Annu. Rev. Astron. Astrophys. 2005. 43: 677-725
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One of the many important advances in our knowledge of the distant, early Universe during the past decade has come from observations of spectral line emission from interstellar molecular gas, the raw material from which stars form, in high-redshift (z > 2) galaxies. For convenience, we call these objects Early (Universe) Molecular (Line Emission) Galaxies, or EMGs. The molecular interstellar medium (ISM) plays a critical role in the evolution of galaxies; these observations provide the first evidence of the location and mass of molecular clouds during the epoch of galaxy formation. To date, observations of rotational transitions of carbon monoxide (CO) have been reported for 36 sources with redshift z > 1, unequivocally demonstrating that molecular clouds, an extreme Population I component, appeared early in the history of the Universe. (For completeness, we have included three galaxies with CO detections at redshifts 1 < z < 2 in this review.) The jump from detecting CO in local (z leq 0.3) galaxies to high-redshift observations was made possible by the increased sensitivity of millimeter-wave telescopes and arrays. It was also facilitated by the large masses of molecular gas associated with EMGs, a "negative K-corrrection" (see Section 2.1) for CO emission, gravitational lensing of many of the sources, and selection of sources with strong FIR emission, which is often associated with star-forming molecular gas.

Almost all candidate galaxies successfully detected in high-redshift CO emission were first identified as strong FIR/submillimeter sources with FIR luminosity in excess of 1012 Lodot. Given the relatively narrow instantaneous bandwidth of millimeter-wave receivers and spectrometers, an important selection criterion for CO emission line searches has been the availability of accurate redshifts from optical line spectroscopy. This situation changes as instrumental bandwidths increase.

Two main techniques have been applied to find most EMGs. The first employs large optical surveys of bright high-z quasars as a potential source list followed by observations of the flux at 1.2 mm or 0.85 mm. At these wavelengths the continuum of an EMG is dominated by thermal dust emission rather than an extension of the nonthermal radio continuum. CO emission has now been observed from 16 quasars, including the most distant known quasar at z = 6.4 (Walter et al. 2003). The second technique identifies highly luminous infrared (IR) galaxies from blank field observations with submillimeter-wave bolometers. Although not targeting individual cases of strong lensing, these observations often take advantage of intermediate-redshift cluster lensing. These techniques have led to the discovery of extremely luminous dusty FIR galaxies at high redshift, similar to local ultraluminous infrared galaxies (ULIRGs), but with a much higher space density. The search for CO in these submillimeter galaxies (SMGs) illustrates the (historical) importance of having good redshifts. Initial searches using Lyalpha redshifts were disappointing; later, the availability of Halpha redshifts led to a success rate of > 50%. A total of 11 SMGs have been reported as having CO emission. There are 73 SMGs with spectroscopic redshifts (Chapman et al. 2005), so a large number of CO detections is possible in the surveys underway. A third detection strategy involved searching IR-luminous radio galaxies for CO emission. Seven such detections have been reported. Finally, one Lyman Break galaxy (LBG) has been observed in CO emission, a detection made possible by strong magnification by a gravitational lens, and one extremely red object (ERO) has been detected.

The discovery of high-redshift CO emission predates these surveys. IRAS F10214 was a source at the detection limit of IRAS in the 60 and 100 µm bands, shown to be of high FIR luminosity when its redshift of z = 2.3 was (serendipitously) measured (Rowan-Robinson et al. 1991). The high FIR luminosity motivated a successful search for the rotational J = 3-2 line of CO with the NRAO 12m Telescope (Brown and Vanden Bout 1991, Brown and Vanden Bout 1992). The (3-2) detection was soon confirmed at the Institut Radioastronomie Millemétrique (IRAM) 30m Telescope, but with a much smaller flux (Solomon, Downes, & Radford 1992a), and the CO(6-5) line was also observed, indicating the presence of warm molecular gas typically associated with star formation. Successful searches for redshifted CO emission in several quasars soon followed: the Cloverleaf at z = 2.6 (Barvainis et al. 1994), BR 1202 at z = 4.7 (Omont et al. 1996b), and BRI 1335 at z = 4.4 (Guilloteau et al. 1997).

The CO observations of EMGs have the potential to answer several important questions about star formation and galaxy evolution in the early Universe: What is the mass of molecular gas and how does it compare with the dynamical mass? Are the EMGs centrally concentrated, as are most local ULIRGs, or are they extended protogalaxies with substantially more molecular mass than that of ULIRGs? What is the star formation lifetime? What is the final evolutionary state of the EMGs?

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