Molecular gas content of the primaeval galaxy IRAS 10214+4724

THE faint IRAS source 10214 + 4724, identified 1 with a distant galaxy at redshift z = 2.286, is one of the most intrinsically luminous objects in the Universe, with L ≈10 14 L ⊙ (where L ⊙ is the solar luminosity). The remarkable detection of emission from neutral CO (ref. 2) leads, by comparison with similar detections in nearby galaxies, to an estimate of 2–6 x 10 11 M ⊙ for the mass of neutral molecular hydrogen in this galaxy. This is 30–90 times less than the estimate given in ref. 2, but still comparable to the total mass (gas, dust and stars) of a large spiral galaxy. This gas mass is consistent with the dynamical mass of the galaxy inferred from the CO emission line-width. The CO line luminosity is twenty times larger than is seen in nearby ( z < 0.3) ultra-luminous ( L >3xl0 11 M ⊙ ) galaxies 3–5 . Although it is extremely gas-rich, with a higher CO luminosity than any other galaxy, the infrared to CO luminosity ratio of 10214 + 4724 is twice that of most infrared-luminous galaxies, and ∼30 times higher than that of normal spirals. Its infrared colours and high infrared/CO luminosity ratio indicate that 10214 + 4724 is similar to nearby ultra-luminous galaxies 5 that are known to be merging. This galaxy is a primaeval molecular galaxy with the mass of a large spiral, but with most of the mass in molecular gas rather than stars.