All Figures

$\begin{figure} \par\includegraphics[width=8cm,clip]{7511f01.ps} \end{figure}$ Figure 1: The contours outlining the three background regions, superimposed on the HST/ACS mosaic image in the F555W passband. The solid lines enclose "high background'' regions, and everything outside the dashed lines indicates the "low background'' regions. The regions in between solid and dashed lines are called "intermediate background''.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f02.ps} \end{figure}$ Figure 2: The aperture correction ( ${\rm AC}_{0.5''}$ ) for the flux between a 5 and 10 pixel (= $0.5\hbox {$^{\prime \prime }$ }$ ) aperture in F435W, versus the effective radius $R_{\rm eff}$ of analytical sources at the distance of M 51 (diamonds). The dashed line is a linear fit to the data of the form of Eq. (1).
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{7511f03.ps} \end{figure}$ Figure 3: The recovered fraction of artificial clusters as a function of magnitude and $R_{\rm eff}$ for the three background regions on the F435W image (i.e. "completeness profiles''). The horizontal dotted line indicates where 90% completeness is reached. The magnitudes at which the completeness profiles reach this 90% completeness limit are summarized in Fig. 4.
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In the text

$\begin{figure} \par\includegraphics[width=7.7cm,clip]{7511f04.ps} \end{figure}$ Figure 4: The 90% completeness limits versus effective radius ( $R_{\rm eff}$ ), determined for F435W, F555W and F814W for the three background regions.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{7511f05.ps} \end{figure}$ Figure 5: The median ( top), 68th percentile ( middle) and 90th percentile ( bottom) of $\delta$ (see Eq. (3)) versus F435W magnitude for the artificial clusters with different radii in the high background region. Below the horizontal line the uncertainty of the radii measurements is smaller than 20%, which is achieved for more than 68% of the clusters up to 9 pc brighter than ${\it F435W}= 22.6$ mag (indicated by the arrow).
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7511f06.ps} \end{figure}$ Figure 6: The mean difference between the measured magnitude and the input magnitude of the recovered artificial clusters versus F435W magnitude for different sizes in the high background region. The truncated line for the largest and faintest clusters means none of these artificial clusters were recovered.
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In the text

$\begin{figure} \par\includegraphics[width=27mm]{7511f07.ps}\includegraphics[widt... ...ics[width=27mm]{7511f14.ps}\includegraphics[width=27mm]{7511f15.ps} \end{figure}$ Figure 7: A selection of F435W images centered on sources that were rejected from the sample after visual inspection. The size of each image is $60\times 60$ pixels or 3 $^{\prime \prime }$ $\times$ 3 $^{\prime \prime }$ . Rejection was based on the following criteria: a) neighbour too close by, b) likely blended, c) and d) clearly blended, e) too irregular, f) crowded, g) interfering background, h) radius fit on the background and i) likely a background galaxy.
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In the text

$\begin{figure} \par\includegraphics[width=27mm]{7511f16.ps}\includegraphics[widt... ...ics[width=27mm]{7511f23.ps}\includegraphics[width=27mm]{7511f24.ps} \end{figure}$ Figure 8: A selection of isolated stellar cluster candidates in F435W that passed the visual inspection. The size of each image is 60 $\times$ 60 pixels or 3 $^{\prime \prime }$ $\times$ 3 $^{\prime \prime }$ .
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f25.ps} \end{figure}$ Figure 9: $R_{\rm eff}$ versus $V\!-\!I$ colour for the total sample of 1284 clusters. The upper-right corner of the dashed lines indicate the region of the sub-group of faint fuzzy candidates (filled circles). These cuts are the same as used by Hwang & Lee (2006) to select faint fuzzy clusters in NGC 5195.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{7511f26.ps} \end{figure}$ Figure 10: Cluster "212995'', the largest cluster in our sample with a $\mbox{$R_{\rm eff}$ }=21.6$ pc (centre of the inset), and its location in M 51 (north is up, east is to the right). The projected galactocentric distance is 1.02 kpc. The scale of the inset image is $\sim$ $60\times 60$ pixels or $\sim$ $122\times 122$ pc. The scale of the big image is $\sim$ $16\times 16$ kpc.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{7511f27.ps} \end{figure}$ Figure 11: The differences between the colours of clusters matched between our ACS dataset and the WFPC2 dataset of Bastian et al. (2005), versus the colour in the ACS dataset.The dashed lines show a difference of 0 as a reference for the eye.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7511f28.ps} \end{figure}$ Figure 12: The differences between the effective radii of the clusters matched between our ACS dataset and the WFPC2 dataset of Bastian et al. (2005), versus the effective radius in the ACS dataset.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7511f29.ps} \end{figure}$ Figure 13: The effective radius distribution of the total sample of 1284 clusters, measured on the F435W image, using linear radius bins (solid line). Also shown are the radius distribution of only the sources in the low background region (dotted line) and the sources in the high background region (dashed line). For better readability, only the (Poissonian) error bars of the total sample are shown.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f30.ps} \end{figure}$ Figure 14: The effective radius distribution of the total sample of 1284 clusters, measured on the F435W image, using logarithmic radius bins (solid line). Also shown are the radius distribution of only the sources in the low background region (dotted line) and the sources in the high background region (dashed line). For better readability, only the (Poissonian) error bars of the total sample are shown. For comparison we also show a power law with a slope of -2.3, typical for a fractal radius distribution of star-forming gas clouds, and a slope of -1.2 as determined by B05 for the radius distribution of 407 star clusters in M 51 with WFPC2 data.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{7511f31.ps} \end{figure}$ Figure 15: The normalized $B\!-\!V$ colour distribution for clusters in the low (dotted) and high (dashed) background region. The high background region has a higher fraction of blue clusters than the low background region.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f32.ps} \end{figure}$ Figure 16: The effective radius distribution split in a sample with $\mbox{$B\!-\!V$ }< 0.1$ (blue, dashed line, 645 clusters) and $\mbox{$B\!-\!V$ }> 0.1$ (red, dotted line, 639 clusters). The radius distribution of the red sample is shifted towards larger radii.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{7511f33.ps} \end{figure}$ Figure 17: The median $R_{\rm eff}$ versus colour for bins containing an equal number of clusters for the complete cluster sample ( top) and the low ( middle) and high ( bottom) background region. The vertical error bars indicate the range in which 68% of the clusters is contained.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f34.ps} \end{figure}$ Figure 18: The radius distribution of the 271 matched clusters for which we have age estimates from B05, split in a sample with log (age) > 7.5 and log (age) < 7.5.
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In the text

$\begin{figure} \par\includegraphics[width=6.7cm,clip]{7511f35.ps} \end{figure}$ Figure 19: The median $R_{\rm eff}$ versus galactocentric distance, for bins containing an equal number of clusters. The vertical error bars indicate the range in which 68% of the clusters is contained.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f36.ps} \end{figure}$ Figure 20: The effective radius distributions for three different galactocentric distance regions in M 51. The solid line shows clusters close to the centre of M 51 (distance < 3 kpc), the dotted line shows clusters at intermediate distances (3 < distance < 5.5 kpc) and the dashed line shows clusters at large galactocentric distances (>5.5 kpc). The changes in the distributions are consistent with a slight increase in $R_{\rm eff}$ with distance.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f37.ps} \end{figure}$ Figure 21: The median $B\!-\!V$ colour versus galactocentric distance of the 1284 clusters in our sample for bins containing an equal number of clusters. The vertical error bars indicate the range in which 68% of the clusters is contained. No obvious trend is visible in the data.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f38.ps} \end{figure}$ Figure 22: The median $R_{\rm eff}$ versus F435W magnitude for our sample of 317 clusters in the low background region ( top) and for our sample of 728 clusters in the high background region. The bins contain an equal number of clusters and the vertical error bars indicate the range in which 68% of the clusters is contained.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f39.ps} \end{figure}$ Figure 23: $R_{\rm eff}$ versus mass for the 271 clusters that were matched with the cluster sample of B05.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{7511f01.ps} \end{figure}$	Figure 1: The contours outlining the three background regions, superimposed on the HST/ACS mosaic image in the F555W passband. The solid lines enclose "high background'' regions, and everything outside the dashed lines indicates the "low background'' regions. The regions in between solid and dashed lines are called "intermediate background''.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f02.ps} \end{figure}$	Figure 2: The aperture correction ( ${\rm AC}_{0.5''}$ ) for the flux between a 5 and 10 pixel (= $0.5\hbox {$^{\prime \prime }$ }$ ) aperture in F435W, versus the effective radius $R_{\rm eff}$ of analytical sources at the distance of M 51 (diamonds). The dashed line is a linear fit to the data of the form of Eq. (1).
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{7511f03.ps} \end{figure}$	Figure 3: The recovered fraction of artificial clusters as a function of magnitude and $R_{\rm eff}$ for the three background regions on the F435W image (i.e. "completeness profiles''). The horizontal dotted line indicates where 90% completeness is reached. The magnitudes at which the completeness profiles reach this 90% completeness limit are summarized in Fig. 4.
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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{7511f04.ps} \end{figure}$	Figure 4: The 90% completeness limits versus effective radius ( $R_{\rm eff}$ ), determined for F435W, F555W and F814W for the three background regions.
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$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7511f06.ps} \end{figure}$	Figure 6: The mean difference between the measured magnitude and the input magnitude of the recovered artificial clusters versus F435W magnitude for different sizes in the high background region. The truncated line for the largest and faintest clusters means none of these artificial clusters were recovered.
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$\begin{figure} \par\includegraphics[width=27mm]{7511f16.ps}\includegraphics[widt... ...ics[width=27mm]{7511f23.ps}\includegraphics[width=27mm]{7511f24.ps} \end{figure}$	Figure 8: A selection of isolated stellar cluster candidates in F435W that passed the visual inspection. The size of each image is 60 $\times$ 60 pixels or 3 $^{\prime \prime }$ $\times$ 3 $^{\prime \prime }$ .
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$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f25.ps} \end{figure}$	Figure 9: $R_{\rm eff}$ versus $V\!-\!I$ colour for the total sample of 1284 clusters. The upper-right corner of the dashed lines indicate the region of the sub-group of faint fuzzy candidates (filled circles). These cuts are the same as used by Hwang & Lee (2006) to select faint fuzzy clusters in NGC 5195.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{7511f26.ps} \end{figure}$	Figure 10: Cluster "212995'', the largest cluster in our sample with a $\mbox{$R_{\rm eff}$ }=21.6$ pc (centre of the inset), and its location in M 51 (north is up, east is to the right). The projected galactocentric distance is 1.02 kpc. The scale of the inset image is $\sim$ $60\times 60$ pixels or $\sim$ $122\times 122$ pc. The scale of the big image is $\sim$ $16\times 16$ kpc.
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$\begin{figure} \par\includegraphics[width=7.4cm,clip]{7511f27.ps} \end{figure}$	Figure 11: The differences between the colours of clusters matched between our ACS dataset and the WFPC2 dataset of Bastian et al. (2005), versus the colour in the ACS dataset.The dashed lines show a difference of 0 as a reference for the eye.
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$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7511f28.ps} \end{figure}$	Figure 12: The differences between the effective radii of the clusters matched between our ACS dataset and the WFPC2 dataset of Bastian et al. (2005), versus the effective radius in the ACS dataset.
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$\begin{figure} \par\includegraphics[width=7.4cm,clip]{7511f31.ps} \end{figure}$	Figure 15: The normalized $B\!-\!V$ colour distribution for clusters in the low (dotted) and high (dashed) background region. The high background region has a higher fraction of blue clusters than the low background region.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f32.ps} \end{figure}$	Figure 16: The effective radius distribution split in a sample with $\mbox{$B\!-\!V$ }< 0.1$ (blue, dashed line, 645 clusters) and $\mbox{$B\!-\!V$ }> 0.1$ (red, dotted line, 639 clusters). The radius distribution of the red sample is shifted towards larger radii.
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$\begin{figure} \par\includegraphics[width=7.4cm,clip]{7511f33.ps} \end{figure}$	Figure 17: The median $R_{\rm eff}$ versus colour for bins containing an equal number of clusters for the complete cluster sample ( top) and the low ( middle) and high ( bottom) background region. The vertical error bars indicate the range in which 68% of the clusters is contained.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f34.ps} \end{figure}$	Figure 18: The radius distribution of the 271 matched clusters for which we have age estimates from B05, split in a sample with log (age) > 7.5 and log (age) < 7.5.
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$\begin{figure} \par\includegraphics[width=6.7cm,clip]{7511f35.ps} \end{figure}$	Figure 19: The median $R_{\rm eff}$ versus galactocentric distance, for bins containing an equal number of clusters. The vertical error bars indicate the range in which 68% of the clusters is contained.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f37.ps} \end{figure}$	Figure 21: The median $B\!-\!V$ colour versus galactocentric distance of the 1284 clusters in our sample for bins containing an equal number of clusters. The vertical error bars indicate the range in which 68% of the clusters is contained. No obvious trend is visible in the data.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f38.ps} \end{figure}$	Figure 22: The median $R_{\rm eff}$ versus F435W magnitude for our sample of 317 clusters in the low background region ( top) and for our sample of 728 clusters in the high background region. The bins contain an equal number of clusters and the vertical error bars indicate the range in which 68% of the clusters is contained.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{7511f39.ps} \end{figure}$	Figure 23: $R_{\rm eff}$ versus mass for the 271 clusters that were matched with the cluster sample of B05.
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