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Astron. Astrophys. 345, 59-72 (1999)

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6. Radial density profiles of cluster systems

As an attempt to investigate how cluster formation correlates with the general characteristics of galaxies, we have compared the surface densities of YMCs (number of clusters per unit area) as a function of galactocentric radius with the surface brigthness in U, V, I and [FORMULA]. Obviously, such a comparison only makes sense for relatively rich cluster systems, and is shown in Fig. 8 for four of the most cluster-rich galaxies in our sample. We did not include data for the apparently quite cluster-rich galaxy NGC 6946 in Fig. 8 because of the numerous Galactic foreground stars in the field of this galaxy which make the cluster identifications less certain.

[FIGURE] Fig. 8. Radial cluster distributions compared with surface brightness profiles in U, V, I and [FORMULA]. The dots with error bars show the "surface density" of clusters, and length of the error bars correspond to poisson statistics.

The surface brightnesses were measured directly on our CCD images using the phot task in DAOPHOT. In the case of [FORMULA] we used continuum-subtracted images, obtained by scaling an R-band frame so that the flux for stellar sources was the same in the R-band and [FORMULA] images, and subtracting the scaled R-band image from the [FORMULA] image. The flux was measured through a number of apertures with radii of 50, 100, 150 [FORMULA] pixels, centered on the galaxies, and the background was measured in an annulus with an inner radius of 850 pixels and a width of 100 pixels. The flux through the i'th annular ring was then calculated as the flux through the i'th aperture minus the flux through the (i-1)'th aperture, and the surface brightness was finally derived by dividing with the area of the i'th annular ring. No attempt was made to standard calibrate the surface brightnesses, so the y-axis units in Fig. 8 are arbitrary. The cluster "surface densities" were obtained by normalising the number of clusters within each annular ring to the area of the respective rings. Finally, all profiles were normalised to the V-band surface brightness profile.

For all the galaxies in Fig. 8 the similarity between the surface brightness profiles and the cluster surface densities is quite striking. In the cases of NGC 2997 and NGC 5236, where the [FORMULA] profiles are markedly different from the broad-band profiles, the cluster surface densities seem to follow the [FORMULA] profiles rather than the broad-band profiles. Accordingly the presence of massive clusters must be closely linked with the process of star formation in general in those galaxies where YMCs are present. In order to get a complete picture one should include the clusters in the central starbursts of NGC 2997 and NGC 5236, but this would, in any case, affect the conclusions only for the innermost bin.

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999