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arXiv 1507.07558astro-ph.HEastro-ph.GA
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红移区间z=3-5活动星系核的X射线光度函数

The X-ray luminosity function of Active Galactic Nuclei in the redshift interval z=3-5

A. Georgakakis, J. Aird, J. Buchner, M. Salvato, M. -L. Menzel, W. N. Brandt, I. D. McGreer, T. Dwelly, G. Mountrichas, K. Koki, I. Georgantopoulos, L. -T. Hsu,… 展开作者

A. Georgakakis, J. Aird, J. Buchner, M. Salvato, M. -L. Menzel, W. N. Brandt, I. D. McGreer, T. Dwelly, G. Mountrichas, K. Koki, I. Georgantopoulos, L. -T. Hsu, A. Merloni, Z. Liu, K. Nandra, N. P. Ross

摘要:

我们结合Chandra天文台的深X射线测量数据和宽域/浅域XMM-XXL场,估算了红移z=3-5范围内活动星系核X射线光度函数。该样品由近340个光源组成,其光度(212)或光谱(128)红移在上述范围内。深、浅测量场的组合提供了三个数量级的亮度基线,Lx(2- 10 keV)~ 1 e43 - 1 e46 erg/s(z>3)。我们遵循贝叶斯方法来确定分箱AGN空间密度,并探讨其演变的模型无关的方式。我们的方法占泊松误差的测定X射线通量和光度红移估计的不确定性。我们证明,后者是必不可少的空间密度的无偏测量。我们发现活动星系核X射线光度函数在红移区间z=3-4和z=4-5之间有强烈的演化。也有暗示性的证据表明,这种演变的幅度是光度依赖。Lx<1 e ~(45)erg/s的活动星系核的空间密度在上述红移区间内下降了5倍,而较亮的活动星系核的演化似乎较温和。将我们的X射线光度函数与类似红移的紫外/光学选择类星体的光度函数进行比较,在明亮的光度下,Lx> 1 e 45 erg/s,两者有很大的一致性。然而,UV/光学光度函数的暗端斜率比X射线选择活动星系核的暗端斜率更陡。这意味着在z>3时,I型活动星系核分数随着亮度的降低而增加,这与较低红移时建立的趋势相反。我们还评估了活动星系核在保持氢的高红移电离的意义。我们的X射线光度函数产生的电离光子速率密度不足以使宇宙在红移z>4时保持电离。计算中的一个不确定性来源是X射线选定活动星系核的紫外光子逃逸分数。

英文摘要:

We combine deep X-ray survey data from the Chandra observatory and the wide-area/shallow XMM-XXL field to estimate the AGN X-ray luminosity function in the redshift range z=3-5. The sample consists of nearly 340 sources with either photometric (212) or spectroscopic (128) redshift in the above range. The combination of deep and shallow survey fields provides a luminosity baseline of three orders of magnitude, Lx(2-10keV)~1e43-1e46erg/s at z>3. We follow a Bayesian approach to determine the binned AGN space density and explore their evolution in a model-independent way. Our methodology accounts for Poisson errors in the determination of X-ray fluxes and uncertainties in photometric redshift estimates. We demonstrate that the latter is essential for unbiased measurement of space densities. We find that the AGN X-ray luminosity function evolves strongly between the redshift intervals z=3-4 and z=4-5. There is also suggestive evidence that the amplitude of this evolution is luminosity dependent. The space density of AGN with Lx<1e45erg/s drops by a factor of 5 between the redshift intervals above, while the evolution of brighter AGN appears to be milder. Comparison of our X-ray luminosity function with that of UV/optical selected QSOs at similar redshifts shows broad agreement at bright luminosities, Lx>1e45erg/s. The faint-end slope of UV/optical luminosity functions however, is steeper than for X-ray selected AGN. This implies that the type-I AGN fraction increases with decreasing luminosity at z>3, opposite to trends established at lower redshift. We also assess the significance of AGN in keeping the hydrogen ionised at high redshift. Our X-ray luminosity function yields ionising photon rate densities that are insufficient to keep the Universe ionised at redshift z>4. A source of uncertainty in this calculation is the escape fraction of UV photons for X-ray selected AGN.

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