Structure and Evolution of the Opacity of Spiral Disks

TL;DR

This paper investigates the structure and evolution of dust opacity in spiral galaxy disks using multiple measurement techniques, revealing insights into dust distribution, typical cloud optical depth, and potential evolution over cosmic time.

Contribution

It combines galaxy count methods, SED modeling, and occulting galaxy pairs to provide new measurements of disk opacity and its evolution, offering a comprehensive view of dust effects in spiral disks.

Findings

Disks are semi-transparent with more opaque arms.

Typical ISM cloud optical depth is about 0.4.

Initial evidence suggests disk opacity evolves with redshift.

Abstract

The opacity of a spiral disk due to dust absorption influences every measurement we make of it in the UV and optical. Two separate techniques directly measure the total absorption by dust in the disk: calibrated distant galaxy counts and overlapping galaxy pairs. The main results from both so far are a semi-transparent disk with more opaque arms, and a relation between surface brightness and disk opacity. In the Spitzer era, SED models of spiral disks add a new perspective on the role of dust in spiral disks. Combined with the overall opacity from galaxy counts, they yield a typical optical depth of the dusty ISM clouds: 0.4 that implies a size of ~60 pc. Work on galaxy counts is currently ongoing on the ACS fields of M51, M101 and M81. Occulting galaxies offer the possibility of probing the history of disk opacity from higher redshift pairs. Evolution in disk opacity could influence distance measurements (SN1a, Tully-Fisher relation). Here, we present first results from spectroscopically selected occulting pairs in the SDSS. The redshift range for this sample is limited, but does offer a first insight into disk opacity evolution as well as a reference for higher redshift measurements.