Fingers of God

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Fingers of God in a portion of the Sloan Digital Sky Survey; image from the Cosmus Open Source Science Outreach project [1].

Fingers of God is an effect in observational cosmology that causes clusters of galaxies to be elongated in redshift space, with an axis of elongation pointed toward the observer (Jackson, 1972). It is caused by a Doppler shift associated with the peculiar velocities of galaxies in a cluster. The large velocities that lead to this effect are associated with the gravity of the cluster by means of the virial theorem; they change the observed redshifts of the galaxies in the cluster. The deviation from the Hubble's law relationship between distance and redshift is altered, and this leads to inaccurate distance measurements.

The effect can be seen in the image to the right. The Earth is at the apex of the survey, on the left edge of the image; the individual "fingers", each one actually a cluster of galaxies all at the same distance, point towards it. At greater distances the fractional effect decreases as the peculiar velocities remain roughly constant, and the actual redshift increases. In a plot of "true" distance, instead of the displayed distance in the figure calculated from naïve application of Hubble's law, these fingers would be collapsed back to small spheres at the true cluster sites.

A closely related effect is the Kaiser effect (Kaiser, 1987). It is caused, again, by peculiar velocities lending an additional Doppler shift to the cosmological redshift, and it leads also to a kind of line-of-sight distortion. It is not caused, however, by the random internal motions of the cluster predicted by the virial theorem; rather, it arises from coherent motions as the galaxies fall inwards towards the cluster center as the cluster assembles. Depending on the particular dynamics of the situation, the Kaiser effect usually leads not to an elongation, but an apparent flattening ("pancakes of God"), of the structure. It is a much smaller effect than the fingers of God, and can be distinguished by the fact that it occurs on larger scales. [2]

[edit] References

• Jackson, J.C. (1972). "A critique of Rees's theory of primordial gravitational radiation". Monthly Notices of the Royal Astronomical Society, 156, 1P-6P.

• Kaiser, N. (1987). "Clustering in real space and in redshift space". Monthly Notices of the Royal Astronomical Society, 227, 1-21.