2002ApJ...580..896K

We present position-velocity strip maps of the Galactic center region in the CO J=7⟶6 and J=4⟶3 transitions observed with the Antarctic Submillimeter Telescope and Remote Observatory located at the Amundsen-Scott South Pole Station. Emission from the two rotational transitions of ¹²CO was mapped at b=0° for 3°.5>l>-1°.5 on a 1' grid with a FWHM beam size of 58" at 806 GHz and 105" at 461 GHz. Previous observations of CO J=4⟶3 (C. L. Martin et al., in preparation) and of [C I] emission (Ojha et al.) from this region show that these lines are distributed in a manner similar to CO J=1⟶0 (Stark et al.); the (CO J=4⟶3)/(CO J=1⟶0) line ratio map is almost featureless across the entire Galactic center region. In contrast, the CO J=7⟶6 emission from the Galactic center is strongly peaked toward the Sgr A and Sgr B molecular complexes. A large velocity gradient analysis shows that, aside from the two special regions Sgr A and Sgr B, the photon-dominated regions within a few hundred parsecs of the Galactic center are remarkably uniform in mean density and kinetic temperature at n=2500-4000 cm^–3 and T=30-45 K. The (CO J=7⟶6)/(CO J=4⟶3) line temperature ratios near Sgr B are a factor of 2 higher than those observed in the nuclear region of the starburst galaxy M82 (Mao et al.), while the CO(J=7⟶6)/CO(J=4⟶3) line temperature ratios around Sgr A are similar to M82. The line ratio on large scales from the Galactic center region is an order of magnitude less than that from M82.