Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003rmxac..17..226b&link_type=abstract
Galaxy Evolution: Theory & Observations (Eds. Vladimir Avila-Reese, Claudio Firmani, Carlos S. Frenk & Christine Allen) Revista
Astronomy and Astrophysics
Astronomy
2
Cosmology: Observations, Galaxies: Active, Galaxies: Distances And Redshifts, Galaxies: Evolution
Scientific paper
We obtained B, V, R, I, and z' magnitudes for the 370 X-ray point sources in the ~ 1 Ms observation of the Chandra Deep Field North (CDF-N), HK' magnitudes for 276, and spectroscopic redshifts for 182. The redshift distribution shows indications of structures at z = 0.843 and z = 1.0175 (also detected in optical surveys) which could account for a part of the field-to-field variation seen in the X-ray number counts. The flux contributions separated into unit bins of redshift show that the z < 1 spectroscopically identified sources already contribute about one-third of the total flux in both the hard and soft bands. Thus, major accretion onto supermassive black holes has occurred since the Universe was half its present age. We find from ratios of the X-ray counts that the X-ray spectra are well-described by absorption of an intrinsic Gamma = 1.8 power-law, with N[H] values ranging from about 10^21 cm-2 to 5×10^23 cm-2. We estimate that the Chandra sources which produce 87% of the HEAO-A X-ray background (XRB) at 3 keV produce 57% at 20 keV, provided that at high energies the spectral shape of the sources continues to be well-described by a Gamma = 1.8 power-law. However, when the Chandra contributions are normalized to the BeppoSAX XRB at 3 keV, the shape matches fairly well the observed XRB at both energies. Thus, whether a substantial population of as-yet undetected Compton-thick sources is required to completely resolve the XRB above 10 keV depends critically on how the currently discrepant XRB measurements in the 1 - 10 keV energy range tie together with the higher energy XRB.
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