Unveiling the evolution of the CO excitation ladder through cross-correlation of CONCERTO-like experiments and galaxy redshift surveys

TL;DR

The paper presents a cross-correlation framework between millimeter LIM data and spectroscopic galaxy surveys to isolate individual CO lines, reconstruct the CO background SLED, and infer the cosmic molecular gas density $\rho_{\mathrm{H2}}(z)$ up to $z=3$. Using 12 SIDES-Uchuu light cones, the authors recover the SLED up to $J_{\mathrm{up}}=6$ with $\lesssim 20\%$ uncertainty and derive bias-weighted intensities to estimate $\rho_{\mathrm{H2}}$, while assessing interloper effects and the feasibility for a CONCERTO-like instrument. The study finds that interlopers and [CI] contamination can inflate high-$J$ estimates (notably $J_{\mathrm{up}}=7$) and that the CO(4-3) cross-power is challenging to detect with current facilities, yet the cross-correlation method provides a powerful, instrumentally robust path to constraining the ISM and molecular gas content across cosmic time. The results offer guidance for designing future LIM surveys to exploit three-dimensional mode sampling and to combine with CO(1-0) measurements for a more complete view of the cosmic cold gas reservoir.

Abstract

Context: Rotational CO transitions, while acting as a foreground for [C II] line-intensity mapping (LIM) experiments, trace the physical conditions of cold gas in galaxies at lower redshifts. Studying these transitions is also crucial for improving component-separation methods as LIM sensitivity increases. Aims: Galaxy-evolution models have so far predicted only the total CO LIM signal. We explore the potential of cross-correlating millimeter-wave LIM data with spectroscopic galaxy surveys to constrain individual CO-line contributions, measure the CO-background spectral line energy distribution (SLED), and derive the cosmic molecular gas density, $ρ_{\mathrm{H2}}(z)$, up to $z = 3$. Methods: We built 12 light cones of $9~\mathrm{deg}^2$ from the Simulated Infrared Extragalactic Sky (SIDES) simulation. By analyzing cross-power spectra between different CO transitions and the galaxy density field, we recovered the CO background SLED. Combining it with bias-weighted line intensities yielded $ρ_{\mathrm{H2}}(z)$. We also assessed the detectability of the CO(4--3) cross-power spectrum with a CONCERTO-like experiment. Results: For a realistic spectroscopic depth, the CO background SLED is accurately recovered up to $J_{\mathrm{up}} = 6$ with $\leq 20%$ uncertainties. Reconstructing $ρ_{\mathrm{H2}}$ from millimeter LIM data requires an excitation correction relative to CO(1--0). Interloper-induced variance does not prevent precise $ρ_{\mathrm{H2}}$ estimation. In the two-star-formation-mode SIDES model, starbursts dominate the SLED at $J_{\mathrm{up}} \geq 6$ but do not bias $ρ_{\mathrm{H2}}$ estimates from $2 \leq J_{\mathrm{up}} \leq 6$. However, CONCERTO lacks the sensitivity to detect the CO$\times$galaxy cross-power on relevant scales, even under ideal conditions.

Figures (11)

• Figure 1: Right axis: Mean number of galaxies per galaxy cube covering 9-deg$^2$ and $\Delta z = 0.25$, as a function of redshift. The error bars come from the dispersion over the twelve independent light cones. Left axis: The associated shot noise of galaxy's number density contrast in the 9-deg$^2$ fields.
• Figure 2: From left to right: Images of SIDES intrinsic CO(4–3) cube integrated along spectral axis over $\Delta z = 0.25$, the same CO(4–3) cube including contamination from line interlopers, and the galaxy cube integrated over the same redshift range. The top row corresponds to $z = 0.5$ and the bottom row to $z = 3.0$. Continuum contamination is assumed to have been removed beforehand.
• Figure 3: Upper panel: CO(4--3) auto-power spectrum at $z=0.5$. The dashed gray line shows the simulation-based auto-power spectrum $\rm P^{tot}(k)$. It is the average over the five auto-power spectra of CO(4--3) maps, each map spanning $\delta z=0.05$. The dotted gray line shows the average shot noise component. Subtracting the shot noise yields the clustering $P^\mathrm{clust}(k)$ shown with the solid gray line. The clustering (Eq. \ref{['eq:autocluste']}) is fit to $P^\mathrm{clust}$ at large scales only, using the red points. From this fit, the effective clustering bias in SIDES, $b^\mathrm{fit}_\mathrm{eff}$, is estimated. Lower panel: Same procedure for galaxies' auto-power spectra and $b^\mathrm{fit}_\mathrm{gal}$, still at $z=0.5$.
• Figure 4: CO SLED and its evolution with redshift. Points have been shifted in the x-axis for clarity. The transition used for normalization is $\rm J_{up}$=3. Colors encode the redshift, from z=0.5 to 3.0. In the upper panel, solid lines and shaded areas are the mean and $1\sigma$ deviation of the background SLED from the SIDES-Uchuu catalog. Points are obtained from Eq. \ref{['eq:sled_from_cross']} (and thus from the ratio of clustering power spectra) and the associated error bars from the dispersion over the 12 subfields of 9$\rm\,deg^2$. The lower panel shows the relative difference between the mean SLED from the catalog and the measurement from the cross-power spectra.
• Figure 5: Left axis and colored dashed curves: Starburst's contribution to mean brightness of transitions, as function of redshift up to $\rm J_{up}$=8, inferred from the SIDES-Uchuu catalog. Right axis and solid black curve: Starburst fraction in SIDES catalog.