The Cluster Population of the Irregular Galaxy NGC 4449 as Seen by the Hubble Advanced 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 cluster properties such as size, ellipticity, and total magnitude. 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 H II regions, we find that the clusters have ages distributed quite continuously over a Hubble time, and they have masses from ~103 M sun up to ~2 × 106 M sun, assuming a Salpeter initial mass function down to 0.1 M sun. Young clusters are preferentially located in regions of young star formation (SF), while old clusters are distributed over the whole NGC 4449 field of view, like the old stars (although we note 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 Large Magellanic Cloud (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(LV )vpropL -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 also tend to be more elliptical.
Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute which is operated by AURA Inc. for NASA under contract NAS5-26555.- Publication:
-
The Astronomical Journal
- Pub Date:
- October 2011
- DOI:
- 10.1088/0004-6256/142/4/129
- arXiv:
- arXiv:1108.0694
- Bibcode:
- 2011AJ....142..129A
- Keywords:
-
- galaxies: dwarf;
- galaxies: individual: NGC 4449;
- galaxies: irregular;
- galaxies: starburst;
- galaxies: star clusters: general;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- Accepted for publication on AJ, one data point changed in Fig. 17