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Session 82 - Spirals & Irregulars.
Display session, Friday, January 09
Exhibit Hall,
The mechanism of structure formation in galaxies is not completely determined yet. In the case of grand design spiral galaxies, the spiral density wave theory is favored, despite its lack of clearly observed age gradients across the spiral arms (although there is one exception (Gonzalez amp; Graham 1996, ApJ 460, 651)). Another mechanism that may cause coherent structure on both small and large scales is self-propagating star formation (SPSF). Frequently, SPSF is able to explain structure formation on small scales (\leq a few 100 pc; e.g. Oey amp; Massey (1995, ApJ 452, 210); Hanawa et al. (1994, ApJ 420, 318); Parker et al. (1992, AJ 103, 1205)). However, doubts remain about the large scales; in the well-studied LMC supershell LMC III, for example, conflicting conclusions have been reached (Dopita et al., 1985, ApJ 297, 599 and Braun et al., astro-ph/9708081). We have obtained HST WFPC2 images of the Magellanic irregular NGC 3664 in four filters to perform the first detailed study of this object. Prominent shell-like structures are clearly visible in groundbased optical broad band and H\alpha images, so this galaxy was considered a prime candidate in which to find SPSF. We have searched for age gradients across these shells and investigated the age structure of the unresolved stellar component. We found no sign for SPSF on large scales (> 300pc), although on smaller scales, we see at least one age gradient, with a resulting propagation velocity similar to those found in the Milky Way and LMC. Concerning the global age structure, it is remarkable that such young star forming regions (some 10^6 yrs) are spread out over the entire disk in several large spatial complexes. This clearly requires a global triggering mechanism, e.g. an encounter with the 35 kpc distant dwarf galaxy NGC 3664A. A slightly older stellar component (up to a few 10^7 yrs) is distributed more homogeneously over the disk, but the number statistics and systematic errors are large for this older component. Another striking feature are five nearly equallly spaced star forming regions (with center to center distances of \sim 600 pc), a feature typical for grand design spiral galaxies (Elmegreen amp; Elmegreen 1983, MNRAS, 203, 182).
