Antarctic Submillimeter Telescope and Remote Observatory Observations of COJ = 7→6 andJ = 4→3 Emission toward the Galactic Center Region

Abstract

We present position-velocity strip maps of the Galactic center region in the CO J ¼ 7 ! 6a ndJ ¼ 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 12 CO was mapped at b ¼ 0 � for 3=5 > l > � 1= 5o n a1 0 grid with a FWHM beam size of 58 00 at 806 GHz and 105 00 at 461 GHz. Previous observations of CO J ¼ 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 ¼ 1 ! 0 (Stark et al.); the (CO J ¼ 4 ! 3)/(CO J ¼ 1 ! 0) line ratio map is almost featureless across the entire Galactic center region. In contrast, the CO J ¼ 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 ¼ 2500–4000 cm � 3 and T ¼ 30–45 K. The (CO J ¼ 7 ! 6)/(CO J ¼ 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 ¼ 7 ! 6)/ CO(J ¼ 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. Subject headings: Galaxy: abundances — Galaxy: center — ISM: general — ISM: molecules — submillimeter