Centimeter Searches for Molecular Line Emission from High‐Redshift Galaxies

Abstract

We consider the capabilities for detecting low-order CO emission lines from high-redshift (z) galaxies using the next generation of radio telescopes operating at 22 and 43 GHz. Low-order CO emission studies provide critical insight into the nature of high-redshift galaxies, including (1) determining gas masses, (2) study of large-scale structure through three-dimensional redshift surveys over cosmologically relevant volumes, (3) imaging gas kinematics on kiloparsec scales, and, in conjunction with observations of higher order transitions using future millimeter telescopes, (4) constraining the excitation conditions of the gas. Particular attention is paid to the impact on such studies of the high-frequency limit for future centimeter telescopes. We employ models for the evolution of dusty star-forming galaxies based on source counts at (sub)millimeter wavelengths and on the observed millimeter through infrared backgrounds, to predict the expected detection rate of low-order CO (2-1) and CO (1-0) line-emitting galaxies for optimal centimeter-wave surveys using future radio telescopes, such as the Square Kilometer Array (SKA) and the Expanded Very Large Array (EVLA). We then compare these results to surveys that can be performed with the next-generation millimeter-wave telescope, the Atacama Large Millimeter Array (ALMA). Operating at 22 GHz, the SKA will be competitive with the ALMA in terms of the detection rate of lines from high-z galaxies and will be potentially superior by an order of magnitude if extended to 43 GHz. Perhaps more importantly, centimeter-wave telescopes are sensitive to lower excitation gas in higher redshift galaxies and so provide a complementary view of conditions in high-redshift galaxies to millimeter-wave surveys. We have also included in our models emission from HCN. The number of HCN (1-0) detections will be about 5% of the CO detections in the (CO-optimized) 22 GHz surveys and about 1.5% for 43 GHz surveys. In order not to overresolve the sources, brightness temperature limitations require that future large-area centimeter telescopes have much of their collecting area on baselines shorter than 10 km.