AstroProjects Galaxy colours

The colours of galaxies in SDSS images

Galaxy colours

By and large, elliptical and lenticular galaxies tend to be yellow-white in colour, while galaxies where stars are actively forming, such as spiral galaxies and some irregular galaxies, tend to appear much bluer.

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Richard Beare, 14th December, 2007 Version 1.00 Page 1 of 1

AstroProjects Galaxy colours

A selection of different types of galaxy
The difference in colour between different galaxies can be clearly seen in SDSS images, which actually accentuate the difference in colour compared with what the human eye. This is because the colour images produced by SDSS are composed using somewhat different wavebands to the blue, green and red seen by the human eye. The SDSS g, r and i filters used instead of blue, green and red, actually cover the following parts of the spectrum: violet, blue and green; yellow, orange and red; extreme red and near infrared.

The colour index (g - r)

Thus blue galaxies look bluer and red ones look redder in SDSS. Fortunately, astronomers do not have to rely on the subjective judgements of the human eye in order to measure the colours of astronomical objects, because there are objective accurate ways of measuring colour numerically using a 'colour index'. This depends on comparing an object's brightness in one colour with that in another, e.g. to compare brightness in the SDSS g and r wavebands, (blue-green compared with yellow-orange-red), one calculates the colour index g–r. This has been done for 2671 galaxies in the graph below.

g–r colour indices for 2671 galaxies in the RC3 catalogue which were imaged by SDSS.

Colour index is a ratio

As this is a difference in magnitudes and magnitude is a logarithmic scale, g–r actually depends on the ratio of the light intensity in the g and r wavebands.

This can be seen by looking at the standard formula for comparing two magnitudes given the ratio of the light intensities (or fluxes) which is:

m1–m2=-2.5log10(F1/F2)

Using this to compare magnitudes in the g and r wavebands:

g–r=-2.5log10(Fg/Fr)

shows that g–r actually measures the ratio of the brightness (or energy flux) in the two wavebands.

For further information, read the separate sheet Measuring colour in astronomy.

An alternative colour index: (u - i)

In practice, u–i is often used instead (comparing near ultraviolet and near infrared) because it differentiates much more strongly between different types of galaxies, as the graph below shows.

u–i colour indices for 2671 galaxies in the RC3 catalogue which were imaged by SDSS.

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Richard Beare, 14th December, 2007 Version 1.00 Page 1 of 1