Empirical determination of the shape of dust attenuation curves in
  star-forming galaxies
  We present a systematic study of the shape of the dust attenuation curve in
star-forming galaxies from the far ultraviolet to the near infrared
(0.15-2microns), as a function of specific star formation rate (sSFR) and axis
ratio (b/a), for galaxies with and without a significant bulge. Our sample
comprises 23,000 (15,000) galaxies with a median redshift of 0.07, with
photometric entries in the SDSS, UKIDSS-LAS (and GALEX-AIS) survey catalogues
and emission line measurements from the SDSS spectroscopic survey. We develop a
new pair-matching technique to isolate the dust attenuation curves from the
stellar continuum emission. The main results are: (i) the slope of the
attenuation curve in the optical varies weakly with sSFR, strongly with b/a,
and is significantly steeper than the Milky Way extinction law in
bulge-dominated galaxies; (ii) the NIR slope is constant, and matches the slope
of the Milky Way extinction law; (iii) the UV has a slope change consistent
with a dust bump at 2175AA which is evident in all samples and varies strongly
in strength with b/a in the bulge-dominated sample; (iv) there is a strong
increase in emission line-to-continuum dust attenuation with both decreasing
sSFR and increasing b/a; (v) radial gradients in dust attenuation increase
strongly with increasing sSFR, and the presence of a bulge does not alter the
strength of the gradients. These results are consistent with the picture in
which young stars are surrounded by dense `birth clouds' with low covering
factor which disperse on timescales of ~1e7 years and the diffuse interstellar
dust is distributed in a centrally concentrated disk with a smaller scaleheight
than the older stars that contribute the majority of the red and NIR light.
[abridged]
