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

Observations of ^(12)CO 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 ^(13)C. Testing this idea, we used the IRAM Interferometer to tentatively detect the ^(13)CO 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 ^(13)C at high redshift. The ^(12)CO/^(13)CO J = 3 → 2 luminosity ratio is with 40_(-8)^(+25) 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 ^(12)CO transitions and the ^(13)CO line yield ^(12)CO/^(13)CO 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 ^(13)CO line and the background. It is more likely, however, that the ratio is caused by a real deficiency of ^(13)CO. 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.