| Abstract
| - Aims. We study the analogy between local luminous and ultraluminous infrared galaxies (U/LIRGs) and high- z massive star forming galaxies (SFGs) by comparing their basic H α structural characteristics, such as size and luminosity surface density, in an homogeneous way (i.e. same tracer, size definition, and similar physical scales). Methods. We use integral field spectroscopy (IFS) based H α emission maps for a representative sample of 54 local U/LIRGs (66 galaxies) observed with INTEGRAL/WHT and VIMOS/VLT. From this initial sample, we select 26 objects with similar H α luminosities ( L(H α)) to those of massive (i.e. M⋆ ~ 10 10 M⊙ or larger) SFGs at z ~ 2, and observed on similar physical scales. We then directly compare the sizes, and luminosity (and SFR) surface densities of these local and high- z samples. Results. The size of the H α emitting region in the local U/LIRGs that we study has a wide range of values, with r1/2(H α) from 0.2 kpc to 7 kpc. However, about two-thirds of local U/LIRGs with Lir > 10 11.4 L⊙ have compact H α emission (i.e. r1/2 < 2 kpc). The comparison sample of local U/LIRGs also contains a larger fraction (59%) of objects with compact H α emission than the high- z sample (25%). This gives further support to the idea that for this luminosity range the size of the star forming region is a distinctive factor when comparing local and distant galaxies of similar SFRs. However, when using H α as a tracer for both local and high- z samples, the differences are smaller than those reported using a variety of other tracers. In the L(H α) - L(H α) surface density (Σ H α) plane, most of the local U/LIRGs and high- z SF galaxies follow the same trend (i.e. higher luminosity for higher surface density) and cover a similar range, except for about 20-40% of local U/LIRGs, which have a higher Σ H α by a factor of about 10. This is considerably smaller than the factors of 1000 or more reported in similar planes (i.e. L(TIR) versus Σ TIR). Despite of the higher fraction of galaxies with compact H α emission, a sizable group (about one-third) of local U/LIRGs are large (i.e. r1/2 > 2 kpc). These are systems that show evidence of pre-coalescence merger activity and are indistinguishable from the massive high- z SFGs galaxies in terms of their H α sizes, and luminosity and SFR surface densities.
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