EDP Sciences
Free Access
Volume 397, Number 2, January II 2003
Page(s) 473 - 486
Section Galactic structure and dynamics
DOI https://doi.org/10.1051/0004-6361:20021384
Published online 17 December 2002

A&A 397, 473-486 (2003)
DOI: 10.1051/0004-6361:20021384

Clusters in the inner spiral arms of M 51: The cluster IMF and the formation history

A. Bik1, 2, H. J. G. L. M. Lamers1, 3, N. Bastian1, N. Panagia4, 5 and M. Romaniello6

1  Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
2  Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
    e-mail: bik@astro.uva.nl
3  SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD21218, USA
    e-mail: panagia@stsci.edu
5  On assignment from the Space Science Department of ESA
6  European Southern Observatory, Karl-Schwarzschild Strasse 2, Garching-bei-Muenchen 85748, Germany
    e-mail: mromanie@eso.org

(Received 19 June 2002 / Accepted 16 September 2002 )

We present the results of an analysis of the HST-WFPC2 observations of the interacting galaxy M 51. From the observations in 5 broadband filters ( UBVRI) and two narrowband filters (H $\alpha$ and [OIII]) we study the cluster population in a region of 3.2 $\times 3.2$ kpc 2 in the inner spiral arms of M 51, at a distance of about 1 to 3 kpc from the nucleus. We found 877 cluster candidates and we derived their ages, initial masses and extinctions by means of a comparison between the observed spectral energy distribution and the predictions from cluster synthesis models for instantaneous star formation and solar metallicity. The lack of [OIII] emission in even the youngest clusters with strong H $\alpha$ emission, indicates the absence of the most massive stars and suggests a mass upper limit of about 25 to 30  $M_{\odot}$. The mass versus age distribution of the clusters shows a drastic decrease in the number of clusters with age, much more severe than can be expected on the basis of evolutionary fading of the clusters. This indicates that cluster dispersion is occurring on a timescale of 10 Myr or longer. The cluster initial mass function has been derived from clusters younger than 10 Myr by a linear regression fit of the cumulative mass distribution. This results in an exponent $\alpha = -{\rm d} \log~ N(M) /{\rm d} \log~(M) =
2.1 \pm 0.3 $ in the range of $2.5\times 10^3 < M < 5\times 10^4~\mbox{$M_{\odot}$ }$ but with an overabundance of clusters with $M > 2\times 10^4$  $M_{\odot}$. In the restricted range of $2.5\times 10^3 < M < 2\times 10^4$  $M_{\odot}$ we find $\alpha = 2.0 \pm 0.05$. This exponent is very similar to the value derived for clusters in the interacting Antennae galaxies, and to the exponent of the mass distribution of the giant molecular clouds in our Galaxy. To study the possible effects of the interaction of M 51 with its companion NGC 5195 about 400 Myr ago, which triggered a huge starburst in the nucleus, we determined the cluster formation rate as a function of time for clusters with an initial mass larger than 104  $M_{\odot}$. There is no evidence for a peak in the cluster formation rate at around 200 to 400 Myr ago within 2 $\sigma$ accuracy, i.e. within a factor two. The formation rate of the detected clusters decreases strongly with age by about a factor 102 between 10 Myr and 1 Gyr. For clusters older than about 150 Myr this is due to the evolutionary fading of the clusters below the detection limit. For clusters younger than 100 Myr this is due to the dispersion of the clusters, unless one assumes that the cluster formation rate has been steadily increasing with time from 1 Gyr ago to the present time.

Key words: galaxies: individual: M 51 -- galaxies: interactions -- galaxies: spiral -- galaxies: starburst -- galaxies: star clusters

Offprint request: H. J. G. L. M. Lamers, lamers@astro.uu.nl

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© ESO 2003

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