Astronomers at the Keio University, Japan, have discovered a helical molecular cloud near the center of the Milky Way Galaxy some 30,000 light-years away.
The astronomers, led by Shinji Matsumura and Prof Tomoharu Oka, named it the ‘pigtail’ molecular cloud.
Giant molecular clouds in this region orbit the Galactic center along two closed orbits. At the bottom of the pigtail molecular cloud, these two orbits intersect. By analyzing multiple molecular spectral lines in detail, the researchers have revealed that the two giant molecular clouds collide with one another at exactly the bottom of the ‘pigtail’ molecular cloud. These findings suggest that the helical structure of the ‘pigtail’ molecular cloud formed when the two molecular clouds with different orbits frictionally collided and the magnetic tube was twisted.
Within a 600-light-year radius from the center of our galaxy, there is a high density of stars and molecular gas. The molecular gas becomes a dense molecular cloud, and it is thought to move mainly along two elliptical orbits around the galactic nucleus.
Team member Dr Tetsuo Hasegawa of the ALMA Observatory in Santiago, Chile, noticed the existence of a helical molecular cloud when he carefully examined the mass data of 115-GHz rotational spectral lines emitted by carbon monoxide molecules. The helical structure measures about 60 x 60 light years.
“In order to solve the mystery of the pigtail molecular cloud, we carried out high-resolution spectroscopic observations of rotational spectral lines for six other molecules,” explained Matsumura, lead author of a paper published in the Astrophysical Journal (arXiv.org version). “Those molecules are clues to understanding the physical state. We were amazed by the clear and beautiful helical structure of the ‘pigtail’ molecular cloud in the data taken by the follow-up observation. The data revealed that the pigtail molecular cloud has a huge volume of gas, several hundreds of thousands times greater than the Sun has.”
The team has proposed the following scenario on how the pigtail molecular cloud was formed: the magnetic tube perpendicular to the galactic disc exists between two giant molecular clouds, those clouds move along the main two elliptical orbits around the galactic nucleus; the magnetic tube is twisted and squeezed to become a helical structure during the frictional contact; the molecular gas is captured by the twisted magnetic tube, and then forms the ‘pigtail’ molecular cloud.
“We emphasize two important points on our study,” Matsumura said. “First, it proves that two orbit groups caused by the bar-like structure actually intersect at this point in the galactic center. Second, it shows that the perpendicular magnetic field of approximately one milligauss is locally confined.”
So far, two other helical structures have been found around the galactic center. The ‘pigtail’ molecule cloud, however, has a much clearer helical structure than the others. Furthermore, the ‘pigtail’ molecular cloud is an important clue in probing molecular cloud dynamics in the galactic disc, as well as the structure of magnetic fields, due to its close location to the galactic disc.
Bibliographic information: Shinji Matsumura et al. 2012. Discovery of the Pigtail Molecular Cloud in the Galactic Center. ApJ, vol. 756, no. 1; doi: 10.1088/0004-637X/756/1/87