Why Can We Detect Quasar BR1202$-$0725 in Co ?

Abstract

We present CO luminosity evolution of both elliptical and spiral galaxies based on a galactic wind model and a bulge-disk model, respectively. We have found that the CO luminosity peaks around the epoch of galactic wind caused by collective supernovae $\sim$ 0.85 Gyr after the birth of the elliptical with $M = 2 \cdot 10^{12} M_\odot$ while $\sim$ 0.36 Gyr after the birth of the bulge with $M = 2 \cdot 10^{11} M_\odot$. After these epochs, the CO luminosity decreases abruptly because the majority of molecular gas was expelled from the galaxy system as the wind. Taking account of typical masses of elliptical galaxies and bulges of spiral galaxies, we suggest that CO emission can be hardly detected from galaxies with redshift $z \sim 1 - 4$ unless some amplification either by galaxy mergers and/or by gravitational lensing is working. Therefore, our study explains reasonably why CO emission was detected from the high-redshift quasar BR 1202$-$0725 at $z =4.7$ while not detected from the powerful radio galaxies with $1 < z < 4$.