Dust lanes are often the most prominent part of the interstellar medium detectable in an optical image of a galaxy. Figure 25 shows different classes of dust lanes, using direct optical images on the left for each galaxy, and a color index map on the right. The color index maps are coded such that blue features are dark while red features (like dust lanes) are light.
Figure 25. Examples showing different classes of dust lanes (left to right) - Row 1 NGC 1530, V-band image and V-Ks color index map; NGC 1566, B-band and B-Ks color index map; Row 2 - NGC 7331, B-band image and B-I color index map; NGC 7217, B-band image and B-I color index map; Row 3 - NGC 7814, B-band image and B-I color index map; NGC 891, B-band and B-V color index map.
The bars of intermediate (mainly Sab to Sbc) spirals often show leading dust lanes, that is, well-defined lanes that lie on the leading edges of the bars, assuming the spiral arms trail. The example shown in Figure 25, NGC 1530, has an exceptionally strong bar and the lanes are very straight, regular, and well-defined. These dust lanes are a dynamical effect associated with the bar. The lanes may be curved or straight. Athanassoula (1992) derived models of bar dust lanes and tied the curvature to the strength of the bar in the sense that models with stronger bars developed straighter dust lanes. Comerón et al. (2009) recently tested this idea by measuring the curvature of actual dust lanes as well as quantitative values of the bar strengths for 55 galaxies. They found that strong bars can only have straight dust lanes, while weaker bars can have straight or curved lanes.
In the same manner as bars, a strong spiral often has dust lanes on the concave sides of the inner arms. This is shown for NGC 1566 in the upper right panels of Figure 25. Both bar and spiral dust lanes are face-on patterns. Another type of face-on pattern is the dust ring. The inner dust ring of NGC 7217 is shown in the right, middle frames of Figure 25, and it appears as the dark, inner edge of a stellar ring having the same shape. Dust rings can also be detected in more inclined galaxies.
An inclined galaxy with a significant bulge also can show another dust effect: in such a case, the bulge is viewed through the dust layer on the near side of the disk, while the dust is viewed through the bulge on the far side of the disk. This leads to a reddening and extinction asymmetry across the minor axis such that the near side of the minor axis is more reddened and extinguished than the far side. In conjunction with rotation data, this near side/far side asymmetry was used by Hubble (1943) and de Vaucouleurs (1958) to show that most spirals trail the direction of rotation.
The lower frames of Figure 25 show the planar dust lanes seen in edge-on spiral galaxies. The lane in NGC 7814 (lower left frames) is red which indicates that the galaxy is probably no later in type than Sa. This is consistent with the large bulge seen in the galaxy. However, the planar dust lane seen in NGC 891, type Sb, has a thin blue section in the middle of a wider red section. The blue color is likely due to outer star formation that suffers relatively low extinction. Some individual star forming regions can be seen along the dust lane. In spite of the blue color, we are only seeing the outer edge of the disk in the plane.
Also related to galactic dust distributions are observations of occulting galaxy pairs, where a foreground spiral galaxy partly occults a background galaxy, ideally an elliptical (White & Keel 1992). With such pairs, one can estimate the optical depth of the foreground dust, often in areas where it might not be seen easily in an isolated spiral. An excellent example is described by Holwerda et al. (2009), who are able to trace the dust distribution in an occulting galaxy to 1.5 times than the standard isophotal radius.
Another way of illustrating the dust distribution in galaxies is with Spitzer Space Telescope Infrared Array Camera (IRAC) images at 8.0µm wavelength. This is discussed further in section 12.