LOW MILKY-WAY-LIKE MOLECULAR GAS EXCITATION OF MASSIVE DISK GALAXIES AT z ∼ 1.5

We present evidence for Milky-Way-like, low-excitation molecular gas reservoirs in near-IR selected massive galaxies at z~1. 5, based on IRAM Plateau de Bure Interferometer CO3-2 and NRAO Very Large Array CO1-0 line observations for two galaxies that had been previously detected in CO2-1 emission. The CO3-2 flux of BzK-21000 at z=1. 522 is comparable within the errors to its CO2-1 flux, implying that the CO3-2 transition is significantly sub-thermally excited. The combined CO1-0 observations of the two sources result in a detection at the 3 sigma level that is consistent with a higher CO1-0 luminosity than that of CO2-1. Contrary to what is observed in submillimeter galaxies and QSOs, in which the CO transitions are thermally excited up to J>=3, these galaxies have low-excitation molecular gas, similar to that in the Milky Way and local spirals. This is the first time that such conditions have been observed at high redshift. A Large Velocity Gradient analysis suggests that molecular clouds with density and kinetic temperature comparable to local spirals can reproduce our observations. The similarity in the CO excitation properties suggests that a high, Milky-Way-like, CO to H₂ conversion factor could be appropriate for these systems. If such low-excitation properties are representative of ordinary galaxies at high redshift, centimeter telescopes such as the Expanded Very Large Array and the longest wavelength Atacama Large Millimeter Array bands will be the best tools for studying the molecular gas content in these systems through the observations of CO emission lines.