5 The resulting photometries

 

 

Figure 4:   Contour plots of the photometries of the Milky Way at central wavelengths of 280nm (above) and 217nm (below). The units for the given intensity steps are $10^{-11}\frac{W}{\rm m^2\,nm\,sr}$.For clarity, the areas between the contours are coloured as indicated in the greyscales

 

Figure 5:   The photometric map of the Southern Milky Way at a central wavelength of 280 nm. The colour coded intensity range is (black) $\le$ -50 ... 350$10^{-11}\frac{W}{\rm m^2\,nm\,sr}$ $\le$ (white)

 

Figure 6:   The photometric map of the Southern Milky Way at a central wavelength of 217nm. The colour coded intensity range is (black) $\le$ -50 ... 400$10^{-11}\frac{W}{\rm m^2\,nm\,sr}$ $\le$ (white)

In Figs. 5 and 6 the final surface photometries are presented. Both maps cover the Milky Way between Cygnus and Vela ($100^\circ \le l \ge 270^\circ$, $-40^\circ \le b \le 40^\circ$). Figure 4 shows the same maps as contour plots. The appearance of the Milky Way at 217nm and 280nm is dominated strongly by interstellar extinction, leading to high intensity gradients all over the galactic plane. Especially, the effect by local dark clouds is extremely strong. Together with young OB stars emitting high flux densities in the UV, they are responsible for various bright reflection nebulae and other light emissions that can be attributed to scattering phenomena.

Other bright sources on the photometries are near, young open clusters like NGC3114, IC2602, M6, M7, M8, and M20. Since these objects consist mainly of OB stars, the bright appearence of them is to be expected.

The region around $\eta$Car, where the tangential point of the Sgr-Car spiral arm is assumed, is one of the brightest areas in both photometries. This can be explained by two effects. First, in this direction the extinction is assumed to be very low (see e.g. Neckel & Klare 1980). Even in the UV, one can see deep into our galaxy onto the inner spiral arm. Second, spiral arms are dominated by young, hot stellar objects. Hence, the integrated star light of many sources bright in the UV forms the high surface intensities in this direction.

The galactic center region is hardly visible at all. Here, the stellar population building the bulge is a relatively old one. Although there is no agreement on the exact age of the bulge, it is generally accepted, that it ought to be older than 5-8 Gyr (Holtzman et al. 1993; Bertelli et al. 1995 and others in there). The resulting low number of young, hot stars, is responsible for the low surface intensity in this region. Additionally, the effects of the large dark clouds of the Ophiuchus complex must not be neglected. The whole region around the galactic center is almost completely hidden by this complex.

Comparing the appearance of the Milky Way in the UV with the earthbound UBVR photometries shows a straight line of order. Towards shorter wavelengths, the image of the Milky Way becomes more and more disrupted due to the increasing effects of interstellar extinction. The galactic center, which is the brightest part in R, becomes fainter, whereas the surface brightness of the Carinae region increases and begins to exceed the galactic center brightness in B.