Systematic biases in determining dust attenuation curves through galaxy SED fitting

While the slope of the dust attenuation curve (delta) is found to correlate with effective dust attenuation (A(V)) as obtained through spectral energy distribution (SED) fitting, it remains unknown how the fitting degeneracies shape this relation. We examine the degeneracy effects by fitting SEDs of a sample of local star-forming galaxies (SFGs) selected from the Galaxy And Mass Assembly survey, in conjunction with mock galaxy SEDs of known attenuation parameters. A well-designed declining starburst star formation history is adopted to generate model SED templates with intrinsic UV slope (beta(0)) spanning over a reasonably wide range. The best-fitting beta(0) for our sample SFGs shows a wide coverage, dramatically differing from the limited range of beta(0) < -2.2 for a starburst of constant star formation. Our results show that strong degeneracies between beta(0), delta, and A(V) in the SED fitting induce systematic biases leading to a false A(V)-delta correlation. Our simulation tests reveal that this relationship can be well reproduced even when a flat A(V)-delta relation is taken to build the input model galaxy SEDs. The variations in best-fitting delta are dominated by the fitting errors. We show that assuming a starburst with constant star formation in SED fitting will result in a steeper attenuation curve, smaller degeneracy errors, and a stronger A(V)-delta relation. Our findings confirm that the A(V)-delta relation obtained through SED fitting is likely driven by the systematic biases induced by the fitting degeneracies between beta(0), delta, and A(V).