2009ApJ...696.1407N

We have determined interstellar extinction law toward the Galactic center (GC) at the wavelength from 1.2 to 8.0 µm, using point sources detected in the IRSF/SIRIUS near-infrared (NIR) survey and those in the Two Micron All Sky Survey (2MASS) and Spitzer/IRAC/GLIMPSE II catalogs. The central region {mid}l {mid} ≲3°.0 and {mid}b {mid} ≲1°.0 has been surveyed in the J, H, and K_S bands with the IRSF telescope and the SIRIUS camera whose filters are similar to the Mauna Kea Observatories (MKO) NIR photometric system. Combined with the GLIMPSE II point source catalog, we made K_Sversus K_S- λ color-magnitude diagrams (CMDs) where λ=3.6, 4.5, 5.8, and 8.0 µm. The K_S magnitudes of bulge red clump stars and the K_S- λ colors of red giant branches are used as a tracer of the reddening vector in the CMDs. From these magnitudes and colors, we have obtained the ratios of total-to-selective extinction for the four IRAC bands. Combined with for the J and H bands derived by Nishiyama et al., we obtain A_J :A_H::A_[3.6]:A_[4.5]:A_[5.8]:A_[8.0]= 3.02:1.73:1:0.50:0.39:0.36:0.43 for the line of sight toward the GC. This confirms the flattening of the extinction curve at λ ≳ 3 µm from a simple extrapolation of the power-law extinction at shorter wavelengths, in accordance with recent studies. The extinction law in the 2MASS J, H, and K_S bands has also been calculated, and good agreement with that in the MKO system is found. Thus, it is established that the extinction in the wavelength range of J, H, and K_S is well fitted by a power law of steep decrease A_λ∝ λ^–2.0 toward the GC. In nearby molecular clouds and diffuse interstellar medium, the lack of reliable measurements of the total-to-selective extinction ratios hampers unambiguous determination of the extinction law; however, observational results toward these lines of sight cannot be reconciled with a single extinction law.