  Star-forming galaxies (SFGs) adhere to a surprisingly tight scaling relation
of dust attenuation parameterized by the infrared excess (IRX=$L_{\rm
IR}/L_{\rm UV}$), being jointly determined by the star formation rate (SFR),
galaxy size ($R_{\rm e}$), metallicity ($Z$/Z$_\odot$) and axial ratio ($b/a$).
We examine how these galaxy parameters determine the effective dust attenuation
and give rise to the universal IRX relation, utilizing a simple two-component
star-dust geometry model in which dust in the dense and diffuse interstellar
medium (ISM) follows exponential mass density profiles, connected with but not
necessarily identical to the stellar mass profiles. Meanwhile, empirical
relations are adopted to link galaxy properties, including the gas--star
formation relation, the dust-to-stellar size relation, as well as the
dust-to-gas ratio versus metallicity relation. By fitting a large sample of
local SFGs with the model, we obtain the best-fitting model parameters as a
function of metallicity, showing that the two-component geometry model is able
to successfully reproduce the dependence of IRX on SFR, $R_{\rm e}$, $b/a$ at
given $Z$/Z$_\odot$, as well as the dependence of power-law indices on
metallicity. Moreover, we also retrieve constraints on the model geometry
parameters, including the optical depth of birth clouds (BCs), BC-to-total dust
mass fraction, BC covering factor of UV-emitting stars, and star-to-total dust
disc radius ratio, which all evolve with galaxy metallicity. Finally, a
consistent picture of how the star-dust geometry in SFGs evolves with galaxy
metallicity is discussed.
