Annu. Rev. Astron. Astrophys. 1989. 27: 139-59 Copyright © 1989 by Annual Reviews. All rights reserved

5.9 IC 1613

The low-surface-brightness irregular galaxy IC 1613 is somewhat similar to NGC 6822, though somewhat less luminous. Its stellar content (44, 64, 110), its HI content (84), its HII regions (108), and its enigmatic star clusters (63a, 68, 126) all show that it has an important young population and that star formation is still going on. Its present metals abundance is very similar to that of the SMC (Z = 0.002), and the small number of Wolf-Rayet stars (one) is consistent with such a low value (11). Sandage (108), from an examination of deep Palomar 5-m plates, found evidence for an underlying component of very old stars, though it was not possible, of course, from this alone to gauge their actual age. A small number of very faint, old clusters have been reported (63a), but these objects have not been examined further, nor have the clusters in this remarkably cluster-free galaxy been fully confirmed independently (44). Figure 13 shows the probable history of stellar populations in IC 1613, with a nearly uniform, slow rate of star formation and a gradual enrichment of heavy elements.

Figure 13. The population history of IC 1613.