The Opacity of Spiral Galaxy Disks. IV. Radial Extinction Profiles from Counts of Distant Galaxies Seen through Foreground Disks

Abstract

Dust extinction can be determined from the number of distant field galaxies seen through a spiral disk. To calibrate this number for the crowding and confusion introduced by the foreground image, González et al. (1998) andHolwerda et al. (2005a) developed the "Synthetic Field Method" (SFM), which analyses synthetic fields constructed by adding various deep exposures of unobstructed background fields to the candidate foreground galaxy field.The advantage of the SFM is that it gives the average opacity for area of galaxy disk without assumptions about either the distribution of absorbers or of the disk starlight. However it is limited by low statistics of the surviving field galaxies, hence the need to combine a larger sample of fields. This paper presents the first results for a sample of 32 deep HST/WFPC2 archival fields of 29 spirals.The radial profiles of average dust extinction in spiral galaxies based on calibrated counts of distant field galaxies is presented here, both for individual galaxies as well as for composites from our sample. The effects of inclination, spiral arms and Hubble type on the radial extinction profile are discussed.The dust opacity of the disk apparently arises from two distinct components; an optically thicker (A I = 0.5 − 4 mag) but radially dependent component associated with the spiral arms and a relatively constant optically thinner disk (A I ≈ 0.5 mag.). These results are completely in agreement with earlier work on occulted galaxies. The early

Radial Distribution of Stars, Gas, and Dust in Sings Galaxies. Ii. Derived Dust Properties

“…Prescott et al (2007) obtained extinction profiles in Hα for the SINGS galaxies by comparing the Hα and 24 μm fluxes of individual star-forming regions. The number of distant galaxies seen through a spiral disk can also provide an independent estimation of the extinction (e.g., Holwerda et al 2005 and references therein). .…”

Astrophysics in 2005

“…• And how well you can see through the disk of a spiral depends mostly on just where you look (Holwerda et al 2005a(Holwerda et al , 2005b.…”

The Opacity of Spiral Galaxy Disks

“…The systematics and uncertainties of the method are also discussed in detail in Holwerda et al (2005a) but we briefly list possible systematics here. The selection of the sample is discussed in Holwerda et al (2005b) but the effects of the smaller sample on the new segmentation of distant galaxy counts are discussed below.…”

“…To combat poor statistics, the numbers of distant galaxies in several images are combined, based on common characteristics of the foreground disks. Holwerda et al (2005b) combined numbers based on radius and Hubble types. In this paper we compare the numbers of distant galaxies for image sections of common surface brightness and color.…”

“…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.…”