THE CLUSTER POPULATION OF THE IRREGULAR GALAXY NGC 4449 AS SEEN BY THEHUBBLEADVANCED CAMERA FOR SURVEYS

Abstract

We present a study of the star cluster population in the starburst irregular galaxy NGC 4449 based on B, V, I, and Hα images taken with the Advanced Camera for Surveys on the Hubble Space Telescope. We derive the cluster properties such as size, ellipticity, and total magnitudes. Cluster ages and masses are derived fitting the observed spectral energy distributions with different population synthesis models. Our analysis is strongly affected by the age-metallicity degeneracy; however, if we assume a metallicity of ∼1/4 solar, as derived from spectroscopy of HII regions, we find that the clusters have ages distributed quite continuously over a Hubble time, and they have masses from ∼ 10 3 M ⊙ up to ∼ 2 × 10 6 M ⊙ , assuming a Salpeters' IMF down to 0.1 M ⊙ . Young clusters are preferentially located in regions of young star formation, while old clusters are distributed over the whole NGC 4449 field of view, like the old stars (although we notice that some old clusters follow linear structures, possibly a reflection of past satellite accretion). The high SF activity in NGC 4449 is confirmed by its specific frequency of young massive clusters, higher than the average value found in nearby spirals and in the LMC (but lower than in other starburst dwarfs such as NGC 1705 and NGC 1569), and by the flat slope of the cluster luminosity function (dN(L V ) ∝ L −1.5 V dL for clusters younger than 1 Gyr). We use the upper envelope of the cluster log(mass) versus log(age) distribution to quantify cluster disruption, and do not find evidence for the high (90%) long-term infant mortality found by some studies. For the red clusters, we find correlations between size, ellipticity, luminosity and mass: brighter and more massive clusters tend to be more compact, and brighter clusters tend to be also more elliptical.Subject headings: galaxies: dwarf -galaxies: individual (NGC 4449) -galaxies: -3 -irregular -galaxies: star clusters: general -galaxies: starburst -4 - IntroductionStar clusters are present in all types of galaxies, from the earliest to the latest morphological types, from the quietest to the most turbulent ones, such as merging and starburst systems. They are suggested to be the birth site of many (possibly most) stars, and provide an excellent tool to study the parent galaxy evolution through a long range of epochs. For these reasons, the interest in star cluster studies never fades. While Galactic star clusters were already studied a century ago, extragalactic ones have required the Data acquisition and reduction are summarized in Section 2, while the cluster analysis and the artificial cluster experiments are described in Section 3. The results (luminosity functions, ages and masses, cluster classification, correlations between cluster properties) are presented in Section 4. In Section 5 we investigate cluster disruption. The final results and discussion are presented in Section 6. Observations and data reductionHere we briefly recall the adopted observing strategy and main steps involved in the data reduct...

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