Recent Science Highlights

28 May 1996

EUVE Observations of the Coma Cluster

(the following information provided by Dr. Richard Lieu)

EUVE Deep Survey Lex/B filter Coma 
Cluster Image image created by Martin Sirk

A Maximum Entropy processed image of the Coma cluster as seen through the EUVE deep survey Lex/B filter. A substantial fraction of the EUV emission in this impressive picture cannot be explained by the well known intracluster gas in Coma, which is very hot (kT = 8 keV). The brightest spot is the galaxy NGC 4874, which is also the brightest ROSAT source in the Coma cluster.

Spectrum: EUVE deep survey Lex/B data and ROSAT PSPC data for an annulus 6' to 9' from the X-ray centroid of the Coma emission region are modelled with an optically thin thermal plasma spectrum at kT = 8.2 keV and abundance = 0.2 solar, which are the parameter values of Coma's well-known intracluster gas. Note the strong soft excess in the Lex/B band (first data point) and in the PSPC Carbon band.

We have demonstrated quantitatively that the Lex/B data are not contaminated by UV or X-ray leaks. In fitting the data we assumed Morrison-McCammon Galactic absorption at a Hydrogen column density (NH) determined by high resolution 21 cm observations. The excess emission has a complex spectral shape - it cannot be satisfactorily explained by a reduction in the absorption, which has to be accompanied by either a drastic drop in NH to the Lockman hole value, or 100 % Helium ionization. It can, however, be explained by introducing two additional gas components, one at kT = 0.33 keV (T = 3.8 million K) and the other at kT = 0.07 keV (T = 0.8 million K). The latter component cools within 10% of a Hubble time, and in a steady state situation would have produced 100 trillion solar masses of cold matter by now.

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