Galaxy color-magnitude diagram

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A mock-up of the galaxy color-magnitude diagram with three populations: the red sequence, the blue cloud, and the green valley.

The Galaxy color-magnitude diagram shows the relationship between absolute magnitude, luminosity, and mass of galaxies. A preliminary description of the three areas of this diagram was made in 2003 by Eric F. Bell et al. from the COMBO-17 survey^[

1] that clarified the bimodal distribution of red and blue galaxies as seen in analysis of Sloan Digital Sky Survey data^[2] and even in de Vaucouleurs' 1961 analyses of galaxy morphology^[3] Noticed in this diagram are three main features: the red sequence, the green valley, and the blue cloud. The red sequence includes most red galaxies which are generally elliptical galaxies. The blue cloud includes most blue galaxies which are generally spirals. In between the two distributions is an underpopulated space known as the green valley which includes a number of red spirals. Unlike the comparable HR diagram for stars, galaxy properties are not necessarily completely determined by their location on the color-magnitude diagram. The diagram also shows considerable evolution through time. The red sequence earlier in evolution of the universe was more constant in color across magnitudes and the blue cloud was not as uniformly distributed but showed sequence progression.

[edit]References

^ Bell, Eric F. et al. Nearly 5000 Distant Early‐Type Galaxies in COMBO‐17: A Red Sequence and Its Evolution since z=1, The Astrophysical Journal, 608:752–767, 2004 June 20. [1]
^ Strateva, I., et al. Color Separation of Galaxy Types in the Sloan Digital Sky Survey Imaging Data, 2001, The Astronomical Journal, 122, 1861 [2]
^ de Vaucouleurs, G. Integrated Colors of Bright Galaxies in the u, b, V System. 1961, The Astrophysical Journal Supplement Series, 5, 233. [3]