New Horizons in GC Astronomy and Beyond

Rationale

The central few hundred parsecs of the Milky Way Galaxy, which is often referred to as the central molecular zone (CMZ), is characterized by a strong concentration of stars and interstellar matter. Many surveys in various wavelengths, from radio to gamma-rays, as well as pointed observations have revealed a variety of astrophysical phenomena, most of which remain elusive: i.e., the three-dimensional distribution of gas; suppressed star formation despite the abundance of dense molecular gas; a currently inactive supermassive black hole but rich signatures of past central activities, the high-energy diffuse gamma-ray emission from GeV up to TeV energies, etc. These unsolved issues span wide ranges of spatial scale, time scale, and energy scale and are likely related to each other. Current key issues in Galactic center studies include: (1) the triggering mechanism of central activity; (2) the formation process of the central supermassive black hole; (3) existence proof of the supermassive black hole at Sgr A*; (4) star formation properties in the CMZ; and (5) the properties of stellar remnants and their binaries, which would be gravitational wave progenitors. These issues are common to studies of extragalactic nuclei.
Recently, millimeter and submillimeter observations with the Atacama Large Millimeter/submillimeter Array (ALMA) have detected at least two candidates for intermediate-mass black holes (IMBHs) in the CMZ, as well as evidence for ongoing star formation in the immediate vicinity of the Galactic nucleus, Sgr A*. Millimeter/submillimeter-wave single-dish observations have delineated the large-scale structure/kinematics of molecular gas, and detected several signatures of mass inflow onto the central few parsecs. High-resolution near-infrared observations have provided the orbital parameters of fast-moving stars around Sgr A*, revealing the existence of a "disk" of young stars. A close passage of a dusty stellar object (DSO/G2) near Sgr A* in spring 2014 was an event of great impact. A slight increase in the brightest X-ray flare rate after the pericenter passage of G2 was reported. A large number of compact objects, including a magnetar, were detected in X-rays in the very vicinity of Sgr A*. All of these recent developments are related to at least one of the aforementioned key issues on Galactic center studies. In addition, the Fermi Large Area Telescope has detected an excess at a few GeV toward the Galactic center, the origin of which is currently under intense debate.