VLT-SINFONI sub-kpc study of the star formation in local LIRGs and ULIRGs - Analysis of the global ΣSFR structure and characterisation of individual star-forming clumps

We present a two-dimensional study of star formation at kiloparsec and sub-kiloparsec scales of a sample of local ( z γ and Pa α emission. In agreement with previous studies, we observe a tight linear correlation between the star formation rate (SFR) derived from our extinction-corrected Pa α measurements and that derived from Spitzer 24 μ m data, and a reasonable agreement with SFR derived from L IR . We also compared our SFR Pa α values with optical measurements from H α emission and find that the SFR Pa α is on average a factor ~3 larger than the SFR H α , even when the extinction corrections are applied. Within the angular resolution and sizes sampled by the SINFONI observations, we found that LIRGs have a median-observed star formation rate surface density of Σ LIRGs obs =1.16 M ⊙ yr -1 kpc -2 , and Σ LIRGs corr =1.72 M ⊙ yr -1 kpc -2 for the extinction-corrected distribution. The median-observed and the extinction-corrected Σ SFR values for ULIRGs are Σ ULIRGs obs =0.16 M ⊙ yr -1 kpc -2 and Σ ULIRGs corr =0.23 M ⊙ yr -1 kpc -2 , respectively. These median values for ULIRGs increase up to 1.38 M ⊙ yr -1 kpc -2 and 2.90 M ⊙ yr -1 kpc -2 , when only their inner regions, covering the same size as the average FoV of LIRGs, are considered. For a given fixed angular sampling, our simulations show that the predicted median of the Σ SFR distribution increases artificially with distance, a factor ~2–3 when the original measurements for LIRGs are simulated at the average distance of our ULIRGs. This could have consequences on any estimates of the star formation surface brightness in high- z galaxies, and consequently on the derivation of the universality of star formation laws at all redshifts. We identified a total of 95 individual star-forming clumps in our sample of U/LIRGs, with sizes that range within ~60–400 pc and ~300–1500 pc, and extinction-corrected Pa α luminosities of ~10 5 –10 7 L ⊙ and ~10 6 –10 8 L ⊙ in LIRGs and ULIRGs, respectively. The Σ SFR of the clumps presents a wide range of values within 1–90 M ⊙ yr -1 kpc -2 and 0.1–100 M ⊙ yr -1 kpc -2 for LIRGs and ULIRGs. Star-forming clumps in LIRGs are about ten times larger and thousands of times more luminous than typical clumps in spiral galaxies, which is consistent with expected photon-bounded conditions in ionized nebulae that surround young stellar clusters. Clumps in ULIRGs have sizes similar (×0.5–1) to those of high- z clumps, having Pa α luminosities similar to some high- z clumps, and about 10 times less luminous than the most luminous high- z clumps identified so far. This could be an indication that the most luminous giant clumps in high- z star-forming galaxies are forming stars with a higher surface density rate than low- z compact ULIRGs. We also observed a change in the slope of the L - r relation, from η = 3.04 of local samples to η = 1.88 from high- z observations. A likely explanation is that most luminous galaxies are interacting and merging, and therefore their size represents a combination of the distribution of the star-forming clumps within each galaxy in the system plus the additional effect of the projected distance between the galaxies. As a consequence, this produces an overall size that is larger than that of individual clumps, or galaxies (for integrated measurements)