9 Summary and conclusions

NGC 7673 is a fascinatingly disturbed galaxy, with a total SFR ($\approx$10-20 $M_{\odot}$ yr^-1) and a SF intensity of for Clump B ($\sim$ $1~M_{\odot}$ yr^-1 kpc^-2) that put it in the range populated by Lyman break galaxies (Pettini et al. 2001). Due to its combination of proximity for a galaxy with CNELG properties and favorable face-on orientation, it affords an excellent opportunity to explore the structure of a major starburst. While further study is needed to fully disentangle the effects of age and reddening on star cluster colors, our initial investigation reveals several interesting trends.

1. Working from WFPC2 images, we have identified 50 star cluster candidates where we measured F255W, F555W and F814W magnitudes, and 268 with F555W and F814W photometry. While we find a broad range of star cluster colors, the bluer and brighter clusters are strongly concentrated into the main starburst "clumps''.

2. In our three color sample we mainly measure clusters with ages of <20 Myr. All of the main optical Clumps, A, B, C, D, and F contain star clusters with ages of 5 Myr or younger. However, the nuclear region, Clump A, may contain older clusters than are found in Clumps B and C. Even so we do not find an obvious age ordering of the clumps. Further observations with more filters and deeper data are needed to allow better separation of clusters which are red due to age versus young clusters reddened by interstellar dust.

3. The four brightest clumps are composed of >50 star clusters detected from the UV through the optical, providing $\sim$16 33$\%$ of the luminosity from the clump as a whole. These results are consistent with other estimates for the fraction of light contributed by compact star clusters in starbursts (e.g., Meurer et al. 1995).

Clump F is distinguished by not being an association of star clusters. It is instead dominated by a single object, for which we derive an age of 4-5 Myr and a mass of $3 \times 10^{5}~M_{\odot}$. If the age is 4 Myr or slightly less, it should show the presence of Wolf-Rayet stars in its spectrum. Perhaps Clump F resembles a more luminous version of compact young stellar complexes with a dominant super star cluster, such as the Hodge complex in NGC 6946 (Larsen et al. 2002) or the extended knot S that is composed of luminous young stars in the Antennae (Whitmore et al. 1999).

4. The star formation process in NGC 7673 differs from that seen in normal spirals in that the disk has broken up into relatively well-defined clumps that contain most of the current star-forming activity. The clumps may be understood as the result of large spatial scale instabilities in perturbed gaseous disks, as suggested by Elmegreen et al. (1993). However, it is not yet clear if the current clumps can survive long enough for dynamical friction to act to bring clumps into the center of the galaxy where they can form a bulge, as suggested by Noguchi (1999).

The unusual structure of NGC 7673 and other nearby starburst galaxies indicate that the impact of highly coeval populations of star clusters should be taken into account when considering how best to model galaxies with intense star formation. These issues extend from implications for the integrated rest-frame UV spectra to their impact on the ISM, and even to the subsequent evolution of the galactic disks (Kroupa 2002b). Furthermore, we must take into account that the formation of dense star clusters is a signature of efficient star formation, where $\ge$$20\%$ of the gas is converted into stars (e.g. Hills 1980; Lada 1999). Formation of numerous dense clusters can allow a galaxy to more efficiently convert gas into stars, thereby sustaining high SFRs in starburst systems.

Acknowledgements

This paper was improved by careful comments from the referee, Danielle Alloin. N.H. acknowledges the ESO Studentship Programme and the Wisconsin Space Grant Consortium Graduate Fellowship Program. J.S.G. and N.H. thank the University of Wisconsin Graduate School for partial support. N.H. would also like to thank Ritter Sport for many bright, chocolatey spots in many gray Garching days, and S.E. for additional encouragement and support. This project uses archival HST WFPC2 data obtained by the WFPC2 Investigation Definition Team for studies of luminous blue galaxies, and we thank the IDT for its early support of this effort.