Probing Star-Formation and AGN activity with mid-infrared spectroscopy

强发射特征主导了本机宇宙中星系的光谱，红移为4。这些特征归因于多环芳烃（PAH）分子在吸收UV光子后的IR荧光。多环芳烃在宇宙中普遍存在，并已被用于追踪恒星形成、正常和活动星系核中的星星形成活动。然而，使用多环芳烃跟踪星系中的星星形成主要是基于经验关系，没有太多的了解有关的PAH分子的属性或，如果和如何星系属性（特别是那些ISM）影响PAH的排放。例如，目前还不知道多环芳烃如何对强烈的紫外线/X射线辐射（如在活动星系核周围发现的）作出反应，也不知道它们如何受到紫外线光处理和冲击的影响。因此，活动星系核对PAH发射的确切影响，或者这种影响如何通过活动星系核周围区域的分子氢（H2）来介导，目前还不清楚。此外，缺乏适用于星系恶劣环境的理论模型并没有广泛提供。以前的星系PAH排放的调查利用ISO和斯皮策光谱，并受到差的角度和光谱分辨率的限制。凭借其超光谱和空间分辨率，波长覆盖范围和前所未有的灵敏度，特别是在低亮度区域，詹姆斯韦伯太空望远镜有望彻底改变PAH研究。这些新的前沿观测与我们在牛津的团队开发的新理论模型相结合，意味着我们能够解决星系演化的关键问题。本博士论文的重点将放在以下问题上：1）活动星系核对核区和环核区多环芳烃排放的作用是什么？2)多环芳烃通过光电加热在调节星系星星形成效率中的作用是什么？它是否随红移而变化？3)我们能不能利用遥远星系的PAH辐射来收集这些红移下ISM条件的信息？为了回答这些问题，博士生将使用JWST获得的新观察结果。这些观察结果将使用内部开发的软件进行全面分析，并使用我们自己的PAH理论模型进行建模。这些是我们第一次能够产生的独特计算。此外，哲学博士的学生将研究一种新的方法来模拟PAH排放星系开发“合成”MIR光谱的基础上的理论PAH模型和真实的星系连续相结合。如果成功，这项技术有可能在未来彻底改变MIR光谱的解释方式。调查的结果必将建立多环芳烃作为强大的恒星形成率指标，不仅适用于本地星系，但所有的方式到遥远的星系在红移~7，这将成为与JWST访问。

Strong emission features dominate the spectra of galaxies in the local Universe and out to redshift of 4. These features are attributed to IR fluorescence of Polycyclic Aromatic Hydrocarbons (PAH) molecules upon absorption of UV photons. PAHs are ubiquitous in the Universe and have been used to trace Star Formation activity in star-forming, normal and Active Galactic Nuclei. However, the use of PAHs to trace Star Formation in galaxies is primarily based on empirical relations without much insight about the properties of PAH molecules or, if and how galaxy properties (especially those of the ISM) influence PAH emission. For instance, it is not known how PAHs respond to intense UV/X-ray radiation (as found in the surroundings of an AGN) or how they are affected by UV photo-processing and shocks. As a result, the exact influence of the AGN on PAH emission or how this influence might be mediated by the presence of molecular hydrogen (H2) in the circumnuclear regions of AGN is not yet known. In addition, lack of theoretical models suitable for the harsh environments of galaxies are not widely available.Previous investigations of PAH emission in galaxies utilised spectra from ISO and Spitzer and were limited by poor angular and spectral resolution. With its superbspectral and spatial resolution, wavelength coverage and unprecedented sensitivity especially for low brightness regions, the James Webb Space Telescope is expected to revolutionize PAH research. These new cutting edge observations combined with new theoretical models developed by our group here in Oxford means that we are able to tackle key questions on galaxy evolution. The focus of this DPhil will be on the following questions: 1) what is the role of the AGN on PAH emission in nuclear and circumnuclear regions?2) what is the role of PAHs in regulating a galaxy's star formation efficiency thoughphotoelectric heating and does it change with redshift?3) can we use PAH emission from a distant galaxy to glean information about the conditions of the ISM at those redshifts? To answer these questions the DPhil student will use new observations acquired with the JWST. These observations will be fully analysed with in-house developed software and modelled using our own PAH theoretical models. These are unique computations that we have been able to produce for the very first time. In addition, the DPhil student will investigate a new way to model PAH emission from galaxies by developing `synthetic' MIR spectra based on the combination of theoretical PAH models and real galaxy continua. If successful this technique has the potential to revolutionise the way MIR spectra are interpreted in the future. The outcome of the investigation is bound to establish PAHs as robust Star-Formation Rate indicators applicable not only in local galaxies but all the way to distant galaxies at redshift~7 which will become accessible with the JWST.