Discovery of molecular loop 3 in the Galactic center: Evidence for a positive-velocity magnetically floated loop towards L = 355°-359°

We have discovered a molecular dome-like feature towards 355° ≤ l ≤ 359° and 0° ≤ b ≤ 2°. The large velocity dispersions of 50-100km 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 a 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 H I gas having a more continuous spatial and velocity distribution than that of ¹²CO. 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. H i, ¹²CO, and ¹³CO datasets were used to estimate the total mass and the kinetic energy of loop 3 to be ∼3.0 × 10⁶ M_Odot; and ∼ 1.7 × 10⁵² 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 innerfilled 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 magnetic instabilities.