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Register a new userThe Sunyaev-Zeldovich Effect at 1 and 2 mm towards ROSAT Clusters
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P.Andreani
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An observing campaign was devoted to the search for the Sunyaev-Zeldovich (S-Z) effect towards X-ray ROSAT Clusters in the millimetric spectral domain. A double channel (1.2 and 2 {it mm}) photometer was installed at the focus of the 15m Swedish ESO Submillimeter Telescope (SEST) in Chile in september 1994 and 1995 and observations of the targets S1077, A2744, S295 and RXJ0658-5557 were gathered. Detections were found for A2744 at 1 {it mm} and in both channels (at 1.2 and 2 {it mm}) towards RXJ0658-5557. For the first time there is evidence for the S-Z enhancement and both the latter and the decrement were detected on the same source. We discuss astrophysical and systematic effects which could give origin to these signals.
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We report simultaneous observations at 1.2 and 2 mm, with a double channel photometer on the SEST Telescope, of the X-ray cluster RXJ0658-5557 in search for the Sunyaev-Zeldovich (S-Z). The S-Z data were analyzed using the relativistically correct expression for the Comptonization parameter and we find from the detected decrement (2.60 +/- 0.79) ~ 10^{-4}, which is consistent with that computed using the X-ray (ROSAT and ASCA) observations. The uncertainty includes contributions due to statistical uncertainty in the detection and systematics and calibration. The 1.2 {mm} channel data alone gives rise to a larger Comptonization parameter and this result is discussed in terms of contamination from foreground sources and/or dust in the cluster or from a possible systematic effect. We then make use of the combined analysis of the ROSAT and ASCA X-ray satellite observations to determine an isothermal model for the S-Z surface brightness. Since the cluster is asymmetrical and probably in a merging process, models are only approximate. The associated uncertainty can, however, be estimated by exploring a set of alternative models. We then find as the global uncertainty on the Comptonization parameter a factor of 1.3. Combining the S-Z and the X-ray measurements, we determine a value for the Hubble constant. The 2 mm data are consistent with H_0(q_0 = 1/2)= 53^{+38}_{-28} km/s Mpc^{-1}, where the uncertainty is dominated by the uncertainty in models of the X-ray plasma halo.
WMAP observations at mm wavelengths are sensitive to the Sunyaev-Zeldovich effect in galaxy clusters. Among all the objects in the sky, the Virgo cluster is expected to provide the largest integrated signal. Based on models compatible with the X-ray emission observed in the ROSAT All Sky Survey, we predict a two-sigma detection of the SZ effect from Virgo in the WMAP 3-year data. Our analysis reveals a 3-sigma signal on scales of 5 degrees, although the frequency dependence deviates from the theoretical expectation for the SZ effect. The main sources of uncertainty are instrumental noise, and most importantly, possible contamination from point sources and diffuse back/foregrounds. In particular, a population of unresolved extragalactic sources in Virgo would explain the observed intensity and frequency dependence. In order to resolve this question one needs to wait for experiments like Planck to achieve the required accuracy.
In this paper we investigate the Sunyaev-Zeldovich (SZ) effect and the X-ray surface brightness for clusters of galaxies with a non-spherical mass distribution. In particular, we consider the influence of the shape and the finite extension of a cluster as well as of a polytropic thermal profile on the Compton parameter, the X-ray surface brightness and on the determination of the Hubble constant. We find that the the non-inclusion of such effects can induce errors up to 30 per cent in the various parameters and in particular on the Hubble constant value, when compared with results obtained under the isothermal, infinitely extended and spherical shape assumptions.
Observations of the X-ray band wavelength reveal an evident ellipticity of many galaxy clusters atmospheres. The modeling of the intracluster gas with an ellipsoidal $beta$-model leads to different estimates for the total gravitational mass and the gas mass fraction of the cluster than those one finds for a spherical beta-model. An analysis of a recent Chandra image of the galaxy cluster RBS797 indicates a strong ellipticity and thus a pronounced aspherical geometry. A preliminary investigation which takes into account an ellipsoidal shape for this cluster gives different mass estimates than by assuming spherical symmetry. We have also investigated the influence of aspherical geometries of galaxy clusters, and of polytropic profiles of the temperature on the estimate of the Hubble constant through the Sunyaev-Zeldovich effect. We find that the non-inclusion of such effects can induce errors up to 40 per cent on the Hubble constant value.
We investigate the influence of the finite extension and the aspherical geometry of a galaxy cluster on the estimate of the Hubble constant through the Sunyaev-Zeldovich effect. An analysis of a recent CHANDRA image of the galaxy cluster RBS797 indicates a strong ellipticity and thus a pronounced aspherical geometry. We estimate the total mass of RBS797 assuming spherical or ellipsoidal geometry and show that in the latter case the mass is about 10-17 % less than the one inferred for a spherical shape.
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