Spin Parity of Spiral Galaxies IV -- Differential Reddening of Globular Cluster Systems of Nearby Spiral Galaxies

TL;DR

This study models and confirms differential reddening effects in globular cluster systems of nearby spiral galaxies, supporting the use of dust asymmetries to determine galaxy orientations and spin directions.

Contribution

It introduces a simple geometric model to evaluate differential reddening and applies it to multiple galaxies, reinforcing the link between dust features and galaxy orientation.

Findings

Confirmed differential reddening in M31 and NGC253 globular clusters.

Supported the use of dust asymmetries to identify the near side of spiral galaxies.

Reinforced the method for studying large-scale galaxy spin distribution.

Abstract

The northwest side of the disk of M31 is known to be the near side because of the differential reddening of globular clusters found from their photographic photometry. This paper reports a simple geometric model to evaluate the visibility of the effect and its application to published CCD photometry on globular cluster systems of three spiral galaxies, M31, M33, and NGC253. The color difference of globular cluster systems due to differential reddening was confirmed for M31 and NGC253; however, the data for M33 were insufficient. The analysis reaffirms the currently adopted interpretation that the side on the minor axis of the galactic disk, where more conspicuous dust features and interstellar reddening are visible, is the nearer side to us and provides an additional basis for using spiral galaxies with identified spiral windings, S-wise or Z-wise, to study the large-scale spin distribution of galaxies in the universe.