The star formation history of luminous infrared galaxies

We constrain the past star formation histories of a sample of 25 distant (z~0.7) luminous infrared galaxies (LIRGs) detected with the mid infrared cameras ISOCAM and MIPS onboard the ISO and Spitzer satellites. We use high resolution VLT-FORS2 spectroscopy in addition to a comprehensive library of 200,000 model optical spectra to derive Bayesian likelihood estimates of the star formation histories of these galaxies based on the analysis of Balmer absorption lines and the 4000 A break. The locus of distant LIRGs in the diagram defined by Hdelta_A and D4000 is roughly comparable to that of local LIRGs observed with IRAS, suggesting that no trend for an evolution is detected between the local and distant LIRGs. We obtain similar results when using either the H8 or the H$\delta_A$ Balmer absorption-line indices in combination with D4000. We compute a birthrate parameter (b=SFR/<SFR>) of 4\pm1, confirming that the distant LIRGs are currently experiencing a major phase of star formation. The most likely duration of the bursts is 0.10 Gyr, during which the LIRGs produce 5-10 % of their current stellar mass. No evidence is found for successive starbursts on the scale of a few times $10^7$ yr, such as those predicted by some numerical simulations of major mergers. However, the large number density of those galaxies suggest that they could experience between two and four LIRG phases until the present epoch.