The Opacity of Nearby Galaxies from Colors and Counts of Background Galaxies: I. The Synthetic Field Method and its Application to NGC 4536 and NGC 3664

摘要

We describe a new, direct method for determining the opacity of foregroundgalaxies which does not require any a priori assumptions about the spatialdistribution or the reddening law of the obscuring material. The method is tomeasure the colors and counts of background galaxies which can be identifiedthrough the foreground system. The method is calibrated, and the effects ofconfusion and obscuration are decoupled by adding various versions of asuitable deep reference frame containing only field galaxies with knownproperties into the image of the foreground galaxy, and analyzing these``synthetic field'' images in the same way as the real images. We test themethod on HST WFPC2 archived images of two galaxies which are quite different:NGC 4536 is a large Sc spiral, and NGC 3664 is a small Magellanic irregular.The reference frames are taken from the Hubble Deep Field. From the background galaxy counts, NGC 4536 shows an extinction A_I ~ 1 magin the northwestern arm region, and lower than 0.5 mag in the correspondinginterarm region (no correction for inclination has been attempted). However,from the galaxy colors, the same reddening of E(V - I) ~ 0.2 is observed inboth the arm and the interarm regions. In the interarm region, the combinationof extinction and reddening can be explained by a diffuse component with aGalactic reddening law (R_V ~ 3). In the spiral arm, however, the same diffuse,low opacity component seems to coexist with regions of much higher opacity.Since the exposures are shorter the results for NGC 3664 are less clear, butalso appear to be consistent with a two component distribution.