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. (1996) to the coadded 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 CMB 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 14 microKelvin (95% c.l.), a negligible amount relative to the 35 microKelvin signal observed in the COBE-DMR data.Comment: 4 pages, 3 figures; accepted for publication in MNRAS pink page

Lyman-alpha forest-CMB cross-correlation and the search for the ionized baryons at high redshift

The intergalactic neutral hydrogen which is responsible for the Lyman alpha forest of quasar absorption is a tracer of much larger amounts of ionised hydrogen. The ionised component has yet to be detected directly, but is expected to scatter CMB photons via the Sunyaev-Zel'dovich (SZ) effect. We use hydrodynamic simulations of a LambdaCDM universe to create mock quasar spectra and CMB sky maps. We find that the high-z Lya forest gas causes temperature fluctuations of the order of 1 muK rms in the CMB on arcmin scales. The kinetic and thermal SZ effects have a similar magnitude at z=3, with the thermal effect becoming relatively weaker as expected at higher z. The CMB signal associated with lines of sight having HI column densities > 10^18 cm^-2 is only marginally stronger than that for lower column densities. The strong dependence of rms temperature fluctuation on mean Lya absorbed flux, however, suggests that the CMB signal effectively arises in lower density material. We investigate the use of the cross-correlation of the Lya forest and the microwave background to detect the SZ effect at redshifts 2-4. In so doing we are able to set direct limits on the density of diffuse ionised intergalactic baryons. We carry out a preliminary comparison at a mean redshift z=3 of 3488 quasar spectra from SDSS Data Release 3 and the WMAP first year data. Assuming that the baryons are clustered as in a LambdaCDM cosmology, and have the same mean temperature, the cross-correlation yields a weak limit on the cosmic density of ionised baryons Omega_(b,I), which is Omega_(b,I) < 0.8 at 95% confidence. With data from upcoming CMB telescopes, we anticipate that a direct detection of the high redshift ionised IGM will soon be possible, providing an important consistency check on cosmological models.Comment: 14 pages, 10 figures, submitted to MNRA

Testing the Gaussianity of the COBE-DMR data with spherical wavelets

We investigate the Gaussianity of the 4-year COBE-DMR data (in HEALPix pixelisation) using an analysis based on spherical Haar wavelets. We use all the pixels lying outside the Galactic cut and compute the skewness, kurtosis and scale-scale correlation spectra for the wavelet coefficients at each scale. We also take into account the sensitivity of the method to the orientation of the input signal. We find a detection of non-Gaussianity at $> 99$ per cent level in just one of our statistics. Taking into account the total number of statistics computed, we estimate that the probability of obtaining such a detection by chance for an underlying Gaussian field is 0.69. Therefore, we conclude that the spherical wavelet technique shows no strong evidence of non-Gaussianity in the COBE-DMR data.Comment: latex file 7 pages, 6 figures, submitted to MNRA