Chandra study of an overdensity of X-ray sources around two distant (z ∼ 0.5) clusters

被引:63
作者
Cappi, M
Mazzotta, P
Elvis, M
Burke, DJ
Comastri, A
Fiore, F
Forman, W
Fruscione, A
Green, P
Harris, D
Hooper, EJ
Jones, C
Kaastra, JS
Kellogg, E
Murray, S
McNamara, B
Nicastro, F
Ponman, TJ
Schlegel, EM
Siemiginowska, A
Tananbaum, H
Vikhlinin, A
Virani, S
Wilkes, B
机构
[1] Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
[2] CNR, Ist TeSRE, I-40131 Bologna, Italy
[3] Univ Hawaii, Inst Astron, Honolulu, HI 96822 USA
[4] Osservatorio Astron Bologna, I-40127 Bologna, Italy
[5] Osserv Astron Roma, I-00044 Frascati, Italy
[6] SRON, NL-3584 CA Utrecht, Netherlands
[7] Univ Birmingham, Sch Phys & Astron, Birmingham B15 2TT, W Midlands, England
关键词
galaxies : active; galaxies : clusters : general; X-rays : galaxies; X-rays : general;
D O I
10.1086/318998
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
We present results from a Chandra X-Ray Observatory study of the field X-ray source populations in four different observations: two high-redshift (z similar to 0.5) clusters of galaxies 3C 295 and RX J003033.2+261819; and two noncluster fields with similar exposure time. Surprisingly, the 0.5-2 keV source surface densities (similar to 900-1200 sources deg(-2) at a flux limit of 1.5 x 10(-15) ergs cm(-2) s(-1)) measured in an similar to 8' x 8' area surrounding each cluster exceed by a factor of similar to2 the value expected on the basis of the ROSAT and Chandra log N-log S, with a significance of similar to 2 sigma each, or similar to 3.5 sigma when the two fields are combined (i.e., a probability to be a statistical fluctuation of < 1% and < 0.04%, respectively). The same analysis performed on the noncluster fields and on the outer chips of the cluster fields does not show evidence of such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the detected objects is well fitted by a power law with Gamma similar to 1.7 similar to active galactic nuclei (AGNs) and shows no sign of intrinsic absorption. The few (similar to 10 of 35) optical identifications available to date confirm that most of them are, as expected, AGNs, but the number of redshifts available is too small to allow conclusions on their nature. We discuss possible interpretations of the overdensity in terms of a statistical variation of cosmic background sources; a concentration of AGNs and/or powerful starburst galaxies associated with the clusters; and gravitational lensing of background QSOs by the galaxy clusters. All explanations, however, are difficult to reconcile with the large number of excess sources detected. Deeper X-ray observations and more redshifts measurements are clearly required to settle the issue.
引用
收藏
页码:624 / 638
页数:15
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