4.3. Local Sunyaev-Zel'dovich effects

While the above discussions have concentrated on distant clusters of galaxies and on the integrated Sunyaev-Zel'dovich effects of clusters and the diffuse intergalactic medium, it is also interesting to consider the possibility of distortions of the microwave background radiation induced by gas in the local group.

Suto et al. (1996) have proposed that gas in the local group may contribute to the apparent large-scale anisotropy of the CMBR (specifically, the quadrupolar anisotropy) through the Sunyaev-Zel'dovich effect. If the local group contains a spherical gas halo, described by an isothermal model (equation 64) and with the Galaxy offset a distance x₀ from its center, then the limit on the value of y from the COBE FIRAS data implies that

(77)

if x₀ << r_c. Electron concentrations this small cause a dipole anisotropy of the CMBR that is much smaller than the observed dipole anisotropy, but may produce a significant quadrupole. Suto et al. suggest that this quadrupole may be as large as 40 µK without violating either the X-ray background limits or the COBE FIRAS limits. Since the observed COBE quadrupole is only Q_rms = 6 ± 3 µK (Bennett et al. 1994), significantly less than Q_rms-PS derived from the overall spectrum of fluctuations, a local Sunyaev-Zel'dovich effect may help to explain why we observe an anomalously small quadrupole moment in the CMBR.

This ingenious explanation of the COBE quadrupole in terms of a local Sunyaev-Zel'dovich effect has been criticized by Pildis & McGaugh (1996), who note that to produce a significant quadrupole the electron density in the local group needs to exceed the value typical of distant groups of galaxies by a factor 10. Thus gas in the local group is unlikely to produce a significant contribution to the COBE quadrupole. Furthermore, Banday & Gorski (1996) found that the full model Sunyaev-Zel'dovich effect predicted by Suto et al., when fitted to the COBE dataset, cannot produce a large enough quadrupolar term to be interesting. Nevertheless, it is clear that local gas may cause some small contributions to microwave background anisotropies on angular scales normally thought to be ``cosmological'', and care will be needed in interpreting signals at levels 0.1 µK.