[论文解读] On the Co-Evolution of the AGN and Star-Forming Galaxy Ultraviolet Luminosity Functions at 3 < z < 9

Steven L. Finkelstein, Micaela B. Bagley|arXiv (Cornell University)|Jul 5, 2022

Galaxies: Formation, Evolution, Phenomena被引用 1

一句话总结

本文利用大范围、中等深度的巡天数据，对红移 3 < z < 9 范围内活动星系核（AGNs）和星系形成星系的 rest-frame 紫外（UV）光度函数进行了联合经验建模。研究发现，从 z = 9 到 z = 3，AGN 体积密度上升了五个数量级，远超星系形成星系两数量级的上升，且将 z = 9 时光度函数的亮端平坦化归因于反馈减弱，而非 AGN 活动。

ABSTRACT

Studies of the high-redshift rest-frame ultraviolet (UV) luminosity functions (LFs) have typically treated the star-forming galaxy and active galactic nuclei (AGN) populations separately, as they have different survey depth and area requirements. However, the recent advent of wide-area deep ground-based imaging surveys now probe volumes large enough to discover AGNs, at depths sensitive enough for fainter star-forming galaxies, bridging these two populations. Using results from such surveys as observational constraints, we present a methodology to jointly empirically model the evolution of the rest-UV luminosity functions at z=3-9. We assume both populations have a LF well-described by a double power law modified to allow a flattening at the faint-end, and that all LF parameters evolve smoothly with redshift. This provides a good fit to the observations, and makes predictions to volume densities and redshifts not yet observed. We find that the volume density of bright (M_UV = -28) AGNs rises by five orders of magnitude from z=9 to z=3, while modestly bright (M_UV = -21) galaxies rise by only two orders of magnitude across the same epoch. The observed bright-end flattening of the z=9 LF is unlikely to be due to AGN, and rather is due to a shallowing of the bright-end slope, implying reduced feedback in bright galaxies at early times. Integrating our LFs we find that the intrinsic ionizing emissivity is dominated by galaxies at all z > 3, and this result holds even after applying a notional escape fraction. We compare our AGN LFs to predictions based on different black-hole seeding models, finding decent agreement on average, but that all models are unable to predict the observed abundance of bright AGNs. We make predictions for the upcoming Euclid and Roman observatories, showing that their respective wide-area surveys should be capable of discovering AGNs to z ~ 8.

研究动机与目标

• 联合建模 3 < z < 9 范围内 AGN 与星系形成星系 UV 光度函数的演化。
• 理解早期宇宙中 AGN 与星系形成星系群体的共同演化。
• 确定 AGN 活动是否可解释 z = 9 时观测到的光度函数亮端平坦化现象。
• 评估 AGN 与星系形成星系在高红移时对电离发射率的贡献。
• 预测未来 Euclid 与 Roman 空间望远镜巡天中 AGN 的可探测性。

提出的方法

• 使用带暗端平坦化的修正双幂律模型，经验性地建模 AGN 与星系形成星系的 UV 光度函数。
• 假设所有光度函数参数随红移平滑演化，采用线性或二次红移依赖关系。
• 将 29 个参数的模型拟合至大范围巡天的观测数据（如 SHELA、VIDEO、Hyper-SuprimeCam）。
• 通过积分光度函数计算本征电离发射率，并应用假设的逃逸分数。
• 将推断的 AGN 光度函数与黑洞种子与增长模型的理论预测进行比较。
• 利用推断的光度函数，对 Euclid 与 Roman 空间望远镜巡天中 AGN 的探测进行预测。

实验结果

研究问题

• RQ1 z = 9 时 UV 光度函数的亮端平坦化是由 AGN 活动引起，还是源于星系的本征演化？
• RQ2 从 z = 9 到 z = 3，AGN 与星系形成星系的体积密度如何演化？
• RQ3 在高红移时，哪一类型星体——AGN 还是星系形成星系——主导本征电离发射率？
• RQ4 理论 AGN 光度函数模型能否再现 z ≈ 9 时观测到的明亮 AGN 丰度？
• RQ5 未来如 Euclid 与 Roman 这类大视场巡天能否探测到大量 z > 7 的 AGN？

主要发现

• 从 z = 9 到 z = 3，明亮 AGN（MUV = −28）的体积密度上升了五个数量级。
• 在相同红移范围内，中等亮度星系形成星系（MUV = −21）的体积密度仅上升两个数量级。
• 观测到的 z = 9 光度函数亮端平坦化是由于亮端斜率变缓所致，而非 AGN 污染。
• 即使在应用合理的逃逸分数后，星系形成星系在所有研究的红移上仍主导本征电离发射率。
• 所有理论 AGN 光度函数模型均无法再现 z ≈ 9 时观测到的明亮 AGN 丰度。
• Euclid 宽场巡天与 Roman 高纬度巡天将探测到大量 z ∼ 8 的 AGN，但需更深度的巡天才能实现再电离时期全覆盖。