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

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| < 3deg and |b| < 1deg 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 A(J):A(H):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. 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.

💡 Research Summary

This paper presents a comprehensive determination of the interstellar extinction law toward the Galactic Center (GC) over the wavelength range from 1.2 µm to 8.0 µm. The authors combine three major data sets: (1) near‑infrared point‑source photometry from the IRSF/SIRIUS survey, which uses filters closely matching the Mauna Kea Observatories (MKO) system; (2) the all‑sky 2MASS J, H, and Ks catalog; and (3) the Spitzer/IRAC GLIMPSE II catalog covering the four mid‑infrared bands at 3.6, 4.5, 5.8, and 8.0 µm. The surveyed region is confined to |l| < 3° and |b| < 1°, a zone of extremely high stellar density and severe extinction, making it an ideal laboratory for testing extinction models.

The methodology hinges on constructing color‑magnitude diagrams (CMDs) of Ks versus (Ks – λ), where λ denotes each of the seven bands (J, H, Ks,