As the fuel for star formation, molecular gas plays a crucial role in the evolution of galaxies. Advanced millimeter-wave facilities now make it possible to study the gas properties of large samples of nearby galaxies and to peer into the cold ISM of “typical” star forming galaxies throughout cosmic time. However, many details of the interplay between gas and star formation that drives galaxy evolution remain unsettled. In this talk, I will present my work refining our tools for measuring and interpreting the observational signatures of molecular gas. I will begin by reviewing recent efforts to measure the cosmic abundance of molecular gas, and its evolution over time. Pioneering studies with ALMA, NOEMA, and the VLA have now constrained the global history of molecular gas to cosmic noon and beyond. Achieving precision measurements of this history will require extensive surveys which push the limits of current facilities. I have developed mocks for current and future surveys to help guide the design of the next generation of molecular gas deep fields. Precision measurements of the cosmic molecular gas history require surveys over significantly larger volumes than have been achieved to date, and I will discuss possible avenues for accomplishing such a survey. The novel technique of “line intensity mapping” (LIM), which can constrain the agreagate gas content of galaxies too faint to be individually detected, provides one path forward using existing facilities. I will provide an overview of the LIM technique, highlight a few upcoming line intensity mapping projects, and present upper limits on the CO-galaxy cross-power spectrum at redshift 2 derived from the exploratory CO Power Spectrum Survey, which serve as a proof of concept for future projects.

While much of our understanding of molecular gas comes from studies using the CO(1-0) line, other tracers gas tracers such as the higher energy CO lines, are increasingly popular. In the second half of my talk I will introduce the Arizona Molecular ISM Survey with the SMT (AMISS), a multi-line study of 177 nearby galaxies designed cross-calibrate the CO(1-0), CO(2-1), and CO(3-2) lines as tracers of molecular gas. The large sample and careful calibration of AMISS make it possible to robustly measure correlations between the excitation of CO emitting gas and host galaxy properties on a galaxy integrated scale. The three lines are not perfectly interchangeable tracers and the luminosity ratios between them vary systematically over the galaxy population. Using the AMISS sample, I characterize these trends, providing an empirical prescription for translating between gas measurements made with different CO lines, and then explore the utility of CO line ratios as diagnostics of conditions in the interstellar media of galaxies.