4.2. Mid-type Spirals

In general, spiral arms are more pronounced in the mid-UV, as UIT has shown to be generally true in the far-UV (Bohlin et al. 1991, Hill et al. 1992, Kuchinski et al. 2001, Marcum et al. 2001), but mid-type spirals and star-forming galaxies (e.g., NGC 6753 [Section 3.11] and NGC 3310 [Section 3.13]) appear more similar from the mid-UV to the optical than the early-type galaxies discussed above. However, several appear as later types in the mid-UV (e.g., NGC 7769; Section 3.12), and a few show drastic changes in type from the optical to the mid-UV, equivalent to an apparent change in morphological type T 3 (e.g., NGC 2551 [Section 3.10] and NGC 7685 [Section 3.14]). One galaxy shows a spectacular resonance ring full of hot stars (NGC 6782; Section 3.15), while the remainder of the disk becomes essentially invisible in the mid-UV. This object is discussed in detail by Eskridge et al. (2002a).

We see a variation in color of the galaxy bulges/centers in the spirals, and a considerable range in scale and SB of the individual star-forming forming regions. Dust features in mid-type spirals can be well traced by comparing the F300W to the F814W images. Dust is visible in lanes or patches (possibly trailing spiral density wave patterns?), in pockets, and/or bubbles. A curious feature is that almost without exception, the mid-type spirals in our sample have their small nuclear bulges bisected by a dust-lane, which is often connected to the inner spiral arm structure. This is interesting in the context of the recent finding that all bulge-dominated systems have a central black-hole with an average black hole mass M_bh 0.005 × M_bulge (Magorrian et al. 1998). The small nuclear dust-lane seen by comparing the F814W and mid-UV images of almost all mid-late-type spiral galaxies in our sample may then be involved in feeding the inner accretion disk (which is not visible in our HST images).

Some edge-on galaxies are very faint in F300W when compared to F814W (UGC 10043 [Section 3.16] and ESO 033 - G22 [Section 3.19]), others emit/transmit significantly more in F300W (UGC 06697 [Section 3.17] and IC 3949 [Section 3.18]), while IC 4393 (Section 3.20) is an intermediate case. All show a F300W/F814W ratio increasing from the inside out, as expected for a decreasing dust content from the inside out, and/or a strong radial gradient in the stellar population, if we assume that the dust and light sources are well mixed (i.e., the light sources are not preferentially located in front of most of the dust; see, e.g., Jansen et al. 1994; Witt & Gordon 1996; Kuchinski et al. 1998).