Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess

Abstract

We have used multi-band imaging to investigate the nature of the extreme\nstarburst environment in Haro 11 galaxy. The central starburst region has been\nobserved in 8 HST wavebands and at 2.16 micron at the ESO-VLT. We constructed\nintegrated spectral energy distributions (SEDs) for about 200 star clusters and\ncompared them with single stellar population models in order to derive ages,\nmasses and extinctions of thestar clusters. The present starburst has lasted\nfor 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such\nan extremely young cluster population, Haro 11 represents a unique opportunity\nto investigate the youngest phase of the cluster formation process and\nevolution in starburst systems. Extinction tends to diminish as function of the\ncluster age, but the spread is large and for clusters in partial embedded\nphases (< 5 Myr). A fraction of low-mass (> 10^4 Msun), very young (1-3 Myr)\nclusters is missing, either because they are embedded inthe parental molecular\ncloud and heavily extinguished, or because of blending. Almost half of the\ncluster sample is affected by flux excesses at wavelengths 8000 \\AA which\ncannot be explained by simple stellar evolutionary models. Fitting SED models\nover all wavebands leads to systematic overestimates of cluster ages and\nincorrect masses for the stellar population supplying the light in these\nclusters. We show that the red excess affects also the HST F814W filter, which\nis typically used to constrain cluster physical properties. The clusters which\nshow the red excess are younger than 40 Myr; we propose possible physical\nexplanations for the phenomenon. Finally, we estimate that Haro 11 hasproduced\nbound clusters at a rate almost a factor of 10 higher than the massive and\nregular spirals, like the Milky Way. (Abriged)\n