High Excitation Molecular Gas in the Galactic Center Loops; 12CO (J= 2–1 and J= 3–2) Observations

We have carried out $^{12}$CO ($J$$=$2–1) and $^{12}$CO ($J$$=$3–2) observations at spatial resolutions of 1.0–3.8 pc toward the entirety of loops 1 and 2 and part of loop 3 in the Galactic center with NANTEN2 and ASTE. These new results have revealed detailed distributions of the molecular gas and the line intensity ratio of the two transitions, $R_{3-2/2-1}$. In the three loops, $R_{3-2/2-1}$is in a range from 0.1 to 2.5 with a peak at $\sim\ $0.7, while that in the disk molecular gas is in a range from 0.1 to 1.2 with a peak at 0.4. This supports that the loops are more highly excited than the disk molecular gas. An LVG analysis of three transitions, $^{12}$CO $J$$=$3–2 and 2–1 and $^{13}$CO $J$$=$2–1, toward six positions in loops 1 and 2 shows that the density and temperature are in the range 10$^{2.2}$–10$^{4.7}\ $cm$^{-3}$and 15–100 K or higher, respectively. Three regions, extended by 50–100 pc in the loops, tend to have higher excitation conditions, as characterized by $R_{3-2/2-1}$greater than 1.2. The highest ratio of 2.5 is found in the most developed foot points between loops 1 and 2. This is interpreted that the foot points indicate strongly shocked conditions, as inferred from their large linewidths of 50–100 km s$^{-1}$, confirming a suggestion by Torii et al. (2010, PASJ, 62, 675). The other two regions outside the foot points suggest that the molecular gas is heated up by some additional heating mechanisms, possibly including magnetic reconnection. A detailed analysis of four foot points has shown a U shape, an L shape or a mirrored-L shape in the $b$–$v$distribution. It is shown that a simple kinematical model that incorporates global rotation and expansion of the loops is able to explain these characteristic shapes.