A team of astronomers from Japan, led by Keio University scientist Prof. Tomoharu Oka, has detected signs of an intermediate-mass black hole in the central molecular zone of our Milky Way Galaxy.
Artist’s impression of the clouds scattered by an intermediate mass black hole. Image credit: Tomoharu Oka / Keio University.
Using the Nobeyama Radio Observatory 45-m radio telescope, Prof. Oka and co-authors discovered a weird gas cloud just 200 light-years away from the Galactic center.
What makes this object, named CO-0.40-0.22, unusual is its surprisingly wide velocity dispersion.
According to the team, CO-0.40-0.22 has an elliptical shape and consists of two components: a compact, low density component with a very wide velocity dispersion of 100 km/s, and a dense component extending 10 light-years with a narrow velocity dispersion.
“What makes this velocity dispersion so wide? There are no holes inside of the cloud. Also, X-ray and infrared observations did not find any compact objects,” Prof. Oka and his colleagues said.
“These features indicate that the velocity dispersion is not caused by a local energy input, such as supernova explosions.”
The scientists carried out a simple simulation of gas clouds flung by a strong gravity source.
A model using a gravity source of 100,000 solar masses, inside an area with a radius of 0.3 light-years, provided the best fit to the observed data.
The gas cloud CO-0.40-0.22: (a) the center of the Milky Way seen in the 115 and 346 GHz emission lines of carbon monoxide; the white regions show the condensation of dense, warm gas; (b) close-up intensity map around CO-0.40-0.22 seen in the 355 GHz emission line of HCN molecules; the ellipses indicate shell structures in the gas near C0-0.40-0.22. Sgr A* is a supermassive black hole at the center of the Milky Way Galaxy. Image credit: Tomoharu Oka / Keio University / NAOJ.
“Considering the fact that no compact objects are seen in X-ray or infrared observations, as far as we know, the best candidate for the compact massive object is a black hole,” said Prof. Oka, who is the first author on a study published online December 28 in the Astrophysical Journal Letters (arXiv.org preprint).
“If that is the case, this is the first detection of a so-called intermediate mass black hole, a class of black holes that contain between 100 and one million times the Sun’s mass.”
A previous study suggested that there are 100 million black holes in the Milky Way Galaxy, but X-ray observations have only found dozens so far. Most of the black holes may be ‘dark’ and very difficult to see directly at any wavelength.
“Investigations of gas motion with radio telescopes may provide a complementary way to search for ‘dark’ black holes,” Prof. Oka said.
_____
Tomoharu Oka et al. 2016. Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy. ApJ 816, L7; doi: 10.3847/2041-8205/816/1/L7
