COBE constraints on a Local Group X-ray halo

We investigate the effect of a putative X-ray-emitting halo surrounding the Local Group of galaxies, and specifically the possible temperature anisotropies induced in the COBE-DMR four-year sky maps by an associated Sunyaev-Zel'dovich effect. By fitting the isothermal spherical halo model proposed by Suto et al. to the co-added four-year COBE-DMR 53- and 90-GHz sky maps in Galactic coordinates, we find no significant evidence of a contribution. We therefore reject the claim that such a halo can affect the estimation of the primordial spectral index and amplitude of density perturbations as inferred from the DMR data. We find that correlation with the DMR data imposes constraints on the plausible contribution of such an X-ray-emitting halo to a distortion in the cosmic microwave background spectrum (as specified by the Compton y-parameter), up to a value for R - the ratio of the core radius of the isothermal halo gas distribution to the distance to the Local Group centroid - of 0.68. For larger values of R, the recent cosmological upper limit derived by COBE-FIRAS provides stronger constraints on the model parameters. Over the entire parameter space for R, we find an upper limit to the inferred sky rms anisotropy signal of similar to 14 mu K (95 per cent confidence), a negligible amount relative to the similar to 35-mu K signal observed in the COBE-DMR data.