Discovery of Molecular Loop 3 in the Galactic Center: Evidence for a Positive-Velocity Magnetically Floated Loop towards $L=355^{\circ }-359^{\circ }$

Abstract

We have discovered a molecular dome-like feature towards $355^{\circ }\le l\le 359^{\circ }$ and $0^{\circ }\le b\le 2^{\circ }$. The large velocity dispersions of 50--100 km s${}^{-1}$ of this feature are much larger than those in the Galactic disk and indicate that the feature is located in the Galactic center, probably within $\sim 1$ kpc of Sgr A${}^{\ast }$. The distribution has a projected length of $\sim 600$ pc and height of $\sim 300$ pc from the Galactic disk and shows a large-scale monotonic velocity gradient of $\sim 130$ km s ${}^{-1}$ per $\sim 600$ pc. The feature is also associated with HI gas having a more continuous spatial and velocity distribution than that of ${}^{12}$CO. We interpret the feature as a magnetically floated loop similar to loops 1 and 2 and name it "loop 3". Loop 3 is similar to loops 1 and 2 in its height and length but is different from loops 1 and 2 in that the inner part of loop 3 is filled with molecular emission. We have identified two foot points at the both ends of loop 3. HI, ${}^{12}$CO and ${}^{13}$CO datasets were used to estimate the total mass and kinetic energy of loop 3 to be $\sim3.0 \times 10^{6} \Mo$ and $\sim 1.7\times 10^{52}$ ergs. The huge size, velocity dispersions and energy are consistent with the magnetic origin the Parker instability as in case of loops 1 and 2 but is difficult to be explained by multiple stellar explosions. We argue that loop 3 is in an earlier evolutionary phase than loops 1 and 2 based on the inner-filled morphology and the relative weakness of the foot points. This discovery indicates that the western part of the nuclear gas disk of $\sim 1$ kpc radius is dominated by the three well-developed magnetically floated loops and suggests that the dynamics of the nuclear gas disk is strongly affected by the magnetic instabilities.

30 pages, 10 figures. High resolution figures are available at http://www.a.phys.nagoya-u.ac.jp/~motosuji/fujishita09_figs/