[0809.3734] A measurement of large-scale peculiar velociti
Posted: October 01 2008
(This is a companion to the longer paper 0809.3733, where the authors provide more details.)
The authors claim to have detected a coherent bulk flow over a scale of at least 300 h^{-1}Mpc. While this might not be completely surprising (see 0809.4041, for example), their method leaves me baffled.
They claim to make the detection using the kinetic SZ effect. They argue as follows, as far as I understand (which is not very far). Take the CMB sky and a cluster catalogue. Remove the cosmological contribution from the CMB sky. Calculate the dipole and monopole at the location of the clusters from this subtracted sky. Now you have the thermals and kinetic SZ components, and from the dipole you identify the kinetic SZ effect, which you can use to measure the peculiar velocity of the cluster.
If their claim is true, it's important as a detection of the kinetic SZ effect at a high significance, apart from the implications for bulk flows. But there are a number of things I don't understand. One of them is: how is it possible to subtract the cosmological CMB sky from the observed one? I understand doing simulations (given a cosmological model) to assess whether something is likely or unlikely to be present in a realisation of a certain cosmology, but that does not seem to be what they are doing. They write that they use a Wiener filter (page 3 of this paper, page 12 of the longer paper), which doesn't say a lot to me.
The authors claim to have detected a coherent bulk flow over a scale of at least 300 h^{-1}Mpc. While this might not be completely surprising (see 0809.4041, for example), their method leaves me baffled.
They claim to make the detection using the kinetic SZ effect. They argue as follows, as far as I understand (which is not very far). Take the CMB sky and a cluster catalogue. Remove the cosmological contribution from the CMB sky. Calculate the dipole and monopole at the location of the clusters from this subtracted sky. Now you have the thermals and kinetic SZ components, and from the dipole you identify the kinetic SZ effect, which you can use to measure the peculiar velocity of the cluster.
If their claim is true, it's important as a detection of the kinetic SZ effect at a high significance, apart from the implications for bulk flows. But there are a number of things I don't understand. One of them is: how is it possible to subtract the cosmological CMB sky from the observed one? I understand doing simulations (given a cosmological model) to assess whether something is likely or unlikely to be present in a realisation of a certain cosmology, but that does not seem to be what they are doing. They write that they use a Wiener filter (page 3 of this paper, page 12 of the longer paper), which doesn't say a lot to me.