Morphology and Size Differences between Local & High Redshift Luminous
  Infrared Galaxies
  We show that the star-forming regions in high-redshift luminous and
ultraluminous infrared galaxies (LIRGs and ULIRGs) and submillimeter galaxies
(SMGs) have similar physical scales to those in local normal star-forming
galaxies. To first order, their higher infrared (IR) luminosities result from
higher luminosity surface density. We also find a good correlation between the
IR luminosity and IR luminosity surface density in starburst galaxies across
over five orders of magnitude of IR luminosity from local normal galaxies to z
~ 2 SMGs. The intensely star-forming regions of local ULIRGs are significantly
smaller than those in their high-redshift counterparts and hence diverge
significantly from this correlation, indicating that the ULIRGs found locally
are a different population from the high-redshift ULIRGs and SMGs. Based on
this relationship, we suggest that luminosity surface density should serve as a
more accurate indicator for the IR emitting environment, and hence the
observable properties, of star-forming galaxies than their IR luminosity. We
demonstrate this approach by showing that ULIRGs at z ~ 1 and a lensed galaxy
at z ~ 2.5 exhibit aromatic features agreeing with local LIRGs that are an
order of magnitude less luminous, but have similar IR luminosity surface
density. A consequence of this relationship is that the aromatic emission
strength in star-forming galaxies will appear to increase at z > 1 for a given
IR luminosity compared to their local counterparts.
