WHY CAN WE DETECT THE HIGH-REDSHIFT QUASAR BR-1202-0725 IN CO

We present CO luminosity evolutions of both elliptical and spiral gala xies based on a galactic-wind model and a bulge-disk model, respective ly. We have found that the CO luminosity peaks at around the epoch of the galactic wind caused by collective supernovae similar to 0.85 Gyr after the birth of the elliptical with M = 2.10(12)M., while similar t o 0.36 Gyr after the birth of the bulge with M = 2.10(11)M. After thes e epochs, the CO luminosity decreased abruptly because the majority of the molecular gas was expelled from the galaxy system as wind. Taking account of typical masses of elliptical galaxies and the bulges of sp iral galaxies, we suggest that CO emission can be hardly detected from galaxies with a redshift of z similar to 1-4 unless some amplificatio n either by galaxy mergers and/or by gravitational lensing is working. Therefore, our study explains reasonably well why CO emission was det ected from the high-redshift quasar BR 1202-0725 at z = 4.7, while it was not detected from powerful radio galaxies with 1 < z < 4.