The Second Galactic Center Black Hole? A Possible Detection of Ionized Gas Orbiting around an IMBH Embedded in the Galactic Center IRS13E Complex

Abstract

The Galactic Center is the nuclear region of the nearest spiral galaxy, the Milky Way, and contains the supermassive black hole with , Sagittarius A * (Sgr A * ). One of the basic questions about the Galactic Center is whether or not Sgr A * is the only “massive” black hole in the region. The IRS13E complex is a very intriguing infrared (IR) object that contains a large dark mass comparable to the mass of an intermediate mass black hole (IMBH) from the proper motions of the main member stars. However, the existence of the IMBH remains controversial. There are some objections to accepting the existence of the IMBH. In this study, we detected ionized gas with a very large velocity width ( km s −1 ) and a very compact size ( au) in the complex using the Atacama Large Millimeter/submillimeter Array (ALMA). We also found an extended component connecting with the compact ionized gas. The properties suggest that this is an ionized gas flow on the Keplerian orbit with high eccentricity. The enclosed mass is estimated to be by the analysis of the orbit. The mass does not conflict with the upper limit mass of the IMBH around Sgr A * , which is derived by the long-term astrometry with the Very Long Baseline Array (VLBA). In addition, the object probably has an X-ray counterpart. Consequently, a very fascinating possibility is that the detected ionized gas is rotating around an IMBH embedded in the IRS13E complex.