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

Abstract

We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO 350−IG038) by means of its stellar cluster population. The central starburst region has been observed in eight different high‐resolution Hubble Space Telescope (HST) wavebands, sampling the stellar and gas components from UV to near‐infrared. Photometric imaging of the galaxy was also carried out at 2.16 μm by NaCo AO instrument at the ESO Very Large Telescope. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters located in the active star‐forming regions and compared them with single stellar population models (suitable for physical properties of very young cluster population) in order to derive ages, masses and extinctions of the star clusters. The cluster age distribution we recover confirms that the present starburst has lasted for 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. We looked for possible relations between cluster ages, extinctions and masses. Extinction tends to diminish as a function of the cluster age, but the spread is large and reaches the highest dispersion for clusters in partial embedded phases (<5 Myr). A fraction of low‐mass (below 104 M⊙), very young (1–3 Myr) clusters is missing, either because they are embedded in the parental molecular cloud and heavily extinguished, or because of blending with neighbouring clusters. The range of the cluster masses is wide; we observe that more than 30 per cent of the clusters have masses above 105 M⊙, qualifying them as super star clusters. Almost half of the cluster sample is affected by flux excesses at wavelengths >8000 Å which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40 Myr; we discuss possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 has produced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. The present cluster formation efficiency is ∼38 per cent of the galactic star formation rate.

Analyzing low signal-to-noise FUSE spectra

“…Thus the dominating hydrogen phases could be H + and H 2 , providing more favourable conditions for escape than in normal galaxies where H I serves as an effective shield for the LyC radiation. Bergvall et al (2006) also investigated the ionizing stellar population and found signatures of Wolf Rayet stars in the optical as well as P Cygni profiles in the far-UV, revealing the presence of O supergiants and an ongoing burst or a burst that has only very recently ended, in agreement with the ages derived in Adamo et al (2010).…”

“…Haro 11 has a complex morphology with three main star-forming knots and seems to be dynamically unrelaxed, remniscent of a merger event (Östlin et al 2001). The knots each contain numerous super star clusters, for which Adamo et al (2010) derived young ages, between 1 and 40 Myr and masses between 10 3 and 10 7 M . Haro 11 is also found to have a low metallicity, with an oxygen abundance 12+log(O/H) = 7.9 (Bergvall & Östlin 2002), and a surprisingly low H I content.…”

Star Forming Dwarf Galaxies

“…It has no neighbors but morphologically and kinematically the galaxy appears to be involved in a merger [116,117,121]. It has a gas consumption timescale of ∼ 100 Myr a birthrate parameter b ∼ 15 and is involved in a vigorous global starburst, producing a large number of super star clusters [1]. The cluster formation efficiency is about 10 times higher than in large spiral galaxies.…”

“…The major difference between the bursting state and the normal one seems rather to be the mode of star formation, the tendency for stars to form in super star clusters (SSCs) [e.g. 1,111] and possibly switch to a flatter IMF [45,139,164].…”

The effect of spatial resolution on optical and near-IR studies of stellar clusters: implications for the origin of the red excess

“…This is caused by the contribution of neighbouring sources as well as the nebular emission from nearby (often related) ionised gas which is stronger at redder wavelengths (e.g., Reines et al 2010;Adamo et al 2010a). Models that include nebular emission can explain some of this excess near-IR emission, however, for some clusters the models cannot account for the additional flux.…”