Natsuko Kudo

Formation of galactic center magnetic loops

A survey of the molecular clouds in the Galaxy with the NANTEN mm telescope has discovered molecular loops in the galactic center region. They show monotonic gradients of the line-of-sight velocity along the loops and large velocity dispersions towards their foot-points. It is suggested that these loops can be explained in terms of a buoyant rise of magnetic loops due to a Parker instability. We carried out global three-dimensional magnetohydrodynamic simulations of the gas disk in the galactic center. The gravitational potential was approximated by an axisymmetric potential proposed by Miyamoto and Nagai (1975, PASJ, 27, 533). At the initial state, we assumed aw arm (� 10 4 K) gas torus threaded by azimuthal magnetic fields. Self-gravity and radiative cooling of the gas were ignored. We found that buoyantly rising magnetic loops are formed above the differentially rotating, magnetically turbulent disk. By analyzing the results of global MHD simulations, we identified individual loops, about 180 in the upper half of the disk, and studied their statistical properties, such as their length, width, height, and velocity distributions along the loops. The typical length and height of a loop are 1 kpc and 200 pc, respectively. The line-ofsight velocity changes linearly along a loop, and shows large dispersions around the foot-points. Numerical results indicate that loops emerge preferentially from the region where the magnetic pressure is large. We argue that these properties are consistent with those of molecular loops discovered by NANTEN.

A detailed observational study of molecular loops 1 and 2 in the galactic center

Fukui et al. (2006) discovered two molecular loops in the Galactic center that are likely created by the magnetic flotation due to the Parker instability with an estimated field strength of ∼150 µG. Following the discovery, we present here a detailed study of the two loops based on NANTEN 12 CO(J=1–0) and 13 CO(J=1–0) datasets. The two loops are located in l = 355 ◦ – 359 ◦ and b = 0 ◦ – 2 ◦ at a velocity range of -20 – -180 km s −1 . They have a projected total length of ∼ 600 pc and heights of ∼250 – 300 pc from the Galactic disk at a distance of 8.5 kpc. They have loop-like filamentary distributions of 30 pc width and show bright foot points in the 12 CO emission in the edges of the loops at b ∼ 0.8 ◦ – 1.0 ◦ . These foot points are characterized by velocity dispersions of 50 – 100 km s −1 , much larger than those in the Galactic disk, supporting that the loops are located in the Galactic center within ∼1 kpc of Sgr A*. The loops also show large-scale velocity gradients of ∼30 – 50 km s −1 per ∼100 pc. We present an attempt to determine geometrical parameters and velocities of the loops by assuming that the loops having the same size are expanding and rotating at a constant radius R. The analysis yields that the loops are rotating at 50 km s −1 and expanding at 150 km s −1 at a radius of 670 pc from the center.

Similarity between the molecular loops in the Galactic center and the solar chromospheric arch filaments

We carried out two-dimensional magnetohydrodynamic simulations of the Galactic gas disk to show that the dense loop-like structures discovered by the Galactic center molecular cloud survey by NANTEN 4 m telescope can be formed by the buoyant rise of magnetic loops due to the Parker instability. At the initial state, we assumed a gravitationally stratified disk consisting of the cool layer (T ∼ 10 3 K), warm layer (T ∼ 10 4 K), and hot layer (T ∼ 10 5 K). Simulation box is a local part of the disk containing the equatorial plane. The gravitational field is approximated by that of a point mass at the galactic center. The self-gravity, and the effects of the galactic rotation are ignored. Numerical results indicate that the length of the magnetic loops emerging from the disk is determined by the scale height of the hot layer (∼ 100 pc at 1 kpc from the Galactic center). The loop length, velocity gradient along the loops and large velocity dispersions at their foot points are consistent with the NANTEN observations. We also show that the loops become top-heavy when the curvature of the loop is sufficiently small, so that the rising loop accumulates the overlying gas

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

Fukui et al. (2006) discovered two molecular loops in the Galactic center and argued that the foot points of the molecular loops, two bright spots at both loops ends, represent the 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 deg at a few pc resolution in the six rotational transitions of CO; 12CO(J=1-0, 3-2, 4-3, 7-6), 13CO(J=1-0) and C18O(J=1-0). The high resolution image of 12CO (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 a range from 30-100 K and 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 the shock heating under C-shock condition caused by the 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 ~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 these new findings further reinforce the magnetic floatation interpretation.

High Excitation Molecular Gas in the Galactic Center Loops; 12CO (J = 2–1 and J = 3–2) Observations

We have carried out 12CO(J =2-1) and 12CO(J =3-2) observations at spatial resolutions of 1.0-3.8 pc toward the entirety of loops 1 and 2 and part of loop 3 in the Galactic center with NANTEN2 and ASTE. These new results revealed detailed distributions of the molecular gas and the line intensity ratio of the two transitions, R3-2/2-1. In the three loops, R3-2/2-1 is in a range from 0.1 to 2.5 with a peak at ~ 0.7 while that in the disk molecular gas is in a range from 0.1 to 1.2 with a peak at 0.4. This supports that the loops are more highly excited than the disk molecular gas. An LVG analysis of three transitions, 12CO J =3-2 and 2-1 and 13CO J =2-1, toward six positions in loops 1 and 2 shows density and temperature are in a range 102.2 - 104.7 cm-3 and 15-100 K or higher, respectively. Three regions extended by 50-100 pc in the loops tend to have higher excitation conditions as characterized by R3-2/2-1 greater than 1.2. The highest ratio of 2.5 is found in the most developed foot points between loops 1 and 2. This is interpreted that the foot points indicate strongly shocked conditions as inferred from their large linewidths of 50-100 km s-1, confirming the suggestion by Torii et al. (2010b). The other two regions outside the foot points suggest that the molecular gas is heated up by some additional heating mechanisms possibly including magnetic reconnection. A detailed analysis of four foot points have shown a U shape, an L shape or a mirrored-L shape in the b-v distribution. It is shown that a simple kinematical model which incorporates global rotation and expansion of the loops is able to explain these characteristic shapes.

Molecular loops in the Galactic centre; evidence for magnetic floatation accelerating molecular gas

The new molecular image obtained by NANTEN telescope in the galactic center has revealed the existence of the two loop like structures, loop 1 and loop 2, which have never been seen before toward l = 355° to 358°. The velocities of loop 1 and loop 2 are −180 to −90 km s −1 and −90 and −40 km s −1 , respectively, and these two loops have strong velocity gradients. The foot points of the loops show a very broad linewidth of ~40 to 80 km s −1 whose large velocity spans are characteristic of the molecular gas near the galactic center. Therefore, we classified the loops as being located in the galactic center and adopt a distance of 8.5 kpc. Then, the projected lengths of loop 1 and loop 2 were estimated as ~500 and ~300 pc, respectively and velocity gradients corresponds to ~80 km s −1 per 250 pc along loop 1 and ~60 km s −1 per 150 pc along loop 2. The heights of these loops are also estimated as ~220 to ~300 pc from the galactic plane, significantly higher than the typical scale height in the nuclear disk.