5. Notes on individual galaxies
5.1. NGC 1156
This is a Magellanic-type irregular galaxy, currently undergoing an episode of intense star formation. Ho et al. 1995 noted that the spectrum of NGC 1156 resembles that of the "W-R galaxy" NGC 4214. NGC 1156 is a completely isolated galaxy, so the starburst could not have been triggered by interaction with other galaxies. We have found a number of massive star clusters in NGC 1156.
5.2. NGC 1313
This is an SB(s)d galaxy of absolute B magnitude . de Vaucouleurs 1963 found a distance modulus of , which we adopt. The morphology of NGC 1313 is peculiar in the sense that many detached sections of star formation are found, particularly in the south-western part of the galaxy. There is also a "loop" extending about 1.5 Kpc (projected) to the east of the bar with a number of HII regions and massive star clusters. Another interesting feature is that one can see an extended, elongated diffuse envelope of optical light, with the major axis rotated relative to the central bar of NGC 1313, embedding the whole galaxy. It has been suggested by Ryder et al. (1995) that the diffuse envelope surrounding NGC 1313 is associated with galactic cirrus known to exist in this part of the sky (Wang & Yu 1995), but this explanation does not seem likely since it would require a very perfect alignment of the centre of NGC 1313 with the diffuse light. Also, the outer boundary of the active star-forming parts of galaxy coincide quite well with the borders of the more luminous parts of the envelope. In our opinion the most likely explanation is that the diffuse envelope is indeed physically associated with NGC 1313 itself.
Walsh & Roy (1997) determined O/H abundances for 33 HII regions in NGC 1313, and found no radial gradient. This makes NGC 1313 the most massive known barred spiral without any radial abundance gradient.
NGC 1313 hosts a rich population of massive star clusters. When looking at the plot in Fig. 6 it seems that there is a concentration of clusters at log(Age) 8.3 or roughly 200 Myr. We emphasize that this should be confirmed by a more thorough study of the cluster population in this galaxy, and in particular it would be very useful to be able to detect fainter clusters in order to improve the statistics. If this is real it could imply that some kind of event stimulated the formation of massive star clusters in NGC 1313 a few hundred Myr ago, perhaps the accretion of a companion galaxy. A second "burst" of cluster formation seems to have been taking place very recently, and is maybe going on even today.
5.3. NGC 2403
NGC 2403 is a nearby spiral, morphologically very similar to M 33 apart from the fact that NGC 2403 lacks a distinct nucleus. It is a textbook example of an Sc-type spiral, and it is very well resolved on our NOT images. A photographic survey of star clusters in NGC 2403 was already carried out by Battistini et al. 1984, who succeeded in finding a few YMC candidates. NGC 2403 spans more than 20 20 arcminutes in the sky, so we have been able to cover only the central parts using the ALFOSC. Within the central (about 66 Kpc) we have located 14 clusters altogether, but the real number of clusters in NGC 2403 should be significantly higher, taking into account the large fraction of the galaxy that we have not covered, and considering the fact that in the other galaxies we have studied, many clusters are located at considerable distances from the centre.
5.4. NGC 2997
NGC 2997 is an example of a "hot spot" galaxy (Meaburn & Terrett 1982) with a number of UV luminous knots near the centre. Walsh et al. (1986) studied the knots and concluded that they are in fact very massive star clusters, and Maoz et al. (1996) further investigated the central region of NGC 2997 using the HST. On an image taken with the repaired HST through the F606W filter they identified 155 compact sources, all with diameters of a few pc. Of 24 clusters detected in the F606W filter as well as in an earlier F220W image, all have colours implying ages less than 100 Myr and masses . Maoz et al. (1996) conclude that the clusters in the centre of NGC 2997 will eventually evolve into objects resembling globular clusters as we know them in the Milky Way today.
In our study we have found a number of massive star clusters also outside the centre of NGC 2997. Taking the numbers at face value, the cluster system does not appear to be as rich as that of NGC 5236, but with better and more complete data we would expect to see a number of YMCs in NGC 2997 that could rival that in NGC 5236.
5.5. NGC 3621
This galaxy is at first sight a quite ordinary late-type spiral, and has not received much attention. It was observed with the HST by Rawson et al. (1997) as part of the Extragalactic Distance Scale Key Project , and cepheids were discovered and used to derive a distance modulus of 29.1.
Our data show that NGC 3621 contains a surprisingly high number of massive star clusters. The galaxy is rather inclined (, Rawson et al. 1997), and nearly all the clusters are seen projected on the near side of the galaxy, so a number of clusters on the far side may be hidden from our view. Ryder & Dopita (1993) noted a lack of HII regions on the far side of the galaxy, and pointed out that there is also a quite prominent spiral arm on the near side that does not appear to have a counterpart on the far side. So it remains possible that the excess of young clusters and HII regions on the near side is real.
5.6. NGC 5204
NGC 5204 is a companion to the giant Sc spiral M 101. The structure of the HI in this galaxy is that of a strongly warped disk (Sicotte & Carignan 1997), and one could speculate that this is related to tidal interaction effects with M 101. Sicotte & Carignan (1997) also find that the dark matter halo of NGC 5204 contributes significantly to the mass even in the inner parts.
The high value of this galaxy is a consequence of its very low rather than a high absolute number of clusters - we found only 7 clusters in this galaxy. Curiously, all of the 7 clusters belong to the "red" class, suggesting that no new clusters are being formed in NGC 5204 at the moment.
5.7. NGC 5236
NGC 5236 (M 83) is a grand-design barred spiral of type SBc, striking by its regularity and its very high surface brightness - the highest among the galaxies in our sample. The absolute visual magnitude is (de Vaucouleurs et al. 1983). NGC 5236 is currently undergoing a burst of star formation in the nucleus as well as in the spiral arms.
A study in the rocket UV (Bohlin et al. 1990) has already revealed the presence of a number of very young massive star clusters inside the HII regions of NGC 5236, and HST observations of the nucleus (Heap et al. 1993) showed an arc of numerous OB clusters near the centre of the galaxy. These clusters were found to have absolute visual magnitudes in the range from to . and typical radii of the order of 4 pc. Masses were estimated to be between and .
Our investigation adds a large number of massive star clusters in NGC 5236 also outside the centre and the HII regions. In terms of absolute numbers the cluster system of NGC 5236 is by far the richest in our sample, and in particular there is a large number of clusters in the "red" group. This may be partly due to reddening effects although Fig. 6 shows that there is in fact a large intrinsic age spread among the clusters in NGC 5236.
5.8. NGC 6946
The study of NGC 6946 is complicated by the fact that it is located at low galactic latitude (), and there is an interstellar absorption of magnitudes and a large number of field stars towards this galaxy. NGC 6946 is nevertheless a well-studied galaxy, and we also chose to include it in our sample, reasoning that star clusters should be recognizable as extended objects on the NOT data.
The chemical abundances of HII regions in NGC 6946 were studied by Ferguson et al. (1998), who concluded that their data were consistent with a single log-linear dependence on the radius. At 1.5-2 optical radii (defined by the B-band 25th magnitude isophote) they measured abundances of O/H of about 10%-15% of the solar value, and N/O of about 20% - 25% of the solar value.
Among the approximately 100 clusters we have identified in NGC 6946, one stands out as particularly striking (Fig. 7). This cluster is apparently a very young object, located in one of the spiral arms at a distance of 4.4 Kpc from the centre, and with an impressive visual luminosity of . Using a deconvolution-like algorithm (Larsen 1999), the effective radius was estimated to be about 15 pc. The cluster is located within a bubble-like structure with a diameter of about 550 pc, containing numerous bright stars and perhaps some less massive clusters. On optical images this structure is very conspicuous, but it is not visible on the mid-IR ISOCAM maps by Malhotra et al. (1996). There are no traces of emission either, except for a small patch at the very centre of the structure.
5.9. LMC and M 33
For these galaxies, we have adopted data from the literature.
As mentioned in the introduction, both the LMC and M 33 contain young star clusters that are more massive than the open clusters seen in the Milky Way. However, as is evident from Table 2, only one cluster in M 33 is a YMC according to our criteriae. The LMC, on the other hand, contains a relatively rich cluster population, with 7 clusters in the "red" group and 1 cluster in the "blue" group. The cluster R136 in the 30 Doradus nebula of the LMC has not been included in the data for Table 2 because of its location within a giant HII region. Compared to the other galaxies in our sample, the LMC ranks among the relatively cluster-rich ones, but it is also clear that a cluster population like the one of the LMC is by no means unusual.
Because the LMC is so nearby, the limiting magnitude for detection of clusters is obviously much fainter than in the other galaxies in our sample, and the Bica et al. (1996) catalogue should certainly be complete down to our limit of , corresponding to (taking into account an absorption of about 0.25 mags. towards the LMC). If the LMC was located at the distance of most of the galaxies in our sample we would probably not have detected 8 clusters, but a somewhat smaller number, and the value would have been correspondingly lower. This should be kept in mind when comparing the data for the LMC with data for the rest of the galaxies in the sample.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999