The Opacity of Spiral Galaxy Disks: dust opacity from calibrated counts of distant galaxies

The opacity of foreground spiral disks can be probed from the number of distant galaxies seen through them. To calibrate this number for effects other than the dust extinction, Gonzalez et al (1998) developed the "Synthetic Field Method". A synthetic field is an extincted Hubble Deep Field added to the science field. The relation between the dimming and the number of retrieved synthetic galaxies calibrates the number found in the science field. Here I present results from counts in 32 HST/WFPC2 fields. The relation between opacity and radius, arm and disk, surface brightness and HI are presented. The opacity is found to be caused by a clumpy distribution of clouds in the disk. The brighter parts of the disk -the center and arms- are also the more opaque ones. The dust distribution in spiral disks is found to be more extended than the stellar disk. A comparison between HI column densities and opacity shows little relation between the two