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 $A_V$.
