Research Highlights

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  • Space & astronomy

Published online: 15 July 2009 | doi:10.1038/nchina.2009.146

Galactic astronomy: Red is not so dead

Felix Cheung

Introduction

© (2009) NASA/JPL-Caltech/Las Campanas

Astronomers like to classify galaxies according to their colour. In general, blue galaxies correspond to young 'late type' galaxies that are actively producing stars, while red galaxies correspond to dead 'early type' galaxies with little or no star formation. Most conventional galaxies comply with this classification system; those that do not — red late types and blue early types — are called 'unconventional galaxies'. Yingping Ding and co-workers at Nanchang University¹ have now estimated the percentage of unconventional galaxies in the universe.

The researchers used data from the Sloan Digital Sky Survey — a major galaxy redshift survey — to explore the properties of galaxies that can be seen from Earth. In the region of lowest galaxy density, the percentages of red early types, red late types, blue late types and blue early types were 23.85%, 26.03%, 47.08% and 3.04%, respectively. In the highest density region, these percentages were 44.86%, 35.40%, 18.39% and 1.35%, respectively.

The findings show that the percentage of red late types rises considerably with increasing local density. In addition, blue galaxies are predominantly late types, whereas only about half of red galaxies are dead, early types. The information will help piece together the mechanisms behind galaxy formation and evolution.

The authors of this work are from:
School of Science, Nanchang University, Nanchang, China.

Reference

1. Deng, X. F., He, J. Z., Wu, P. & Ding, Y. P. The environmental dependence of the fraction of "unconventional" galaxies: red late types and blue early types. Astrophys. J. 699, 948–952 (2009).
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