Characterizing Ultraviolet and Infrared Observational Properties for
  Galaxies. II. Features of Attenuation Law
  Variations in the attenuation law have a significant impact on observed
spectral energy distributions for galaxies. As one important observational
property for galaxies at ultraviolet and infrared wavelength bands, the
correlation between infrared-to-ultraviolet luminosity ratio and ultraviolet
color index (or ultraviolet spectral slope), i.e., the IRX-UV relation (or
IRX-beta relation), offered a widely used recipe for correcting dust
attenuation in galaxies, but the usability appears to be in doubt now because
of considerable dispersion in this relation found by many studies. In this
paper, on the basis of spectral synthesis modeling and spatially resolved
measurements of four nearby spiral galaxies, we provide an interpretation of
the deviation in the IRX-UV relation with variations in the attenuation law.
From both theoretical and observational viewpoints, two components in the
attenuation curve, the linear background and the 2175 Angstrom bump, are
suggested to be the parameters in addition to the stellar population age
(addressed in the first paper of this series) in the IRX-UV function; different
features in the attenuation curve are diagnosed for the galaxies in our sample.
Nevertheless, it is often difficult to ascertain the attenuation law for
galaxies in actual observations. Possible reasons for preventing the successful
detection of the parameters in the attenuation curve are also discussed in this
paper, including the degeneracy of the linear background and the 2175 Angstrom
bump in observational channels, the requirement for young and dust-rich systems
to study, and the difficulty in accurate estimates of dust attenuations at
different wavelength bands.
