Charting molecular gas through cosmological time: Present results and future directions

Molecular gas, observed through tracers such as CO rotational transitions, is a vital component of galactic evolution and star formation. Recent detections of the CO molecule in massive galaxies at redshifts as high as z = 6:42 have demonstrated its existence in the early Universe, and have motivated its use as a means of exploring large-scale structure and as a probe of galaxy evolution in the early Universe. But many questions about molecular gas and the evolution of galaxies in the early Universe still remain: its distribution at high redshift understood is so poorly that theoretical models of the mean abundance of CO for z ≳ 2 span orders of magnitude. Direct detection of molecular gas in galaxies at these redshifts (with instruments like the VLA) have only found the largest and most luminous of galaxies at these redshifts (typically containing 10¹⁰M_⊙ of molecular gas and star formation rates of 100M_⊙yr^-1), whereas the bulk of the molecular gas is expected to be in the unseen masses of smaller galaxies (M_gas ~ 10⁸M_⊙; SFR ~ 1M_⊙ yr^-1). While difficult to detect individually, these smaller galaxies are likely detectable as an integrated ensemble with the technique of “intensity mapping”. This technique, similar to those employed by HI epoch of reionization experiments, utilizes measurements of different 3D Fourier modes to construct a power spectrum.