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We present a new upper limit on the cosmic molecular gas density at z=2.4−3.4 obtained using the first year of observations from the CO Mapping Array Project (COMAP). COMAP data cubes are stacked on the 3D positions of 243 quasars selected from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) catalog, yielding a 95% upper limit for flux from CO(1-0) line emission of 0.129 Jy km/s. Depending on the balance of the emission between the quasar host and its environment, this value can be interpreted as an average CO line luminosity L′CO of eBOSS quasars of ≤1.26×1011 K km pc2 s−1, or an average molecular gas density ρH2 in regions of the universe containing a quasar of ≤1.52×108 M⊙ cMpc−3. The L′CO upper limit falls among CO line luminosities obtained from individually-targeted quasars in the COMAP redshift range, and the ρH2 value is comparable to upper limits obtained from other Line Intensity Mapping (LIM) surveys and their joint analyses. Further, we forecast the values obtainable with the COMAP/eBOSS stack after the full 5-year COMAP Pathfinder survey. We predict that a detection is probable with this method, depending on the CO properties of the quasar sample. Based on the achieved sensitivity, we believe that this technique of stacking LIM data on the positions of traditional galaxy or quasar catalogs is extremely promising, both as a technique for investigating large galaxy catalogs efficiently at high redshift and as a technique for bolstering the sensitivity of LIM experiments, even with a fraction of their total expected survey data. 

Line-intensity mapping (LIM) is a promising technique to constrain the global distribution of galaxy properties. To combine LIM experiments probing different tracers with traditional galaxy surveys and fully exploit the scientific potential of these observations, it is necessary to have a physically motivated modeling framework. As part of developing such a framework, in this work, we introduce and model the conditional galaxy property distribution (CGPD), i.e., the distribution of galaxy properties conditioned on the host halo mass and redshift. We consider five galaxy properties, including the galaxy stellar mass, molecular gas mass, galaxy radius, gas-phase metallicity, and star formation rate (SFR), which are important for predicting the emission lines of interest. The CGPD represents the full distribution of galaxies in the five-dimensional property space; many important galaxy distribution functions and scaling relations, such as the stellar mass function and SFR main sequence, can be derived from integrating and projecting it. We utilize two different kinds of cosmological galaxy simulations, a semi-analytic model and the IllustrisTNG hydrodynamic simulation, to characterize the CGPD and explore how well it can be represented using a Gaussian mixture model (GMM). We find that with just a few (approximately three) Gaussian components, a GMM can describe the CGPD of the simulated galaxies to high accuracy for both simulations. The CGPD can be mapped to LIM or other observables by constructing the appropriate relationship between galaxy properties and the relevant observable tracers, which will be discussed in future works.

 

We present a new upper limit on the cosmic molecular gas density atz= 2.4–3.4 obtained using the first year of observations from the CO Mapping Array Project (COMAP). COMAP data cubes are stacked on the 3D positions of 243 quasars selected from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) catalog, yielding a 95% upper limit for flux from CO(1–0) line emission of 0.129 Jy km s⁻¹. Depending on the balance of the emission between the quasar host and its environment, this value can be interpreted as an average CO line luminosity$L_{\mathrm{CO}}^{\prime }$of eBOSS quasars of ≤1.26 × 10¹¹K km pc²s⁻¹, or an average molecular gas density${\rho }_{{\mathrm{H}}_2}$in regions of the Universe containing a quasar of ≤1.52 × 10⁸M_⊙cMpc⁻³. The$L_{\mathrm{CO}}^{\prime }$upper limit falls among CO line luminosities obtained from individually targeted quasars in the COMAP redshift range, and the${\rho }_{{\mathrm{H}}_2}$value is comparable to upper limits obtained from other line intensity mapping (LIM) surveys and their joint analyses. Further, we forecast the values obtainable with the COMAP/eBOSS stack after the full 5 yr COMAP Pathfinder survey. We predict that a detection is probable with this method, depending on the CO properties of the quasar sample. Based on the achieved sensitivity, we believe that this technique of stacking LIM data on the positions of traditional galaxy or quasar catalogs is extremely promising, both as a technique for investigating large galaxy catalogs efficiently at high redshift and as a technique for bolstering the sensitivity of LIM experiments, even with a fraction of their total expected survey data.

 

We present the deconvolved distribution estimator (DDE), an extension of the voxel intensity distribution (VID), in the context of future observations proposed as part of the CO Mapping Array Project (COMAP). The DDE exploits the fact that the observed VID is a convolution of correlated signal intensity distributions and uncorrelated noise or interloper intensity distributions. By deconvolving the individual VID of two observables away from their joint VID in a Fourier-space operation, the DDE suppresses sensitivity to interloper emission while maintaining sensitivity to correlated components. The DDE thus improves upon the VID by reducing the relative influence of uncorrelated noise and interloper biases, which is useful in the context of COMAP observations that observe different rotational transitions of CO from the same comoving volume in different observing frequency bands. Fisher forecasts suggest that the theoretical sensitivity in the DDE allows significant improvements in constraining power compared to either the cross power spectrum or the individual VID data, and matches the constraining power of the combination of all other one- and two-point summary statistics. Future work should further investigate the covariance and model-dependent behaviour of this novel one-point cross-correlation statistic.

 

Abstract The Millimeter-wave Intensity Mapping Experiment (mmIME) recently reported a detection of excess spatial fluctuations at a wavelength of 3 mm, which can be attributed to unresolved emission of several CO rotational transitions between z ∼ 1 and 5. We study the implications of these data for the high-redshift interstellar medium using a suite of state-of-the-art semianalytic simulations that have successfully reproduced many other submillimeter line observations across the relevant redshift range. We find that the semianalytic predictions are mildly in tension with the mmIME result, with a predicted CO power ∼3.5 σ below what was observed. We explore some simple modifications to the models that could resolve this tension. Increasing the molecular gas abundance at the relevant redshifts to ∼10 8 M ⊙ Mpc −3 , a value well above that obtained from directly imaged sources, would resolve the discrepancy, as would assuming a CO–H 2 conversion factor α CO of ∼1.5 M ⊙ K −1 (km s −1 ) −1 pc 2 , a value somewhat lower than is commonly assumed. We go on to demonstrate that these conclusions are quite sensitive to the detailed assumptions of our simulations, highlighting the need for more careful modeling efforts as more intensity mapping data become available. 

Abstract Submillimeter emission lines produced by the interstellar medium (ISM) are strong tracers of star formation and are some of the main targets of line intensity mapping (LIM) surveys. In this work we present an empirical multiline emission model that simultaneously covers the mean, scatter, and correlations of [C ii ], CO J = 1–0 to J = 5–4, and [C i ] lines in the redshift range 1 ≤ z ≤ 9. We assume that the galaxy ISM line emission luminosity versus halo mass relations can be described by double power laws with redshift-dependent lognormal scatter. The model parameters are then derived by fitting to the state-of-the-art semianalytic simulation results that have successfully reproduced multiple submillimeter line observations at 0 ≤ z ≲ 6. We cross-check the line emission statistics predicted by the semianalytic simulation and our empirical model, finding that at z ≥ 1 our model reproduces the simulated line intensities with fractional error less than about 10%. The fractional difference is less than 25% for the power spectra. Grounded on physically motivated and self-consistent galaxy simulations, this computationally efficient model will be helpful in forecasting ISM emission-line statistics for upcoming LIM surveys. 

The EXperiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) is a balloon-borne cryogenic telescope that will survey the spectrum of diffuse emission from both the Milky Way and the cosmic web to probe star formation, the interstellar medium, and galaxy evolution across cosmic time. EXCLAIM’s primary extragalactic science survey maps 305 deg2 along the celestial equator with an R = 512 spectrometer over the frequency range ν = 420 − 540 GHz, targeting emission of the [C ii] line over redshifts 2.5 < z < 3.5 and several CO lines for z < 1. Cross-correlation with galaxy redshift catalogues isolates line emission from the large-scale structure at target redshifts. In this paper, we forecast the sensitivity for both the two-point and conditional one-point cross-correlation. We predict that EXCLAIM will detect both the [C ii]-QSO cross-power spectrum and the conditional voxel intensity distribution (CVID) at various redshifts under a broad range of [C ii] intensity models, allowing it to differentiate among these models in the literature. These forecasts for the power spectra include the effects of line interlopers and continuum foreground contamination. We then convert the joint [C ii] constraints from both the cross-power spectrum and the CVID into constraints on the [C ii] halo luminosity–mass relation $L_\mathrm{[C\, \small {II}]}(M)$ model parameters and the star formation rate density (SFRD) from [C ii] emission. We also develop sensitivity estimates for CO, showing the ability to differentiate between models.

 

Abstract Line-intensity mapping observations will find fluctuations of integrated line emission are attenuated by varying degrees at small scales due to the width of the line emission profiles. This attenuation may significantly impact estimates of astrophysical or cosmological quantities derived from measurements. We consider a theoretical treatment of the effect of line broadening on both the clustering and shot-noise components of the power spectrum of a generic line-intensity power spectrum using a halo model. We then consider possible simplifications to allow easier application in analysis, particularly in the context of inferences that require numerous, repeated, fast computations of model line-intensity signals across a large parameter space. For the CO Mapping Array Project and the CO(1–0) line-intensity field at z ∼ 3 serving as our primary case study, we expect a ∼10% attenuation of the spherically averaged power spectrum on average at relevant scales of k ≈ 0.2–0.3 Mpc −1 compared to ∼25% for the interferometric Millimetre-wave Intensity Mapping Experiment targeting shot noise from CO lines at z ∼ 1–5 at scales of k ≳ 1 Mpc −1 . We also consider the nature and amplitude of errors introduced by simplified treatments of line broadening and find that while an approximation using a single effective velocity scale is sufficient for spherically averaged power spectra, a more careful treatment is necessary when considering other statistics such as higher multipoles of the anisotropic power spectrum or the voxel intensity distribution. 

Abstract We present the current state of models for the z ∼ 3 carbon monoxide (CO) line intensity signal targeted by the CO Mapping Array Project (COMAP) Pathfinder in the context of its early science results. Our fiducial model, relating dark matter halo properties to CO luminosities, informs parameter priors with empirical models of the galaxy–halo connection and previous CO (1–0) observations. The Pathfinder early science data spanning wavenumbers k = 0.051–0.62 Mpc −1 represent the first direct 3D constraint on the clustering component of the CO (1–0) power spectrum. Our 95% upper limit on the redshift-space clustering amplitude A clust ≲ 70 μ K 2 greatly improves on the indirect upper limit of 420 μ K 2 reported from the CO Power Spectrum Survey (COPSS) measurement at k ∼ 1 Mpc −1 . The COMAP limit excludes a subset of models from previous literature and constrains interpretation of the COPSS results, demonstrating the complementary nature of COMAP and interferometric CO surveys. Using line bias expectations from our priors, we also constrain the squared mean line intensity–bias product, Tb 2 ≲ 50 μ K 2 , and the cosmic molecular gas density, ρ H2 < 2.5 × 10 8 M ⊙ Mpc −3 (95% upper limits). Based on early instrument performance and our current CO signal estimates, we forecast that the 5 yr Pathfinder campaign will detect the CO power spectrum with overall signal-to-noise ratio of 9–17. Between then and now, we also expect to detect the CO–galaxy cross-spectrum using overlapping galaxy survey data, enabling enhanced inferences of cosmic star formation and galaxy evolution history.