The efficiency and wavelength dependence of near-infrared interstellar
  polarization toward the Galactic center
  Near-infrared polarimetric imaging observations toward the Galactic center
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 degree of polarization to color excess. The interstellar medium
between the Galactic center and us shows the 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 those of
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 the longitudinal magnetic field configuration between the Galactic
center 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 of 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 Galactic center and us. We also derived the ratios of degree
of polarization $p_H/p_J$ = 0.581 $\pm$ 0.004 and $p_{K_S}/p_H$ = 0.620 $\pm$
0.002. The power law indices of the wavelength dependence of polarization are
$\beta_{JH}$ = 2.08 $\pm$ 0.02 and $\beta_{HK_S}$ = 1.76 $\pm$ 0.01. Therefore
the wavelength dependence of interstellar polarization exhibits flattening
toward longer wavelengths in the range of 1.25$-$2.14 $\micron$. The flattening
would be caused by aligned large-size dust grains.
