2 MICRON SPECTROSCOPY WITHIN 0'.3 OF SAGITTARIUS A

摘要

We present moderate- (R ≈ 2700) and high-resolution (R ≈ 22,400) 2.0-2.4 μm spectroscopy of the central 0.1 arcsec~2 of the Galaxy obtained with the facility near-infrared spectrometer (NIRSPEC) for the Keck II telescope. The composite spectra do not have any features attributable to the brightest stars in the central cluster; i.e., after background subtraction, W_(~12co(2-0)) < 2 A. This stringent limit leads us to conclude that the majority, if not all, of the stars are hotter than typical red giants. Coupled with previously reported photometry, we conclude that the sources are likely OB main-sequence stars. In addition, the continuum slope in the composite spectrum is bluer than that of a red giant and is similar to that of the nearby hot star IRS 16NW It is unlikely that they are late-type giants stripped of their outer envelopes because such sources would be much fainter than those observed. Given their inferred youth (τ_age < 20 Myr), we suggest the possibility that the stars have formed within 0.1 pc of the supermassive black hole. We find a newly identified broad-line component (V_FWHM ≈1000 km s~-1) toward the 2.2178 μm [Fe III] line located within a few arcseconds of Sagittarius A*. A similar component is not seen in the Brγ emission.