Accretion and gas flows near Sagittarius A*: Toward an understanding of the central parsec of the Milky Way

Abstract

The strong radio emission from Sgr A*, an object located at the dynamical center of the Milky Way, has been attributed to accretion of interstellar gas by a supermassive compact object or dense cluster of objects. We show that any dynamically stable cluster of objects cannot compress the ambient magnetic field or heat the accreting gas sufficiently to reproduce the spectrum of Sgr A*, reaffirming the paradigm that Sgr A* is a single supermassive black hole. We investigate how such a black hole would interact with its surroundings and attempt to determine observational consequences of this interaction. The complexity of the gas, dust, and stellar dynamics of the central parsec of the Galaxy complicates this problem, however. Focusing our attention on the black hole itself but being constrained by observations of the surrounding gas and stars, we have constructed models of the accretion process. We examine two types of accretion models. The first, involving a cold, massive, fossilized accretion disk, is found to generate too much infrared radiation as infalling gas impacts the disk. The second model is spherical accretion, in which the radio emission from Sgr A* is dominated by magnetic bremsstrahlung. Such a model requires accurate emissivities for a wide range of temperatures and field strengths. In this work, we derive the magnetic bremsstrahlung emissivities and apply them to the spherical accretion model, yielding a spectrum that is fully consistent with the radio emission from Sgr A*. We empirically determine a magnetic field profile that suggests the presence of other phenomena, such as a central magnetic dynamo. In addition, the model predicts that the observed high energy emission from the Galactic Center region is not dominated by emission from the central black hole and its environs.