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

Javier Piqueras López, Luis Colina, Santiago Arribas, Miguel Pereira-Santaella, Almudena Alonso-Herrero. 2016. VLT-SINFONI sub-kpc study of the star formation in local LIRGs and ULIRGs Analysis of the global Sigma(SFR) structure and characterisation of individual star-forming clumps. Astronomy and Astrophysics 590, DOI: 10.1051/0004-6361/201527671

We present a two-dimensional study of star formation at kiloparsec and sub-kiloparsec scales of a sample of local (z < 0.1) luminous (10) and ultraluminous (7) infrared galaxies (U/LIRGs), based on near-infrared VLT-SINFONI integral field spectroscopy (IFS). We obtained integrated measurements of the star formation rate and star formation rate surface density, together with their 2D distributions, based on Br gamma and Pa alpha emission. In agreement with previous studies, we observe a tight linear correlation between the star formation rate (SFR) derived from our extinction-corrected Pa alpha measurements and that derived from Spitzer 24 mu m data, and a reasonable agreement with SFR derived from L-IR. We also compared our SFRPa alpha values with optical measurements from H alpha emission and find that the SFRPa alpha is on average a factor similar to 3 larger than the SFRH alpha, 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 Sigma(obs)(LIRGs) = 1.16 M-circle dot yr(-1) kpc(-2), and Sigma(corr)(LIRGs) = 1.72 M-circle dot yr(-1) kpc(-2) for the extinction-corrected distribution. The median-observed and the extinction-corrected Sigma(SFR) values for ULIRGs are Sigma(obs)(ULIRGs) = 0.16 M-circle dot yr(-1) kpc(-2) and Sigma(corr)(ULIRGs) = 0.23 M-circle dot yr(-1) kpc(-2), respectively. These median values for ULIRGs increase up to 1.38 M-circle dot yr(-1) kpc(-2) and 2.90 M-circle dot 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 Sigma(SFR) distribution increases artificially with distance, a factor similar to 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 similar to 60-400 pc and similar to 300-1500 pc, and extinction-corrected Pa alpha luminosities of similar to 10(5)-10(7) L-circle dot and similar to 10(6)-10(8) L-circle dot in LIRGs and ULIRGs, respectively. The Sigma(SFR) of the clumps presents a wide range of values within 1-90 M-circle dot yr(-1) kpc(-2) and 0.1-100 M-circle dot 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 (x0.5-1) to those of high-z clumps, having Pa alpha 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 eta = 3.04 of local samples to eta = 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)

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