Molecular Gas in High Redshift Galaxies

Abstract

Study of molecular gas in distant galaxies during the last twenty years has followed the steady progress in mm wave receiver sensitivity. In 1975, CO was detected in M 82, NGC 253, and several other galaxies with redshifts of a few hundred km s^-1 (Rickard et al. 1975; Solomon & de Zafra 1975). Over the next fifteen years, the CO detection horizon increased steadily, reaching z ≈ 0.22 by 1990 (Downes et al. 1991). The discovery that the large population of infrared luminous galaxies detected by IRAS are very gas rich (e.g., Sanders, Scoville, & Soifer 1991) was especially significant. In the last few years, there has been a breakthrough; CO has been observed in two high redshift objects, IRAS FSC 10214+4724 at z = 2.3 (Brown & Vanden Bout 1992b) and the Cloverleaf quasar (H 1413+117) at z = 2.6 (Barvainis et al. 1994; Barvainis 1996). These objects offer glimpses of galaxies’ properties when the Universe was only about 15% of its present age. The presence and conditions of molecular gas in galaxies at such an early epoch are clues to understanding galaxy formation and evolution in the early Universe. Despite much observational effort, however, no other high redshift sources have confirmed detections of CO. Indeed molecular gas in both 10214+4724 and the Cloverleaf is visible only because they are gravitationally lensed. In intrinsic molecular content, gas distribution, and IR luminosity, these galaxies resemble gas rich, ultraluminous IR galaxies in the local universe.