We re–emphasize that we stand at a cusp in knowledge, with the breath–taking promise of the ALMA and JVLA poised to revolutionize the study of the cool gas in early galaxies, and we have presented some of the early science results from these telescopes herein. Later in the decade, telescopes such as CCAT and NOEMA will also make important contributions.
We are no longer limited by numbers of high–z sources to study – there are thousands of CSG, quasars, and SMGs from cosmological deep fields. These populations have been delineated in terms of their space density, cosmic environment, and redshift distribution in remarkable detail. At this stage, we feel it is the high resolution imaging capabilities of ALMA and the JVLA that will be most incisive for unraveling the complex processes involved in early galaxy formation. Some of the key questions that need to be addressed include:
• For interpreting CO observations, the key uncertainty remains the conversion factor, especially in low metallicity systems where lack of dust shielding may dramatically reduce the CO content. We expect calibrating this relationship will follow the current path using multiple methods, including dynamics, radiative transfer modeling, and dust–to–gas modeling, leading to a concordance of estimates, and likely multi–parameter models involving eg. metallicity.
• Spatially resolved imaging (sub–kpc) of multiple CO transitions, as well as of the thermal dust continuum emission, is needed to study the relative distribution and excitation of the fuel for star formation with respect to regions of active star formation. Such imaging also allows for a study of star formation laws as a function of surface brightness, and not integrated quantities.
• Observations of high dipole moment molecules, and other complex molecules, will allow for detailed astrochemical modeling of the dense gas immediately involved in active star formation in distant galaxies.
• An inventory of fine structure lines is required to set the ISM gas cooling budget, AGN versus star formation indicators, and for metallicity determinations.
• We expect that studies of the [C II] 158µm line will play an important role in the determination of redshifts and dynamics of the first galaxies, well into cosmic reionization.
The new telescopes coming on–line open up the very real possibility of performing blind, deep field surveys for molecular gas, and atomic fine structure line emission, from distant galaxies. Over the next few years, we expect that the dense gas history of the Universe diagram will be fully populated, as the CO luminosity functions and conversion factors are quantified out to the highest redshifts. Coupled with the near–IR through X–ray studies of stars, star formation, and AGN, such pan–chromatic deep fields will provide a complete picture of the conversion of gas into stars over the history of the Cosmos.
Acknowledgements: The authors are indebted to their many long–term collaborators who have all greatly contributed to the field of high–redshift molecular gas emission. They include D. Riechers, R. Wang, A. Weiß, E. Daddi, P. Cox, R. Neri, K. Menten, F. Bertoldi, J. Wagg, M. Aravena, J. Hodge, R. Decarli. We thank D. Riechers, D. Narayanan, L. Tacconi, E. van Dishoeck, G. Stacey, R. Decarli, A. Weiß, E. da Cunha, A. van der Wel, M. Sargent for their valuable detailed input regarding this manuscript. We thank R. Decarli in particular for his help with preparing the figures. FW thanks the Aspen Center for Physics, where part of this work was conducted. CC thanks the Astrophysics Group, Cavendish Laboratory, Cambridge for support while much of this maunscript was written.