The X-Ray Halo of the Local Group and Its Implications for Microwave and Soft X-Ray Backgrounds

Since recent X-ray observations have revealed that most clusters of galaxiesare surrounded by an X-ray emitting gaseous halo, it is reasonable to expectthat the Local Group of galaxies has its own X-ray halo. We show that such ahalo, with temperature $\sim 1$keV and column density $\sim O(10^{21}) {\rmcm}^{-2}$, is a possible source for the excess low-energy component in theX-ray background. The halo should also generate temperature anisotropies in themicrowave background via the Sunyaev-Zel'dovich effect. Assuming an isothermalspherical halo with the above temperature and density, the amplitude of theinduced quadrupole turns out to be comparable to the COBE data withoutviolating the upper limit on the $y$-parameter. The induced dipole isnegligible compared to the peculiar velocity of the Local Group, and multipoleshigher than quadrupole are generally much smaller than the observed ones.However non-sphericity and/or clumpiness of the halo will produce a strongereffect. Therefore the gaseous halo of the Local Group, if it exists, willaffect the estimate of the primordial spectral index $n$ and the amplitude ofthe density fluctuations deduced from the {\sl COBE} data.Comment: 13 pages. uuencoded, gzipped tar file including LaTeX text and 4 postcript figures. ApJ(Letters) in pres