Empirical determination of the shape of dust attenuation curves in star-forming galaxies

Abstract

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