Interstellar Extinction Law toward the Galactic Center III : J, H, KS bands in the 2MASS and the MKO systems, and 3.6, 4.5, 5.8, 8.0 micron in the Spitzer/IRAC system

Shogo Nishiyama(1), Motohide Tamura(2), Hirofumi Hatano(3), Daisuke Kato(4), Toshihiko Tanabe(5), Koji Sugitani(6), and Tetsuya Nagata(1)

(1) Department of Astronomy, Kyoto University, Kyoto, 606-8502, Japan
(2) National Astronomical Observatory of Japan, Mitaka, Tokyo, 181-8588, Japan
(3) Department of Astrophysics, Nagoya University, Nagoya, 464-8602, Japan
(4) Department of Astronomy, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, 113-0033, Japan
(5) Institute of Astronomy, Graduate School of Science, The University of Tokyo, Mitaka, Tokyo, 181-0015, Japan
(6) Graduate School of Natural Sciences, Nagoya City University, Nagoya, 464-8602, Japan

Paper: ApJ, accepted


EPrint Server: 0902.3095


We have determined interstellar extinction law toward the Galactic center (GC) at the wavelength from 1.2 to 8.0 micron , using point sources detected in the IRSF/SIRIUS near-infrared survey and those in the 2MASS and Spitzer/IRAC/GLIMPSE II catalogs. The central region | l | < 3.^o0 and | b | < 1.^o0 has been surveyed in the J, H and KS bands with the IRSF telescope and the SIRIUS camera whose filters are similar to the Mauna Kea Observatories (MKO) near-infrared photometric system. Combined with the GLIMPSE II point source catalog, we made KS versus KS - lambda color-magnitude diagrams where lambda = 3.6, 4.5, 5.8, and 8.0 micron . The KS magnitudes of bulge red clump stars and the KS - lambda colors of red giant branches are used as a tracer of the reddening vector in the color-magnitude diagrams. From these magnitudes and colors, we have obtained the ratios of total to selective extinction A_KS/E_KS- lambda for the four IRAC bands. Combined with A_ lambda /A_KS for the J and H bands derived by Nishiyama et al., we obtain AJ : AH : A_KS : 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 lambda > 3 micron from a simple extrapolation of the power-law extinction at shorter wavelengths, in accordance with recent studies. The extinction law in the 2MASS JHKS bands has also been calculated, and a 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 KS is well fitted by a power law of steep decrease A lambda \propto lambda -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.

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