Published in PASP 70, 1958.

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A PRELIMINARY CLASSIFICATION OF THE FORMS OF GALAXIES ACCORDING TO THEIR STELLAR POPULATION

W. W. Morgan


Yerkes Observatory, University of Chicago


Abstract. A revised system of classification of the forms of galaxies is described. The principal criterion of type is the degree of central concentration of the light of each galaxy; in this respect the new system can be considered a modification of that of Hubble.

The basis for the new classification is the spectroscopic work on composite spectra of galaxies by Morgan and Mayall. The correlation between form and stellar population found by these investigators has been utilized to set tip a form classification which would be indicative of the general kind of stellar population encountered in the majority of galaxies classified.

A total of 608 galaxies have been classified on the new system from plates obtained principally by Edwin Hubble in the Mount Wilson-Palomar collection.

Possible applications of the new classification to problems of galactic distribution and evolution are outlined. There is a definite indication of large-scale fluctuations in the average stellar population characteristics of galaxies within distances corresponding to redshifts of around 2000 km/sec.


Table of Contents

INTRODUCTION

REASONS FOR A NEW FORM CLASSIFICATION

OUTLINE OF THE NEW FORM CLASSIFICATION
The Terminology for the Fundamental Parameter
Secondary Parameters: the "Form Family"
Secondary Parameters: the "Inclination Class"
Generalities Concerning the Classification System

SOME PHOTOGRAPHIC CONSIDERATIONS
The Amount of Information in a Photographic Image of a Galaxy
The Photographic Gradation of the Prints

A DESCRIPTION OF THE NEW CLASSIFICATION IN TERMS OF THE STANDARD GALAXIES
The I Systems (Plates I-V)
The S Systems (Plates VI-XVI)
The B Systems (Plates XVII-XIX)
The D Systems (Plates XX-XXIV)
Systems With Peculiar or Complex Nuclei (Plates XXV-XXVII)

REASONS FOR A NEW CLASSIFICATION. CONCLUSION

TABLE I

NOTES TO TABLE I

PLATE 1 PLATE 2 PLATE 3
PLATE 4 PLATE 5 PLATE 6
PLATE 7 PLATE 8 PLATE 9
PLATE 10 PLATE 11 PLATE 12
PLATE 13 PLATE 14 PLATE 15
PLATE 16 PLATE 17 PLATE 18
PLATE 19 PLATE 20 PLATE 21
PLATE 22 PLATE 23 PLATE 24
PLATE 25 PLATE 26 PLATE 27

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