Is There a Supermassive Black Hole at the Center of the Milky Way?

Abstract

This review outlines the observations that now provide an overwhelming scientific case that the center of our Milky Way Galaxy harbors a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M ) of 4 × 10 6 M ⊙ within a radius of 100 AU. Orbital speeds have been observed to exceed 5,000 km s −1 . At the focal position there is an extremely compact radio source (Sgr A*), whose apparent size is near the Schwarzschild radius (2GM/c 2 ). This radio source is motionless at the ∼ 1 km s −1 level at the dynamical center of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon.

Microarcsecond Radio Astrometry

“…This technique, called Very Long Baseline Interferometry (VLBI), lead to many discoveries, including superluminal motion in jets from active galactic nuclei (AGN) and upper limits of ∼ 1 pc on the size of the emitting regions. Both results provided strong evidence for super-massive black holes as the engines for AGN (Reid 2009). Early VLBI observations with intrinsic angular resolution better than 1 mas offered absolute position accuracy of ∼ 0.3 mas using groupdelay observables (Clark et al 1976; Ma et al 1986) and relative astrometric accuracy between fortuitously close pairings of QSOs of ∼ 10 µas using phase-delay information (Marcaide et al 1985).…”

Black hole entropy from theSU(2)-invariant formulation of type I isolated horizons

“…Their existence in our nearby universe is by now supported by a great amount of observational evidence [1]. When isolated, these systems are remarkably simple for late and distant observers: once the initial very dynamical phase of collapse is passed the system is expected to settle down to a stationary situation completely described (as implied by the famous results by Carter, Israel, and Hawking [2]) by the three extensive parameters (mass M , angular momentum J, electric charge Q) of the Kerr-Newman family [3].…”

“…There is also the zeroth law stating the uniformity of the surface gravity κ H on the event horizon of stationary black holes, and finally the third law precluding the possibility of reaching an extremal black hole (for which κ H = 0) by means of any physical process 1 . The validity of these classical laws motivated Bekenstein to put forward the idea that black holes may behave as thermodynamical systems with an entropy S = αa/ℓ 2 p and a temperature kT = κ H /(8πα) where α is a dimensionless constant and the dimensionality of the quantities involved require the introduction of leading in turn to the appearance of the Planck length ℓ p , even though in his first paper [5] Bekenstein states "that one should not regard T as the temperature of the black hole; such identification can lead to all sorts of paradoxes, and is thus not useful".…”

The Galaxy in Context: Structural, Kinematic, and Integrated Properties

“…The S2 star has approached the central mass within 125 AU at pericenter, requiring a minimum interior mass density of ρ•(< 125 AU) = 5 × 10 15 M pc −3 . This is so large that one can rule out any known form of compact object other than a black hole (Reid 2009;Genzel, Eisenhauer & Gillessen 2010). From matching the positions of SiO maser stars visible both in the NIR and the radio, the position of the compact mass and the radio source Sgr A * have been shown to coincide within ∼ 2mas = 16 AU (Reid et al 2003;Gillessen et al 2009b).…”