On estimating the cosmic molecular gas density from CO Line Intensity Mapping observations

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 − 5. We study the implications of this data for the high-redshift interstellar medium using a suite of state-of-the-art semianalytic simulations which 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 ∼ 3.5σ below what was observed. We explore some simple modifications to the models which 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 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.

An Intensity Mapping Constraint on the CO-Galaxy Cross Power Spectrum at Redshift ~ 3

“…The models based on Chung (2019) and Yang et al (2021a) are well below the sensitivity of the current data set. The SAM underlying the Yang et al (2021a) model was compared to recent CO intensity mapping observations in Breysse et al (2021) and found to be in tension with current observational results. The Chung (2019) model is also in moderate tension with the tentative detection of the CO auto-power in COPSS, producing an auto-power signal approximately an order of magnitude fainter.…”

“…These results, shown as open circles in Figure 13, show a molecular density higher than reported by direct detection studies at z ∼ 2.5. The auto-power spectra of mmIME and COPSS contain information about all emit-ting galaxies, however the shot power is most sensitive to the brightest objects (Keenan et al 2020), and the constraints are sensitive to modeling assumptions (Breysse et al 2021).…”

Constraining low redshift [CII] Emission by Cross-Correlating FIRAS and BOSS Data

“…This possibility highlights a trend where early intensity mapping results suggest unexpectedly bright cumulative emission compared to direct detection of individual objects. For instance, Breysse et al (2021) suggests that potential bright CO emission detected by mmIME (Keating et al 2020) may be due to a large population of dim CO galaxies. Another possibility is that the assumption of these models, that [C II] emission is directly tied to the star formation rate, is too simple.…”

Atomic and molecular gas from the epoch of reionization down to redshift 2

“…Moreover, exploiting [C II] emission in high-z enriched environments (as lately done by Heintz et al 2021) would possibly open the door to observing Ω neutral at z > 6, where our results predict different scenarios depending on the chosen UV background. On the other hand, estimates of the reservoir of H 2 masses in star forming environments from [C II] or CO observational data are challenging and usually adopted conversion factors bear uncertainties and biases (Madden et al 2020;Gong et al 2020;Breysse et al 2021). The [C II] 158 micron line is a workhorse for (sub-)mm observations and is resolved on kpc-scale by ALMA (Rybak et al 2019).…”