THE EFFICIENCY AND WAVELENGTH DEPENDENCE OF NEAR-INFRARED INTERSTELLAR POLARIZATION TOWARD THE GALACTIC CENTER

Near-infrared polarimetric imaging observations toward the Galactic center (GC) have been carried out to examine the efficiency and wavelength dependence of interstellar polarization. A total area of about 5.7 deg(2) is covered in the J, H, and K-S bands. We examined the polarization efficiency, defined as the ratio of the degree of polarization to color excess. The interstellar medium between the GC and us shows a polarization efficiency lower than that in the Galactic disk by a factor of three. Moreover we investigated the spatial variation of the polarization efficiency by comparing it with that of the color excess, degree of polarization, and position angle. The spatial variations of color excess and degree of polarization depend on the Galactic latitude, while the polarization efficiency varies independently of the Galactic structure. Position angles are nearly parallel to the Galactic plane, indicating a longitudinal magnetic field configuration between the GC and us. The polarization efficiency anticorrelates with dispersions of position angles. The low polarization efficiency and its spatial variation can be explained by the differences in the magnetic field directions along the line of sight. From the lower polarization efficiency, we suggest a higher strength of a random component relative to a uniform component of the magnetic field between the GC and us. We also derived the ratios of degree of polarization p(H)/p(J) = 0.581 +/- 0.004 and p(KS)/p(H) = 0.620 +/- 0.002. The power-law indices of the wavelength dependence of polarization are beta(JH) = 2.08 +/- 0.02 and beta(HKS) = 1.76 +/- 0.01. Therefore, the wavelength dependence of interstellar polarization exhibits flattening toward longer wavelengths in the range of 1.25-2.14 mu m. The flattening would be caused by aligned large-size dust grains.