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Session 75 - Dust & PAHs in Galaxies.
Display session, Friday, January 09
Exhibit Hall,

[75.02] The Opacity of Spiral Disks from Colors and Counts of Background Galaxies: NGC 4536 and NGC 3664

R. A. Gonzalez, R. J. Allen (STScI), B. Dirsch (Sternwarte Bonn), H. C. T. Ferguson, D. Calzetti, N. Panagia (STScI)

We have developed a method for establishing the opacity of spiral galaxy disks by studying the counts and colors of background galaxies seen through the disk of the foreground galaxy. The method will be used to measure the opacity of several galaxies previously observed with HST, and is calibrated using the Hubble Deep Field as a reference. The method has been applied first to WFPC2 images of NGC 4536 and of the Magellanic irregular NGC 3664.

>From the galaxy number counts, NGC 4536 shows an extinction A_I \sim 1 mag in the Northwestern arm region, and lower than 0.5 mag in the corresponding interarm region (no correction for inclination has been attempted). However, from the galaxy colors, the same reddening of E(V- I) \sim 0.2 is observed in both the arm and the interarm regions. The combination of extinction and reddening yields a galactic reddening law in the interarm region, but a much greyer extinction law in the arm. These results are consistent with a diffuse absorbing component everywhere in the disk, and more opaque clumps in the arms. The results for NGC 3664 (a greyer than galactic extinction law) are also consistent with a clumpy dust distribution.

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