Integral field spectroscopy based Hα sizes of local luminous and ultraluminous infrared galaxies A direct comparison with high-z massive star-forming galaxies

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 alpha 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 alpha 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 alpha luminosities (L(H alpha)) to those of massive (i.e. M-star similar to 10(10) M-circle dot or larger) SFGs at z similar to 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 alpha emitting region in the local U/LIRGs that we study has a wide range of values, with r(1/2)(H alpha) from 0.2 kpc to 7 kpc. However, about two-thirds of local U/LIRGs with L-ir > 10(11.4) L-circle dot have compact H alpha emission (i.e. r(1/2) < 2 kpc). The comparison sample of local U/LIRGs also contains a larger fraction (59%) of objects with compact H alpha 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 alpha 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 alpha) - L(H alpha) surface density (Sigma(H alpha)) 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 Sigma(H alpha) 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 Sigma(TIR)). Despite of the higher fraction of galaxies with compact H alpha emission, a sizable group (about one-third) of local U/LIRGs are large (i.e. r(1/2) > 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 alpha sizes, and luminosity and SFR surface densities.