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2011A&A...534A..96S - Astronomy and Astrophysics, volume 534A, 96-96 (2011/10-1)

The population of young stellar clusters throughout the disk of M33.


Abstract (from CDS):

The properties of young stellar clusters (YSCs) in M33, identified from the center out to about twice the size of the bright star-forming disk, are investigated to determine possible spatial and time variations of the star formation process in this Local Group blue galaxy. 915 MIR sources have been extracted from the Spitzer 24µm image. Upon inspection of Hα and GALEX images and exclusion of evolved AGB stars, a sample of 648 objects is selected as candidate YSCs and their luminosity function is examined. The spectral energy distribution of each object, based on aperture photometry, is compared with Starburst99 models to derive age, mass and AV of individual clusters. In the analysis we allow for different values of the upper mass cutoff of the stellar initial mass function (IMF), the porosity of the ISM, and the dustiness of HII regions. We also examine the influence of different dust models and include corrections for incompleteness of the IMF. We find discrete MIR sources as far as the extent of the warped HI disk, i.e. 16kpc from the galaxy center. Their surface density has a steep radial decline beyond 4.5kpc, and flattens out beyond the optical radius at 8.5kpc. We are able to identify YSCs out to 12 kpc. At large galactocentric radii, the paucity of luminous clusters and the relevance of hot dust emission become evident from the analysis of the bolometric and MIR luminosity functions. The YSC mass and size are correlated with a log-log slope of 2.09±0.01, similar to that measured for giant molecular clouds in M33 and the Milky Way, which represent the protocluster environment. Most of the YSCs in our sample have AV∼0-1mag and ages between 3 and 10Myr. In the inner regions of M33 the clusters span a wide range of mass (102<M<3x105M) and luminosity (1038<Lbol<3x1041erg/s), while at galactocentric radii larger than ∼4kpc we find a deficiency of massive clusters. Beyond 7 kpc, where the Hα surface brightness drops significantly, the dominant YSC population has M<103M and a slightly older age (10Myr). This implies the occurrence of star formation events about 10Myr ago as far as 10-12kpc from the center of M33. The cluster LFUV vs. L relation is non-linear for LFUV<1039erg/s, in agreement with randomly sampled models of the IMF which, furthermore, shows no appreciable variation throughout the M33 disk.

Abstract Copyright:

Journal keyword(s): galaxies: individual: M33 - galaxies: ISM - galaxies: star clusters: general - galaxies: star formation

VizieR on-line data: <Available at CDS (J/A+A/534/A96): table1.dat table2.dat>

Nomenclature: Table 2: [VHC2007b] IR NNN (Nos 516-915) added.

Status at CDS:   All or part of tables of objects could be ingested in SIMBAD with priority 2.

Simbad objects: 2

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