The Chandra Deep Field North Survey. VI. The Nature of the Optically Faint X-Ray Source Population
The Chandra Deep Field North Survey. VI. The Nature of the Optically Faint X-Ray Source Population
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钱德拉深场北部勘测。
DOI：
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发表时间：
2001
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通讯作者：
R. E. Griffiths
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作者：
D. Alexander;W. Brandt;A. Hornschemeier;G. Garmire;D. P. Schneider;F. Bauer;R. E. Griffiths
我们提供了包含哈勃深空北区的夏威夷侧翼区内8.′4 × 8.′4区域的1 Ms Chandra曝光的光学微弱（I ≥ 24）X射线源群体的性质的约束。我们探测到47个（2400 deg-2）低至0.5-2.0 keV和2.0-8.0 keV通量的光学微弱源，通量分别为0.3 × 10-17 ergs cm-2 s-1和0.2 × 10-16 ergs cm-2 s-1，这些源分别贡献0.5-2.0 keV和2.0-8.0 keV X射线背景辐射的0.14%和0.21%。从3 × 10-14 ergs cm-2 s-1到X射线通量极限，在0.5-8.0 keV通量范围内，光学微弱X射线源的比例近似恒定（约为35%）。相当一部分（30%）的光学微弱X射线源是非常红的物体（I-K ≥ 4）。对光学和X射线特性的分析表明，大量的光学微弱的X射线源可能在z = 1-3处有活动星系核（AGN）活动。根据这些结果，我们计算出在z ≤ 3时，光学微弱X射线源中有很大一部分（15%-45%）可能是被遮蔽的类星体（静止坐标系未吸收的0.5-8.0 keV光度>3 × 1044 ergs-1）。考虑到没有I波段对应物的X射线源的数量，在z > 6处不太可能有超过1015个源。我们提供的证据表明，z > 6源的真实数量是相当低的。我们研究了光学微弱X射线源的多波长特性。9个光学微弱的X射线源都有对应的μJy射电源，约53%的光学微弱的μJy射电源在这个区域。在这些X射线探测到的光学微弱的μJy射电源中，最有可能的X射线发射来源是被遮蔽的活动星系核活动。然而，其中两个源以前已经在亚毫米波长下被探测到，这些源的X射线发射可能是由于发光的星星形成活动。假设NGC 6240的光谱能量分布，我们估计典型光学微弱X射线源的175 μm通量小于10 mJy;然而，那些具有可检测的亚毫米级对应物（即，f850 μm > 3 mJy）可以显著更亮。因此，大多数光学微弱的X射线源不太可能对远红外（140-240 μm）背景辐射有显著贡献。然而，正如对活动星系核活动源所预期的那样，在ISOCAM HDF-N区域最敏感区域内的两个光学微弱的X射线源有着微弱的（约50 μJy）15 μm对应物。我们还提供了限制的平均X-射线性质的类光学微弱的来源，而不是单独检测到的X-射线能量。对光学微弱的μJy射电源进行叠加分析，没有单独探测到X射线辐射，结果可能在0.5-2.0 keV波段探测到（置信度为98.3%）;这种X射线辐射可能是由z = 1-3的星星形成活动产生的。在HDF-N中，没有一个光学上微弱的活动星系核候选源在X射线能量下被单独探测到，也没有一个被叠加分析探测到，这表明如果这些源确实是活动星系核，它们的X射线光度就很低。
We provide constraints on the nature of the optically faint (I ≥ 24) X-ray source population from a 1 Ms Chandra exposure of a 8.′4 × 8.′4 region within the Hawaii flanking-field area containing the Hubble Deep Field North region. We detect 47 (2400 deg-2) optically faint sources down to 0.5–2.0 keV and 2.0–8.0 keV fluxes of ≈3 × 10-17 ergs cm-2 s-1 and ≈2 × 10-16 ergs cm-2 s-1, respectively; these sources contribute ≈14% and ≈21% of the 0.5–2.0 keV and 2.0–8.0 keV X-ray background radiation, respectively. The fraction of optically faint X-ray sources is approximately constant (at ≈35%) for 0.5–8.0 keV fluxes from 3 × 10-14 ergs cm-2 s-1 down to the X-ray flux limit. A considerable fraction (30%) of the optically faint X-ray sources are Very Red Objects (I-K ≥ 4). Analysis of the optical and X-ray properties suggests a large number of optically faint X-ray sources are likely to host obscured active galactic nucleus (AGN) activity at z = 1–3. From these results we calculate that a significant fraction (≈5%–45%) of the optically faint X-ray source population could be obscured QSOs (rest-frame unabsorbed 0.5–8.0 keV luminosity >3 × 1044 ergs s-1) at z ≤ 3. Given the number of X-ray sources without I-band counterparts, there are unlikely to be more than ≈15 sources at z > 6. We provide evidence that the true number of z > 6 sources is considerably lower. We investigate the multiwavelength properties of optically faint X-ray sources. Nine optically faint X-ray sources have μJy radio counterparts; ≈ 53% of the optically faint μJy radio sources in this region. The most likely origin of the X-ray emission in these X-ray detected, optically faint μJy radio sources is obscured AGN activity. However, two of these sources have been previously detected at submillimeter wavelengths, and the X-ray emission from these sources could be due to luminous star formation activity. Assuming the spectral energy distribution of NGC 6240, we estimate the 175 μm flux of a typical optically faint X-ray source to be less than 10 mJy; however, those sources with detectable submillimeter counterparts (i.e., f850 μm > 3 mJy) could be substantially brighter. Hence, most optically faint X-ray sources are unlikely to contribute significantly to the far-IR (140–240 μm) background radiation. However, as expected for sources with AGN activity, the two optically faint X-ray sources within the most sensitive area of the ISOCAM HDF-N region have faint (≲50 μJy) 15 μm counterparts. We also provide constraints on the average X-ray properties of classes of optically faint sources not individually detected at X-ray energies. Stacking analyses of optically faint μJy radio sources not individually detected with X-ray emission yields a possible detection (at 98.3% confidence) in the 0.5–2.0 keV band; this X-ray emission could be produced by star formation activity at z = 1–3. None of the optically faint AGN-candidate sources in the HDF-N itself are detected at X-ray energies either individually or with stacking analyses, showing that these sources have low X-ray luminosities if they are indeed AGNs.