Dust in galaxies can be mapped by either the FIR/sub-mm emission, the opticalor infrared reddening of starlight, or the extinction of a known backgroundsource. We compare two dust extinction measurements for a set of fifteensections in thirteen nearby galaxies, to determine the scale of the dusty ISMresponsible for disk opacity: one using stellar reddening and the other a knownbackground source. In our earlier papers, we presented extinction measurementsof 29 galaxies, based on calibrated counts of distant background objectsidentified though foreground disks in HST/WFPC2 images. For the 13 galaxiesthat overlap with the Spitzer Infrared Nearby Galaxies Survey (SINGS), we nowcompare these results with those obtained from an I-L color map. Our goal is todetermine whether or not a detected distant galaxy indicates a gap in the dustyISM, and hence to better understand the nature and geometry of the diskextinction. We find that distant galaxies are predominantly in low-extinction sectionsmarked by the color maps, indicating that their number depends both on thecloud cover of {\it Spitzer}-resolved dust structures --mostly the spiralarms--and a diffuse, unresolved underlying disk. We note that our infraredcolor map (E[I-L]) underestimates the overall dust presence in these disksseverely, because it implicitly assumes the presence of a dust screen in frontof the stellar distribution.
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机译：可以通过FIR / sub-mm发射，星光的光学或红外变红或已知背景源的消灭来绘制星系中的尘埃。我们比较了十三个附近星系中一组十五个断面的两次尘埃消灭测量结果，以确定造成磁盘不透明的尘土ISM的规模：一种使用恒星变红，另一种使用已知的背景源。在较早的论文中，我们根据通过HST / WFPC2图像中的前景圆盘识别的远处背景物体的校准计数，介绍了29个星系的消光测量。对于与斯皮策红外邻近星系调查（SINGS）重叠的13个星系，我们现在将这些结果与从I-L色图获得的结果进行比较。我们的目标是确定检测到的遥远星系是否表明尘土飞扬ISM中存在缝隙，从而更好地了解磁盘灭绝的性质和几何形状。我们发现，遥远的星系主要位于彩色图标记的低消光部分，这表明它们的数量既取决于{\ it Spitzer}解析的尘埃结构的云层（主要是螺旋臂），又取决于弥散的，未解析的下层磁盘。我们注意到，我们的红外图（E [I-L]）严重低估了这些磁盘中的总尘埃，因为它隐式地假定了恒星分布前面存在一个防尘网。
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