10 research outputs found
Formation of Galactic Center Magnetic Loops
A survey for the molecular clouds in the Galaxy with NANTEN mm telescope has
discovered molecular loops in the Galactic center region. The loops show
monotonic gradients of the line of sight velocity along the loops and the large
velocity dispersions towards their foot points. It is suggested that these
loops are explained in terms of the buoyant rise of magnetic loops due to the
Parker instability. We have carried out global three-dimensional
magneto-hydrodynamic simulations of the gas disk in the Galactic center. The
gravitational potential is approximated by the axisymmetric potential proposed
by Miyamoto & Nagai (1975). At the initial state, we assume a warm (~ 10^4 K)
gas torus threaded by azimuthal magnetic fields. Self-gravity and radiative
cooling of the gas are 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 have 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. Typical length and height of a loop are 1kpc and 200pc,
respectively. The line of sight 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 magnetic pressure is large.
We argue that these properties are consistent with those of the molecular loops
discovered by NANTEN.Comment: 16pages, 10figures. Accepted for publication in PASJ. Replace to
higher resolution versio
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 ( K), warm layer ( K), and hot layer ( 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 faster than sliding it down along
the loop. This mechanism is similar to that of the formation of solar
chromospheric arch filaments. The molecular loops emerge from the low
temperature layer just like the dark filaments observed in the H image
of the emerging flux region of the sun.Comment: 23 pages, 13 figures. Accepted for publication in PAS
A Detailed Observational Study of Molecular Loops 1 and 2 in the Galactic Center
Fukui et al. (2006) discovered two huge molecular loops in the Galactic
center located in (l, b) ~ (355 deg-359 deg, 0 deg-2 deg) in a large velocity
range of -180-40 km s^-1. Following the discovery, we present detailed
observational properties of the two loops based on NANTEN 12CO(J=1-0) and
13CO(J=1-0) datasets at 10 pc resolution including a complete set of velocity
channel distributions and comparisons with HI and dust emissions as well as
with the other broad molecular features. We find new features on smaller scales
in the loops including helical distributions in the loop tops and vertical
spurs. The loops have counterparts of the HI gas indicating that the loops
include atomic gas. The IRAS far infrared emission is also associated with the
loops and was used to derive an X-factor of 0.7(+/-0.1){\times}10^20 cm^-2 (K
km s^-1)^-1 to convert the 12CO intensity into the total molecular hydrogen
column density. From the 12CO, 13CO, H I and dust datasets we estimated the
total mass of loops 1 and 2 to be ~1.4 {\times} 106 Msun and ~1.9 {\times} 10^6
Msun, respectively, where the H I mass corresponds to ~10-20% of the total mass
and the total kinetic energy of the two loops to be ~10^52 ergs. An analysis of
the kinematics of the loops yields that the loops are rotating at ~47 km s-1
and expanding at ~141 km s^-1 at a radius of 670 pc from the center. Fukui et
al. (2006) presented a model that the loops are created by the magnetic
flotation due to the Parker instability with an estimated magnetic field
strength of ~150 {\mu}G. We present comparisons with the recent numerical
simulations of the magnetized nuclear disk by Machida et al. (2009) and
Takahashi et al. (2009) and show that the theoretical results are in good
agreements with the observations. The helical distributions also suggest that
some magnetic instability plays a role similarly to the solar helical features.Comment: 40 pages, 22 figures, submitted to publication in PAS
Discovery of Molecular Loop 3 in the Galactic Center: Evidence for a Positive-Velocity Magnetically Floated Loop towards
We have discovered a molecular dome-like feature towards and . The large velocity
dispersions of 50--100 km s 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 kpc of Sgr A. The distribution has a
projected length of pc and height of pc from the Galactic
disk and shows a large-scale monotonic velocity gradient of km s
per pc. The feature is also associated with HI gas having a
more continuous spatial and velocity distribution than that of 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, CO and CO datasets were used to estimate the
total mass and kinetic energy of loop 3 to be \sim3.0 \times 10^{6} \Mo and
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 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.Comment: 30 pages, 10 figures. High resolution figures are available at
http://www.a.phys.nagoya-u.ac.jp/~motosuji/fujishita09_figs
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.Comment: 40 pages, 23 figures, accepted by PASJ on Vol.62 No.
Sub-millimeter Observations of Giant Molecular Clouds in the Large Magellanic Cloud: Temperature and Density as Determined from J=3-2 and J=1-0 transitions of CO
We have carried out sub-mm 12CO(J=3-2) observations of 6 giant molecular
clouds (GMCs) in the Large Magellanic Cloud (LMC) with the ASTE 10m sub-mm
telescope at a spatial resolution of 5 pc and very high sensitivity. We have
identified 32 molecular clumps in the GMCs and revealed significant details of
the warm and dense molecular gas with n(H2) 10 cm and
Tkin 60 K. These data are combined with 12CO(J=1-0) and 13CO(J=1-0)
results and compared with LVG calculations. We found that the ratio of
12CO(J=3-2) to 12CO(J=1-0) emission is sensitive to and is well correlated with
the local Halpha flux. We interpret that differences of clump propeties
represent an evolutionary sequence of GMCs in terms of density increase leading
to star formation.Type I and II GMCs (starless GMCs and GMCs with HII regions
only, respectively) are at the young phase of star formation where density does
not yet become high enough to show active star formation and Type III GMCs
(GMCs with HII regions and young star clusters) represents the later phase
where the average density is increased and the GMCs are forming massive stars.
The high kinetic temperature correlated with \Halpha flux suggests that FUV
heating is dominant in the molecular gas of the LMC.Comment: 74 pages, including 41 figures, accepted for publication in ApJ