AAS 204th Meeting, June 2004
Session 40 Galaxies
Poster, Tuesday, June 1, 2004, 10:00am-7:00pm, Ballroom

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[40.22] Arm and inter-arm opacities from counts of background galaxies.

R. J. Allen (STScI), B.W. Holwerda (Kapteyn/STScI), R.A. Gonzalez (UNAM), P.C. van der Kruit (Kapteyn)

The opacity of spiral disks can in principle be determined from the numbers of field galaxies seen through it. To calibrate the numbers found, Gonzalez (1998) and Holwerda (2004) used the "Synthetic Field Method", where the relation between extinction and the numbers of field galaxies is found from a series of simulated fields, essentially dimmed Hubble Deep Fields added to the original data. This calibrates the crowding and confusion unique to each disk field.

From earlier studies using occulting galaxies, it is suspected that the spiral arms are a much more opaque region of the spiral disk. We have applied the "Synthetic Field Method" on a sample of 30 deep HST/WFPc2 archival fields. Here we present our results for the average opacities of arm and inter-arm regions derived from the calibrated numbers of field galaxies seen in these regions.

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