Weak 13CO in the Cloverleaf quasar: evidence for a young, early generation starburst

Observations of 12CO at high redshift indicate rapid metal enrichment in the nuclear regions of at least some galaxies in the early universe. However, the enrichment may be limited to nuclei that are synthesized by short-lived massive stars, excluding classical “secondary” nuclei like 13C. Testing this idea, we used the IRAM Interferometer to tentatively detect the 13CO J = 3 → 2 line at a level of 0.3 Jy km s−1 toward the Cloverleaf quasar at z = 2.5. This is the first observational evidence for 13C at high redshift. The 12CO/13CO J = 3 → 2 luminosity ratio is with 40+25 −8 much higher than ratios observed in molecular clouds of the Milky Way and in the ultraluminous galaxy Arp 220, but may be similar to that observed toward NGC 6240. Large Velocity Gradient models simulating seven 12CO transitions and the 13CO line yield 12CO/13CO abundance ratios in excess of 100. It is possible that the measured ratio is affected by a strong submillimeter radiation field, which reduces the contrast between the 13CO line and the background. It is more likely, however, that the ratio is caused by a real deficiency of 13CO. This is already apparent in local ultraluminous galaxies and may be even more severe in the Cloverleaf because of its young age (<∼2.5 Gyr). A potential conflict with optical data, indicating high abundances also for secondary nuclei in quasars of high redshift, may be settled if the bulk of the CO emission is originating sufficiently far from the active galactic nucleus of the Cloverleaf.