The Dust Attenuation Law in Distant Galaxies: Evidence for Variation
  with Spectral Type
  This letter utilizes composite spectral energy distributions (SEDs)
constructed from NEWFIRM Medium-Band Survey photometry to constrain the dust
attenuation curve in 0.5<z<2.0 galaxies. Based on similarities between the full
SED shapes (0.3-8 micron), we have divided galaxies in 32 different spectral
classes and stacked their photometry. As each class contains galaxies over a
range in redshift, the resulting rest-frame SEDs are well-sampled in wavelength
and show various spectral features including Halpha and the UV dust bump at
2175 Angstrom. We fit all composite SEDs with flexible stellar population
synthesis models, while exploring attenuation curves with varying slopes and UV
bump strengths. The Milky Way and Calzetti law provide poor fits at UV
wavelengths for nearly all SEDs. Consistent with previous studies, we find that
the best-fit attenuation law varies with spectral type. There is a strong
correlation between the best-fit dust slope and UV bump strength, with steeper
laws having stronger bumps. Moreover, the attenuation curve correlates with
specific star formation rate (SFR), with more active galaxies having shallower
dust curves and weaker bumps. There is also a weak correlation with
inclination. The observed trends can be explained by differences in the
dust-to-star geometry, a varying grain size distribution, or a combination of
both. Our results have several implications for galaxy evolution studies.
First, the assumption of a universal dust model leads to biases in derived
galaxy properties. Second, the presence of a dust bump may result in
underestimated values for the UV slope, used to correct SFRs of distant
galaxies.
