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

Abstract

Abstract We have discovered a molecular dome-like feature towards 355$^{\circ}$$\leq$$l$$\leq$ 359$^{\circ}$ and 0$^{\circ}$$\leq$$b$$\leq$ 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 a 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 being 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 both ends of loop 3. HI, ${}^{12}$CO, and ${}^{13}$CO datasets were used to estimate the total mass and the kinetic energy of loop 3 to be $\sim $3.0 $\times$ 10$^{6} M_\odot$ and $\sim $1.7 $\times$ 10$^{52} $erg. The huge size, velocity dispersions, and energy are consistent with the magnetic origin of the Parker instability, as in the 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 magnetic instabilities.