Centimeter searches for molecular line emission from high-redshift galaxies

Abstract

We consider the capabilities for detecting low order CO emission lines from high-z galaxies using the next generation of radio telescopes operating at 22 and 43 GHz. We employ models for the evolution of dusty star forming galaxies based on source counts at (sub)millimeter wavelengths, and on the observed mm 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(cm)-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 done with the next-generation mm-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, cm-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 mm-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 over-resolve the sources, brightness temperature limitations require that a future large area cm telescopes have much of its collecting area on baselines shorter than 10 km.