10}\ \mathrm{K\ km\ s^{-1}\ pc^2}$ at 95\% confidence within an $\sim 18\ \mathrm{cMpc}$ box. We compare this upper limit to models of the CO emission stacked on quasars and find a tentative ($\sim 3 σ$) 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).

Figures (12)

• Figure 1: Spatial distribution of the eBOSS/DESI quasars compared to the footprint of the COMAP data. The COMAP footprint is coloured by the RMS in each voxel, averaged across all spectral channels, and the quasars are coloured by their redshift.
• Figure 2: Redshift distribution of the eBOSS and DESI quasars included in the stack catalogue.
• Figure 3: Models for CO luminosity as a function of halo mass used to generate simulated stacks. We defaulted to the chung2021_comapforecasts fiducial model.
• Figure 4: Models for quasar luminosity used to generate simulated catalogues, created from abundance-matching onto observed luminosity functions. The top panel shows the PAU model from torralba-torregrosa2024_quasarlyalumfunc and the bottom panel shows the palanquedelabrouille2016_ebosslumfunc. We defaulted to the torralba-torregrosa2024_quasarlyalumfunc model.
• Figure 5: Averaged simulated stacks used to account for pipeline signal loss in the stack, shown here for COMAP Field 1 above and Field 2 below. We show the stacked spectra on the left (averaging across the central $5\times 5$ spaxels for presentation purposes) and the stacked images (averaging across the central three spectral channels for presentation purposes) on the left. The apertures over which we sum are shown in black. The spatial and spectral effects of the COMAP analysis pipeline, and their different effects for each field, are visible.