Discovery of Molecular Loop 3 in the Galactic Center: Evidence for a Positive-Velocity Magnetically Floated Loop towards L=355∘-359∘

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We have discovered a molecular dome-like feature towards 355∘ ≤ l ≤ 359∘ and 0∘ ≤ b ≤ 2∘. 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 1 kpc of Sgr A*. The distribution has a projected length of 600 pc and height of 300 pc from the Galactic disk and shows a large-scale monotonic velocity gradient of 130 km s ⁻¹ per 600 pc. The feature is also associated with HI gas having a more continuous spatial and velocity distribution than that of ¹²CO. 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, ¹²CO and ¹³CO datasets were used to estimate the total mass and kinetic energy of loop 3 to be 3.0 × 10⁶ and 1.7 × 10⁵² 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 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.