EDP Sciences
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Volume 443, Number 1, November III 2005
Page(s) 79 - 90
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20053165

A&A 443, 79-90 (2005)
DOI: 10.1051/0004-6361:20053165

Hierarchical star formation in M 51: star/cluster complexes

N. Bastian1, M. Gieles1, Yu. N. Efremov2 and H. J. G. L. M. Lamers1, 3

1  Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
    e-mail: bastian@astro.uu.nl
    e-mail: bastian@astro.uu.nl, gieles@astro.uu.nl
2  Sternberg Astronomical Institute of Moscow State University, Universitetsky Prospect, 13, Moscow, 119899, Russia
    e-mail: efremov@sai.msu.ru
3  SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
    e-mail: lamers@phys.uu.nl

(Received 31 March 2005 / Accepted 28 June 2005)

We report on a study of young star cluster complexes in the spiral galaxy M 51. Recent studies have confirmed that star clusters do not form in isolation, but instead tend to form in larger groupings or complexes. We use HST broad and narrow band images (from both WFPC2 and ACS), along with BIMA-CO observations to study the properties and investigate the origin of these complexes. We find that the complexes are all young (<10 Myr), have sizes between ~85 and ~240 pc, and have masses between 3- $30 \times
10^{4}~ M_{\odot}$. Unlike that found for isolated young star clusters, we find a strong correlation between the complex mass and radius, namely $M\propto R^{2.33 \pm 0.19}$. This is similar to that found for giant molecular clouds (GMCs). By comparing the mass-radius relation of GMCs in M 51 to that of the complexes we can estimate the star formation efficiency within the complexes, although this value is heavily dependent on the assumed CO-to-H2 conversion factor. The complexes studied here have the same surface density distribution as individual young star clusters and GMCs. If star formation within the complexes is proportional to the gas density at that point, then the shared mass-radius relation of GMCs and complexes is a natural consequence of their shared density profiles. We briefly discuss possibilities for the lack of a mass-radius relation for young star clusters. We note that many of the complexes show evidence of merging of star clusters in their centres, suggesting that larger star clusters can be produced through the build up of smaller clusters.

Key words: galaxies: individual: M 51 -- galaxies: star clusters -- galaxies: starbursts

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