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Register a new userHST Observations of the Serendipitous X-ray Companion to Mrk 273: Cluster at z=0.46?
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Authors
K. D. Borne
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We have used HST I-band images to identify Mrk 273X, the very unusual high-redshift X-ray-luminous Seyfert 2 galaxy found by ROSAT in the same field-of-view as Mrk 273. We have measured the photometric properties of Mrk 273X and have also analyzed the luminosity distribution of the faint galaxy population seen in the HST image. The luminosity of the galaxy and the properties of the surrounding environment suggest that Mrk 273X is the brightest galaxy in a relatively poor cluster at a redshift near 0.46. Its off-center location in the cluster and the presence of other galaxy groupings in the HST image may indicate that this is a dynamically young cluster on the verge of merging with its neighboring clusters. We find that Mrk 273X is a bright featureless elliptical galaxy with no evidence for a disk. It follows the de Vaucouleurs (r^{1/4}) surface brightness law very well over a range of 8 magnitudes. Though the surface brightness profile does not appear to be dominated by the AGN, the galaxy has very blue colors that do appear to be produced by the AGN. Mrk 273X is most similar to the IC 5063 class of active galaxies --- a hybrid Sy 2 / powerful radio galaxy.
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shortened) Results obtained from 9 X-ray observations of 3C 273 performed by ASCA are presented (for a total exposure time of about 160 000 s). The analysis and interpretation of the results is complicated by the fact that 4 of these observations were used for on-board calibration of the CCDs spectral response. The present analysis shows that, in agreement with official recommendations, a conservative systematic error (at low energies) of about 2-3 x 10**20cm-2 must be assumed when analyzing ASCA SIS data. A soft-excess, with variable flux and/or shape, has been clearly detected as well as flux and spectral variability. An anti-correlation is found between the spectral index and the flux in the 2-10 keV energy range. Fitting the data with the latest available calibration matrices, we also detect an emission line at ~5.4-5.7 keV (~6.3-6.6 keV in the quasar frame) in (only) the two observations with lowest fluxes where it is weak (EW ~ 20-30 eV), narrow and consistent with being produced by Fe K emission from neutral matter. Overall, the observations are qualitatively consistent with a variable, non-thermal X-ray continuum emission, i.e., a power law with Gamma~1.6 (possibly produced in the innermost regions of the radio-optical jet), plus underlying ``Seyfert-like features, i.e., a soft-excess and Fe K line emission due to a reflection component. When the continuum (jet) emission is in a low state, the spectral features produced by the Seyfert-like spectrum (soft-excess, iron line and possibly a steep power law plus reflection continuum) are more easily seen.
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D.M. Neumann (CEA/Saclay
, Service dAstrophysique
, France andn Max-Planck-Institut fur extraterrestrische Physik - MPE
1999
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The evolution of the properties of the hot gas that fills the potential well of galaxy clusters is poorly known, since models are unable to give robust predictions and observations lack a sufficient redshift leverage and are affected by selection effects. Here, with just two high redshift, z approx 1.8, clusters avoiding selection biases, we obtain a significant extension of the redshift range and we begin to constrain the possible evolution of the X-ray luminosity vs temperature relation. The two clusters, JKC041 at z=2.2 and ISCSJ1438+3414 at z=1.41, are respectively the most distant cluster overall, and the second most distant that can be used for studying scaling relations. Their location in the X-ray luminosity vs temperature plane, with an X-ray luminosity 5 times lower than expected, suggests at the 95 % confidence that the evolution of the intracluster medium has not been self-similar in the last three quarters of the Universe age. Our conclusion is reinforced by data on a third, X-ray selected, high redshift cluster, too faint for its temperature when compared to a sample of similarly selected objects. Our data suggest that non-gravitational effects, such as the baryon physics, influence the evolution of galaxy cluster. Precise knowledge of evolution is central for using galaxy clusters as cosmological probes in planned X-ray surveys such as WFXT or JDEM.
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