Diversity of Galaxy Dust Attenuation Curves Drives the Scatter in the
  IRX-beta Relation
  We study the drivers of the scatter in the IRX-beta relation using 23,000
low-redshift galaxies from the GALEX-SDSS-WISE Legacy Catalog 2 (GSWLC-2). For
each galaxy we derive, using CIGALE and the SED+LIR fitting technique, the
slope of the dust attenuation curve and the strength of the UV bump, plus many
other galaxy parameters. We find that the IRX-beta scatter is driven entirely
by a wide range of attenuation curves - primarily their slopes. Once the slope
and the UV bump are fixed, the scatter in the IRX-beta vanishes. The question
of the IRX-beta scatter is the direct manifestation of a more fundamental
question of the diversity of dust attenuation curves. The predominant role of
the attenuation curve is the consequence of a narrow range of intrinsic UV
slopes of star-forming galaxies. Galaxies with different specific SFRs or
population ages do not show strong trends in the IRX-beta diagram because their
attenuation curves are, on average, similar. Similarly, there is no shift in
the IRX-beta locus between starbursts and normal star-forming galaxies, both
types having, on average, steep attenuation curves. Optical opacity is
identified as the strongest determinant of the attenuation curve slope, and
consequently the IRX-beta diversity. Despite the scatter, the use of an average
IRX-beta relation is justified to correct SFRs, adding a random error of <~0.15
dex. The form of the local correspondence between IRX-beta and attenuation
curves is maintained at high redshift as long as the evolution of the intrinsic
UV slopes stays within a few tenths.
