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

1. Introduction

During the last decade many investigations have revealed the presence of "young massive star clusters" (YMCs) or "super star clusters" in mergers and starburst galaxies, and it has been speculated that these objects could be young analogues of the globular clusters seen today in the Milky Way. It is an intriguing idea that globular clusters could still be formed today in some environments, because the study of such objects would be expected to provide a direct insight into the conditions that were present in the early days of our own and other galaxies when the globular clusters we see today in the halos were formed.

Probably the most famous example of a merger galaxy hosting "young massive clusters" is the "Antennae", NGC 4038/39 where Whitmore & Schweizer (1995) discovered more than 700 blue point-like sources with absolute visual magnitudes up to . Other well-known examples are NGC 7252 (Whitmore et al. 1993), NGC 3921 (Schweizer et al. 1996) and NGC 1275 (Holtzman et al. 1992). All of these galaxies are peculiar systems and obvious mergers. In fact, in all cases investigated so far where star formation is associated with a merger, YMCs have been identified (Ashman & Zepf 1998).

But YMCs exist not only in mergers. They have been located also in starburst galaxies such as NGC 1569 and NGC 1705 (O'Connell et al. 1994), NGC 253 (Watson et al. 1996) and M 82 (O'Connell et al. 1995), in the nuclear rings of NGC 1097 and NGC 6951 (Barth et al. 1995), and in the blue compact galaxy ESO-338-IG04 (Östlin et al. 1998). The magnitudes of YMCs reported in all these galaxies range from to -15, and the effective radii ( = the radius within which half of the light is contained) have been estimated to be of the order of a few parsec to about 20 pc, compatible with the objects deserving the designation "young globular clusters".

All of the systems mentioned above are relatively distant, but in fact one does not have to go farther away than the Local Group in order to find galaxies containing rather similar star clusters. The Magellanic Clouds have long been known to host star clusters of a type not seen in the Milky Way, i.e. compact clusters that are much more massive than Galactic open clusters (van den Bergh 1991, Richtler 1993), and in many respects resemble globular clusters more than open clusters. Some of the most conspicuous examples are the years old cluster in the centre of the 30 Doradus nebula in the LMC (Brandl et al. 1996), shining at an absolute visual magnitude of about -11, and the somewhat older object NGC 1866 (about years), also in the LMC (Fischer et al. 1992), which has an absolute visual magnitude of . Even if these clusters are not quite as spectacular as those found in genuine starburst galaxies, they are still more massive than any of the open clusters seen in the Milky Way today. YMCs have been reported also in M 33 (Christian & Schommer 1988), and in the giant Sc spiral M 101 (Bresolin et al. 1996).

Taking into account the spread in the ages of the YMCs in the Antennae, Fritze - v. Alvensleben (1998) recovered a luminosity function (LF) resembling that of old globular clusters (GC's) to a very high degree when evolving the present LF to an age of 12 Gyr. Elmegreen & Efremov (1997) point out the interesting fact that the upper end of the LF of old GC systems is very similar to that observed for YMCs, open clusters in the Milky Way, and even for HII regions in the Milky Way, and this is one of their arguments in favour of the hypothesis that the basic mechanism behind the formation of all these objects is the same. They argue that massive clusters are formed whenever there is a high pressure in the interstellar medium, due to starbursts or other reasons as e.g. a high virial density (as in nuclear rings and dwarf galaxies). However, this doesn't seem to explain the presence of YMCs in apparently undisturbed disk galaxies like M 33 and M 101.

So it remains a puzzling problem to understand why YMCs exist in certain galaxies, but not in others. In this paper we describe some first results from an investigation aiming at addressing this question. It seems that YMCs can exist in a wide variety of host galaxy environments, and there are no clear systematics in the properties of the galaxies in which YMCs have been identified. And just like it is not clear how YMCs and old globular clusters are related to each other, one can also ask if the very luminous YMCs in mergers and starburst galaxies are basically the same type of objects as those in the Magellanic Clouds, M 33 and M 101.

We therefore decided to observe a number of nearby galaxies and look for populations of YMCs. The galaxies were mainly selected from the Carnegie Atlas (Sandage & Bedke 1994), and in order to minimise the problems that could arise from extinction internally in the galaxies we selected galaxies that were more or less face-on. We tried to cover as wide a range in morphological properties as possible, although the requirement that the galaxies had to be nearby (because we would rely on ground-based observations) restricted the available selection substantially. The final sample consists of 21 galaxies out to a distance modulus of , for which basic data can be seen in Table 1.

Table 1. The galaxies. In the first column each galaxy is identified by its NGC number, the second column gives the morphological classification taken from NED, right ascension and declination for equinox 2000.0 are in columns 3 and 4. Apparent blue magnitude (from RC3) is in the 5th column, the distance modulus is given in column 6, and the absolute blue magnitude is in column 7. The last column indicates which telescope was used for the observations (DK154 = Danish 1.54m. telescope, NOT = Nordic Optical Telescope). The sources for the distances are as follows: ¹ de Vaucouleurs 1963, ² Carignan 1985, ³ Freedman et al. 1992, ⁴ Bottinelli et al. 1985, ⁵ Nearby Galaxies Catalog (Tully 1988) ⁶ de Vaucouleurs 1979a, ⁷ de Vaucouleurs 1979b, ⁸ Shanks 1997, ⁹ Karachentsev & Drozdovsky 1998, ¹⁰ Freedman & Madore 1988 ¹¹ Karachentsev et al. 1996.

In this paper we give an overview of our observations, and we discuss the main properties of the populations of YMCs in the galaxies in Table 1. In a subsequent paper (Larsen & Richtler 1999) we will discuss the correlations between the number of YMCs in a galaxy and various properties of the host galaxies in more detail, and compare our data with data for starburst galaxies and mergers published in the literature.

© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999
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