Planck intermediate results - VIII. Filaments between interacting clusters
2013
Context. About half of the baryons of the Universe are expected to be in
the form of filaments of hot and low-density intergalactic medium. Most
of these baryons remain undetected even by the most advanced X-ray
observatories, which are limited in sensitivity to the diffuse
low-density medium.
Aims. The Planck satellite has provided hundreds of detections of the
hot gas in clusters of galaxies via the thermal Sunyaev-Zel'dovich (tSZ)
effect and is an ideal instrument for studying extended low-density
media through the tSZ effect. In this paper we use the Planck data to
search for signatures of a fraction of these missing baryons between
pairs of galaxy clusters.
Methods. Cluster pairs are good candidates for searching for the hotter
and denser phase of the intergalactic medium (which is more easily
observed through the SZ effect). Using an X-ray catalogue of clusters
and the Planck data, we selected physical pairs of clusters as
candidates. Using the Planck data, we constructed a local map of the tSZ
effect centred on each pair of galaxy clusters. ROSAT data were used to
construct X-ray maps of these pairs. After modelling and subtracting the
tSZ effect and X-ray emission for each cluster in the pair, we studied
the residuals on both the SZ and X-ray maps.
Results. For the merging cluster pair A399-A401 we observe a significant
tSZ effect signal in the intercluster region beyond the virial radii of
the clusters. A joint X-ray SZ analysis allows us to constrain the
temperature and density of this intercluster medium. We obtain a
temperature of kT = 7.1 +/- 0.9 keV (consistent with previous estimates)
and a baryon density of (3.7 +/- 0.2) x 10(-4) cm(-3). Conclusions. The
Planck satellite mission has provided the first SZ detection of the hot
and diffuse intercluster gas.
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