Temperature and Density in the Foot Points of the Molecular Loops in the Galactic Center; Analysis of Multi-<i>J</i> Transitions of 12CO (<i>J</i> = 1–0, 3–2, 4–3, 7–6), 13CO (<i>J</i> = 1–0), and C18O (<i>J</i> = 1–0)

Abstract

Fukui et al. (2006, Science, 314, 106) discovered two molecular loops in the Galactic center, and argued that the foot points of the molecular loops, two bright spots at both loop ends, represent gas accumulated by the falling motion along the loops, subsequent to magnetic flotation by the Parker instability. We have carried out sensitive CO observations of the foot points toward $l =$ 356$^\circ$ at a few pc resolution in the six rotational transitions of CO: $^{12}$CO ($J =$ 1–0, 3–2, 4–3, 7–6), $^{13}$CO ($J =$ 1–0), and C$^{18}$O ($J =$ 1–0). A high-resolution image of $^{12}$CO ($J =$ 3–2) has revealed the detailed distribution of the high-excitation gas, including U shapes, the outer boundary of which shows sharp intensity jumps accompanying strong velocity gradients. An analysis of the multi-$J$ CO transitions shows that the temperature is in the range from 30 to 100 K and the density is around 10$^{3}$–10$^{4} $cm$^{-3}$, confirming that the foot points have high temperature and density, although there is no prominent radiative heating source, such as high-mass stars in or around the loops. We argue that the high temperature is likely due to shock heating under the C-shock condition caused by magnetic flotation. We made a comparison of the gas distribution with theoretical numerical simulations, and note that the U shape is consistent with numerical simulations. We also find that the region of highest temperature of $\sim $100 K or higher inside the U shape corresponds to the spur having an upward flow, additionally heated up either by magnetic reconnection or bouncing in the interaction with the narrow neck at the bottom of the U shape. We note that these new findings further reinforce the magnetic floatation interpretation.