EXTENDED SUBMILLIMETER EMISSION OF THE GALACTIC CENTER AND NEAR-INFRARED/SUBMILLIMETER VARIABILITY OF ITS SUPERMASSIVE BLACK HOLE*

摘要

The innermost tens of parsecs of our Galaxy are characterized by the presence of molecular cloud complexes surrounding Sgr?A*, the radiative counterpart of the supermassive black hole (~4 × 106 M ☉) at the Galactic center. We seek to distinguish the different physical mechanisms that dominate the molecular clouds at the Galactic center, with special emphasis on the circumnuclear disk (CND). We also want to study the energy flow and model the variable emission of Sgr?A*. Our study is based on NIR and submillimeter (sub-mm) observations. Using sub-mm maps, we describe the complex morphology of the molecular clouds and the circumnuclear disk, along with their masses (of order 105-106 M ☉), and derive also the temperature and spectral index maps of the regions under study. We conclude that the average temperature of the dust is 14 ± 4?K. The spectral index map shows that the 20 and 50?km?s–1 clouds are dominated by dust emission. Comparatively, in the CND and its surroundings the spectral indices decrease toward Sgr?A* and range between about 1 and –0.6. These values are mostly explained with a combination of dust, synchrotron, and free-free emission in different ratios. The presence of non-thermal emission also accounts for the apparent low temperatures derived in these areas, indicating their unreliability. The Sgr?A* light curves show significant flux density excursions in both the NIR and sub-mm domains. We have defined a classification system to account for the NIR variability of Sgr?A*. Also, we have modeled on the NIR/sub-mm events. From our modeling results we can infer a sub-mm emission delay with respect to the NIR; we argue that the delay is due to the adiabatic expansion of the synchrotron source components.