Perhaps the most striking fact about the cluster-rich galaxies in our sample is that they do not appear to have a lot of other properties in common. Fig. 9 shows the specific frequency as a function of the "T"-type (Table 1), and does not support the suggestion by Kennicutt & Chu (1988) that the presence of YMCs in galaxies increases along the Hubble sequence. Instead, at wide range of values is seen independently of Hubble type, so even if YMCs might be absent in galaxies of even earlier types than we have studied here the phenomenon cannot be entirely related to morphology.
However, what characterises all these cluster systems is that they do not seem to have been formed during one intense burst of star formation. Instead, their age distributions as inferred from the "S" sequence are quite smooth (possibly with the exception of NGC 1313), so in contrast to starburst galaxies like the Antennae or M 82, the rather "normal" galaxies in our sample have been able to maintain a "production" of clusters over a longer timescale, at least several hundred Myr, in a more quiescent mode than that of the starburst galaxies. The most luminous clusters we have found have absolute visual magnitudes of about , about three magnitudes brighter than the brightest open clusters in the Milky Way, but still somewhat fainter than the to clusters in the Antennae and certain starburst galaxies.
One notable exception is NGC 6946 which is forming such a "super star cluster" just before our eyes. That cluster is located far away from the centre of the galaxy, something which is not unusual at all. Also in NGC 1313 the most massive cluster is located far from the centre of the host galaxy, at a projected galactocentric distance of about 3.7 kpc, and in the Milky Way a number of high-mass (old) open clusters are found in the anticentre direction, e.g. M 67. It can of course not be excluded that a massive cluster like the one in NGC 6946 could be located in a region of the Galactic disk hidden from our view, but in any case the Milky Way does not seem to contain any large number of young massive clusters as seen e.g. in NGC 5236 or NGC 1313.
In general we find, however, that the distribution of YMCs follows the H surface brightness profile, at least for those galaxies where the statistics allow such a comparison. Taking H as an indicator of star formation, it then appears that in certain galaxies the formation of YMCs occurs whenever stars are formed. This raises the question whether the presence of massive cluster formation is correlated with global star formation indicators, such as luminosity or other parameters. These questions will be addressed in more detail in a subsequent paper (Larsen et al. 1999).
Two of the galaxies in our sample, NGC 5236 and NGC 2997, have a lot of properties in common. Both galaxies are grand-design, high surface-brightness spirals although NGC 2997 lacks the impressive bar of NGC 5236, and both were known to contain massive star clusters near their centres also before this study. We have identified rich cluster system throughout the disks of these two galaxies.
In our opinion it is becoming clearer and clearer that a whole continuum of cluster properties (age, mass, size) must exist, one just has to look in the right places. For some reason the Milky Way and many other galaxies were only able to form very massive, compact star clusters during the early phases of their evolution, these clusters are today seen as globular clusters in the halos of these galaxies. Other galaxies such as the Magellanic Clouds, M 33 and NGC 2403 are able to form substantially larger number of massive clusters than the Milky Way even today, and in our sample of galaxies we have at least 5 galaxies that are able to form clusters whose masses reach well into the interval defined by the globular clusters of the Milky Way. Still more massive clusters are being formed today in genuine starburst and merger galaxies such as the Antennae, NGC 7252, M 82 and others, and it seems that the masses of these clusters can easily compete with those of "high-end" globular clusters in the Milky Way. Whether YMCs will survive long enough to one day be regarded as "true" globular clusters is still a somewhat controversial question, whose definitive answer requires a detailed knowledge of the internal structure of the individual clusters and a better theoretical understanding of the dynamical evolution of star clusters in general.
One could also ask if the LF of star clusters really has an upper cut-off that varies from galaxy to galaxy, or if the presence of massive clusters is merely a statistical effect that follows from a generally rich cluster system. In order to investigate this question it is necessary to obtain data with a sufficiently high resolution that the search for star clusters can be extended to much fainter magnitudes than we have been able to do in our study.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999