The opacity of spiral galaxy disks

In this paper we explore the relation between dust extinction and stellar light distribution in disks of spiral galaxies. Extinction influences our dynamical and photometric perception of disks, since it can distort our measurement of the contribution of the stellar component. To characterize the total extinction by a foreground disk, González et al. (1998, ApJ, 506, 152) proposed the “Synthetic Field Method” (SFM), which uses the calibrated number of distant galaxies seen through the foreground disk as a direct indication of extinction. The method is described in González et al. (1998, ApJ, 506, 152) and Holwerda et al. (2005a, AJ, 129, 1381). To obtain good statistics, the method was applied to a set of HST/WFPC2 fields (Holwerda et al. 2005b, AJ, 129, 1396) and radial extinction profiles were derived, based on these counts. In the present paper, we explore the relation of opacity with surface brightness or color from 2MASS images, as well as the relation between the scalelengths for extinction and light in the I band. We find that there is indeed a relation between the opacity (AI) and the surface brightness, particularly at the higher surface brightnesses. No strong relation between near infrared ($H-J$, $H-K$) color and opacity is found. The scalelengths of the extinction are uncertain for individual galaxies but seem to indicate that the dust distribution is much more extended than the stellar light. The results from the distant galaxy counts are also compared to the reddening derived from the Cepheids light-curves (Freedman et al. 2001, ApJ, 553, 47). The extinction values are consistent, provided the selection effect against Cepheids with higher values of AI is taken into account. The implications from these relations for disk photometry, M/L conversion and galaxy dynamical modeling are briefly discussed