Dense and warm molecular gas in Arp 220

We present the results from the observations of Arp 220 in multiple CO spectral line transitions of J=6-5, J=3-2, J=2-1 and the isotope (CO)-C-13 at J=2-1 at 690, 340 and 230 GHz using the Submillimeter Array (SMA) with the angular resolutions of sub-arcsec to a few arcsec. Based on the measured line ratios, we modeled the excitation conditions of the molecular clouds in the nuclear region of Arp 220, suggesting that the CO J=6-5 emission arises from dense (n(H-2)similar to 10(5) cm(-3)) and warm (T-k similar to 60 K) cloud components. Each of the four CO J=6-5 clumps contains at least a few times 10(8) M-circle dot molecular gas. The dense and warm molecular clouds are likely the consequence of inelastic collisions between the molecular clouds in the counterrotating nuclear disks. Properties determined from these high-density regions suggest that they are in the early stage of star formation. The study of Arp 220 could provide us detailed information on the astrophysical processes in the galaxies at high red-shifts.