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Essays

Tri-color Imaging of Selected Arp Galaxies

By Tim Hunter
 

Abstract

Halton Arp published his Atlas of Peculiar Galaxies in 1966. It is considered one of the definitive collections of anomalous galaxies. His images were black and white photographs taken on the Mount Palomar 200-inch telescope. Modern CCD technology now makes it possible to take color CCD images of many of the Arp galaxies using much smaller telescopes. The 24-inch f/5 telescope at the Grasslands Observatory was used to obtain tri-color CCD images of a representative collection of 11 Arp galaxies with a high quantum efficiency back-illuminated CCD system combined with Johnson-Cousins R, V, B filters. These galaxies are discussed in regard to galaxy color, galaxy classification systems, and galaxy interactions. Most of the Arp galaxies represent extreme examples of past and ongoing galaxy interactions.

 

Introduction

In 1966 Halton Arp (1924- ) published his Atlas of Peculiar Galaxies1. This work is now considered a classic in the astronomical community. In the preface to his Atlas, Arp states:

"The present Atlas specifically started from an attempt to better understand spiral galaxies. Despite even recent analyses from a contrary standpoint, I believe that gravitational orbits in a stellar assemblage will not alone furnish satisfactory explanations of galaxies. It is clear that the convolution which spiral arms are seen to undergo in certain galaxies cannot be performed by loci of stellar orbits. In the investigation of these special spiral properties, therefore, galaxies which showed unusual or perturbed arms or filamentary extensions were sampled with high resolution photographs with the Palomar 200-inch telescope. Subjects were first drawn from the pioneering work of Zwicky and Vorontsov-Velyaminov. So many important objects emerged under high-resolution, limiting-magnitude study, however, that the investigation into the nature of spiral arms was temporarily postponed in order to organize systematically these new phenomena into groups and publish a representative sample of the best objects. 1

 

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