We have conducted J , H , and K _ { S } imaging observations for the Coalsack Globule 2 with the SIRIUS infrared camera on the IRSF 1.4 m telescope at SAAO , and determined the color excess ratio , E _ { J - H } / E _ { H - K _ { S } } .
The ratio is determined in the same photometric system as our previous study for the \rho Oph and Cha clouds without any color transformation ; this enables us to directly compare the near-infrared extinction laws among these regions .
The current ratio E _ { J - H } / E _ { H - K _ { S } } = 1.91 \pm 0.01 for the extinction range 0.5 < E _ { J - H } < 1.8 is significantly larger than the ratios for the \rho Oph and Cha clouds ( E _ { J - H } / E _ { H - K _ { S } } = 1.60 –1.69 ) .
This ratio corresponds to a large negative index \alpha = 2.34 \pm 0.01 when the wavelength dependence of extinction is approximated by a power law A _ { \lambda } \propto \lambda ^ { - \alpha } , which might indicate little growth of dust grains , or larger abundance of dielectric non-absorbing components such as silicates , or both in this cloud .
We also confirm that the color excess ratio for the Coalsack Globule 2 has a trend of increasing with decreasing optical depth , which is the same trend as the \rho Oph and Cha clouds have .