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Publications (4)18.55 Total impact

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    ABSTRACT: The cosmological star formation rate in the combined Chandra Deep Fields North and South is derived from our X-ray luminosity function for galaxies in these deep fields. Mild evolution is seen up to redshift order unity with star formation rate ~ (1 + z)2.7. This is the first directly observed normal star-forming galaxy X-ray luminosity function (XLF) at cosmologically interesting redshifts (z > 0). This provides the most direct measure yet of the X-ray-derived cosmic star formation history of the universe. We make use of Bayesian statistical methods to classify the galaxies and the two types of active galactic nuclei (AGNs), finding the most useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and X-ray to optical flux ratio. There is some residual AGN contamination in the sample at the bright end of the luminosity function. Incompleteness slightly flattens the XLF at the faint end of the luminosity function. The XLF has a lognormal distribution and agrees well with the radio and infrared luminosity functions. However, the XLF does not agree with the Schechter luminosity function for the Hα LF, indicating that, as discussed in the text, additional and different physical processes may be involved in the establishment of the lognormal form of the XLF. The agreement of our star formation history points with the other star formation determinations in different wavebands (IR, radio, Hα) gives an interesting constraint on the initial mass function (IMF). The X-ray emission in the Chandra band is most likely due to binary stars, although X-ray emission from nonstellar sources (e.g., intermediate-mass black holes and/or low-luminosity AGNs) remains a possibility. Under the assumption that it is due to binary stars, the overall consistency and correlations between single-star effects and binary-star effects indicate that not only is the one-parameter IMF (M) constant but also the bivariate IMF(M1, M2) must be constant, at least at the high-mass end. Another way to put this, quite simply, is that X-ray observations may be measuring directly the binary-star formation history of the universe. X-ray studies will continue to be useful for probing the star formation history of the universe by avoiding problems of obscuration. Star formation may therefore be measured in more detail by deep surveys with future X-ray missions.
    The Astrophysical Journal 12/2008; 607(2):721. · 6.73 Impact Factor
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    ABSTRACT: In the Chandra Deep Field-South 1 Ms exposure, we have found, at redshift 3.700 ± 0.005, the most distant type 2 active galactic nucleus ever detected. It is the source with the hardest X-ray spectrum with redshift z > 3. The optical spectrum has no detected continuum emission to a 3 σ detection limit of ~3 × 10-19 ergs s-1 cm-2 Å-1 and shows narrow lines of Lyα, C IV, N V, He II, O VI, [O III], and C III]. Their FWHM line widths have a range of ~700-2300 km s-1 with an average of approximately ~1500 km s-1. The emitting gas is metal-rich (Z 2.5-3 Z☉). In the X-ray spectrum of 130 counts in the 0.5-7 keV band, there is evidence for intrinsic absorption with NH 1024 cm-2. An iron Kα line with rest-frame energy and equivalent width of ~6.4 keV and ~1 keV, respectively, in agreement with the obscuration scenario, is detected at a 2 σ level. If confirmed by our forthcoming XMM-Newton observations, this would be the highest redshift detection of Fe Kα. Depending on the assumed cosmology and the X-ray transfer model, the 2-10 keV rest frame luminosity corrected for absorption is ~1045 ± 0.5 ergs cm-2 s-1, which makes our source a classic example of the long-sought type 2 QSO. From standard population synthesis models, these sources are expected to account for a relevant fraction of the black hole-powered QSO distribution at high redshift.
    The Astrophysical Journal 12/2008; 571(1):218. · 6.73 Impact Factor
  • 07/2007;
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    ABSTRACT: To investigate the relationships between dynamical status and other important characteristics of galaxy clusters, we conducted a study of X-ray cluster morphology using a sample of 101 clusters at redshift z=0.05-1 taken from the Chandra archive. The X-ray morphology is quantitatively characterized by a series of objectively measured simple statistics of the X-ray surface brightness distribution, which are designed to be robust against variations of image quality caused by various exposure times and various cluster redshifts. We found: (1) The distorted and non-distorted clusters occupy well-defined loci in the L-T plane, demonstrating the measurements of the global luminosity and temperature for distorted clusters should be interpreted with caution, or alternatively, a rigorous morphological characterization is necessary when we use a sample of clusters with heterogeneous morphological characteristics to investigate the L-T or other related scaling relations. (2) Ellipticity and Off-center show no evolutionary effects between high and low redshift cluster subsets, while there may be a hint of weak evolutions for the Concentration and Asymmetry, in such a way that high-z clusters show more distorted morphology. (3) No correlation is found between X-ray morphology and X-ray luminosity or X-ray morphology and X-ray temperature of clusters, implying that interaction of clusters may not enhance or decrease the luminosity or temperature of clusters for extended period of time. Comment: Accepted for publication in A&A. 16 pages, 18 figures
    Astronomy and Astrophysics 11/2006; · 5.08 Impact Factor