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 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 Law relationship between distance and redshift is altered, and this leads to inaccurate distance measurements. Image File history File links Fingersofgod. ... Image File history File links Fingersofgod. ... The Sloan Digital Sky Survey or SDSS is a major redshift survey using a dedicated 2. ... // Cosmology, from the Greek: κοσμολογία (cosmologia, κόσμος (cosmos) world + λογια (logia) discourse) is the study of the Universe in its totality, and by extension, humanitys place in it. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ... Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left). ... The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ... The term peculiar velocity refers to the components of a receding galaxys velocity that cannot be explained by Hubbles law. ... Gravity is a force of attraction that acts between bodies that have mass. ... In physics, the virial theorem states that the average kinetic energy of a system of particles whose motions are bounded is given by where ri and Fi are the position and force vectors on the i th particle respectively. ... Hubbles law is the statement in astronomy that the redshift in light coming from distant galaxies is proportional to their distance. ...

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 the Hubble 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] 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 cosmology that causes clusters of galaxies to be elongated in redshift space, with an axis of elongation pointed toward the observer. ...

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.