Young stellar clusters and associations in M33

Abstract

We analyse multi-wavelength observations of 32 young star clusters and associations in M33 with known oxygen abundance (8 < 12 + log(O/H) < 8.7), using ultraviolet (UV), optical, mid-infrared (MIR), CO (1-0) and 21-cm line (HI) observations. We derive their spectral energy distribution, and we determine age, bolometric luminosities, masses and the extinction, by comparing the multi-band integrated photometry to single-age stellar population models. The stellar system ages range between 2 and 15 Myr, masses are between 3 x 10^2 and 4 x 10^4 M_sun, and the intrinsic extinction, A_V, varies from 0.3 to 1 mag. We find a correlation between age and extinction, and between the cluster mass and size. The MIR emission shows the presence of a dust component around the clusters whose fractional luminosity at 24 um, L_{24}/L_{Bol}, decreases with the galactocentric distance. However, the total IR luminosity inferred from L_{24} is smaller than what we derive from the extinction corrections. The Halpha luminosity predicted by population synthesis models is larger than the observed one, especially for low-mass systems (M < 10^4 M_sun). Such a difference is reduced, but not erased, when the incomplete sampling of the initial mass function (IMF) at the high-mass end is taken into account. Our results suggest that a non-negligible fraction of UV ionising and non-ionising radiation is leaking into the ISM outside the HII regions. This would be in agreement with the large UV and Halpha diffuse fractions observed in M33, but it implies that stellar systems younger than 3 Myr retain, on average, only 30% of their Lyman continuum photons. However, the uncertainties on cluster ages and the stochastic fluctuations of the IMF do not allow to accurately quantify this issue. We also consider the possibility that this discrepancy is the consequence of a suppressed or delayed formation of the most massive stars.

17 pages, 13 figures. Accepted for publications in A&A; v2 --> Table 2 corrected because of a misprint in the FUV magnitudes

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Cited by in corpus (11)

• Emission-line diagnostics of nearby HII regions including interacting binary populations
• GMC Evolutions in the Nearby Spiral Galaxy M33
• On how leakage can affect the Star Formation Rate estimation using Halpha luminosity
• The population of Young Stellar Clusters throughout the disk of M33
• Millimeter and Submillimeter Excess Emission in M33 revealed by Planck and LABOCA
• Spectral Energy Distributions of HII regions in M33 (HerM33es)
• The dependence of stellar age distributions on GMC environment
• Ionizing stellar population in the disc of NGC 3310 - II. The Wolf-Rayet population
• On the nature of faint mid-infrared sources in M33
• Star formation drivers across the M33 disk
• M^3D: Mosaicking M33 with MUSE datacubes I. Unveiling the diversity of HII regions in M33 with MUSE