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Session 2 - Spiral, Irregular and Elliptical Galaxies.
Display session, Monday, June 09
South Main Hall,

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

R. A. González, R. J. Allen, D. Calzetti, H. C. Ferguson, N. Panagia (STScI)

We measure the dust extinction at various positions through the disk of the Sbc(s)I-II galaxy NGC 4536 (located in the direction of the Virgo Cluster), from the counts and colors of the background galaxies behind it. To this end, we use archival HST WFPC2 images of the northwestern arm in the 555 and 814 nm broad band filters. The resolution of the WFPC2 helps to reduce the confusion with the star clusters and HII regions in the foreground galaxy, by providing more detailed morphology. As a control field, we use the Hubble Deep Field (HDF). Since it is virtually noiseless, we can add it directly to the galaxy images, simulating different amounts of extinction, and compare the colors and counts of the recovered HDF galaxies to those in the original HDF field. This method also allows us tu decouple the effects of extinction from those caused only by confusion due to crowding.

As a by-product, since we start by identifying and subtracting the resolved stars with the goal of diminishing the confusion, we will also obtain information on changes in the mean age of the stellar population at different distances from the arm.

Program listing for Monday