Estimating dust attenuation from galactic spectra. I. methodology and
  tests
  We develop a method to estimate the dust attenuation curve of galaxies from
full spectral fitting of their optical spectra. Motivated from previous
studies, we separate the small-scale features from the large-scale spectral
shape, by performing a moving average method to both the observed spectrum and
the simple stellar population model spectra. The intrinsic dust-free model
spectrum is then derived by fitting the observed ratio of the small-scale to
large-scale (S/L) components with the S/L ratios of the SSP models. The
selective dust attenuation curve is then determined by comparing the observed
spectrum with the dust-free model spectrum. One important advantage of this
method is that the estimated dust attenuation curve is independent of the shape
of theoretical dust attenuation curves. We have done a series of tests on a set
of mock spectra covering wide ranges of stellar age and metallicity. We show
that our method is able to recover the input dust attenuation curve accurately,
although the accuracy depends slightly on signal-to-noise ratio of the spectra.
We have applied our method to a number of edge-on galaxies with obvious dust
lanes from the ongoing MaNGA survey, deriving their dust attenuation curves and
$E(B-V)$ maps, as well as dust-free images in $g$, $r$, and $i$ bands. These
galaxies show obvious dust lane features in their original images, which
largely disappear after we have corrected the effect of dust attenuation. The
vertical brightness profiles of these galaxies become axis-symmetric and can
well be fitted by a simple model proposed for the disk vertical structure.
Comparing the estimated dust attenuation curve with the three commonly-adopted
model curves, we find that the Calzetti curve provides the best description of
the estimated curves for the inner region of galaxies, while the Milky Way and
SMC curves work better for the outer region.
