Astron. Astrophys. 327, 562-568 (1997)

7. Summary

The irregular galaxy NGC 1427A is comparable in size, luminosity and color to the LMC. The brightest stellar populations of the galaxy are offset to the south-west compared to its nearly elliptical/disky shape at low isophotes. At the south-west border we identified a half-ring of OB associations and H II regions, which indicate recent star formation. The colors of the `nob' and the low surface brightness tail in the east are consistent with starburst ages less than some years, showing a different star formation history than the main body of NGC 1427A. We found about 34 cluster-type objects down to mag that are uniformly distributed over the galaxy. Their color as well as their luminosity distribution is comparable to the intermediate age cluster population of the LMC. Assuming a LMC metallicity the clusters have mean ages less than 2 Gyr, except a few clusters that might be genuine old globular clusters. In the X-ray NGC 1427A appears similar to the LMC with a relatively soft and complex spectrum and a luminosity in the order of 10⁴⁰ ergs s^-1.

NGC 1427A appears to be nearly a twin of the LMC in all the investigated properties. It is interesting to note, that NGC 1427A shows also some signs for tidal influence on the star formation history. The ages of clusters point at a dominating cluster formation event a few Gyr ago, which accounts for most of the cluster candidates in our data. A sudden event of cluster formation 2 Gyrs ago is well known in the LMC (e.g. Bomans et al. 1995, Girardi et al. 1995) and is thought to be triggered by interaction with the Milky Way. There are several conceivable scenarios that might be responsible for the present appearence of NGC 1427A. Cellone & Forte (1997) interpreted the `nob' as a separate dwarf galaxy and concluded that the particular appearence of NGC 1427A is due to the collision between two galaxies. A similar scenario was suggested by Freeman & de Vaucouleur (1974) for the folded ring galaxy Arp 144. According to this view the distorted appearence and the elongated ring of this galaxy may be due to the encounter with an intergalactic gas cloud, and possibly triggering starbirth regions both in the galaxy and the remainder of the cloud. In our case the former cloud candidate could be the `nob'. Nevertheless, the fact that the `nob' and the main body of the galaxy are embedded in common and rather symmetrical envelope may support our interpretation of the `nob' being an original part of NGC 1427A.

Alternatively, one may speculate about the passage of NGC 1427A through the gas associated with the Fornax cluster as trigger for the subsequent star formation. If this is the case, the main starburst event must have appeared as the galaxy started crossing the dense region in the cluster core near NGC 1399 or NGC 1404. As judged from the cluster candidates in NGC 1427A, this must have happened about 2 Gyr ago. The question, which of these scenarios should be prefered and whether the `nob' represents an initially distinct object or belongs to NGC 1427A, can only be solved by dynamical analysis from high resolution H I measurements and optical spectra.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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