On the Nature of the Compact Dark Mass at the Galactic Center

We consider a model in which Sgr A*, the 3.7 × 10 6 M ☉ supermassive black hole candidate at the Galactic center, is a compact object with a thermally emitting surface. For very compact surfaces within the photon orbit, the thermal assumption is likely to be a good approximation because of the large number of rays that are strongly gravitationally lensed back onto the surface. Given the very low quiescent luminosity of Sgr A* in the near-infrared, the existence of a hard surface, even in the limit in which the radius approaches the horizon, places a severe constraint on the steady mass accretion rate onto the source: ≲ 10 -12 M ☉ yr -1 . This limit is well below the minimum accretion rate needed to power the observed submillimeter luminosity of Sgr A*: > 10 -10 M ☉ yr -1 . We thus argue that Sgr A* does not have a surface, i.e., that it must have an event horizon. The argument could be made more restrictive by an order of magnitude with microarcsecond resolution imaging, e.g., with submillimeter very long baseline interferometry.