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

星系的光谱主要由强发射特征决定，红移为4。这些特征归因于多环芳烃(PAH)分子吸收紫外光时的红外荧光。多环芳烃在宇宙中普遍存在，已被用来追踪恒星形成、正常和活动星系核的恒星形成活动。然而，使用多环芳烃来追踪星系中恒星的形成主要是基于经验关系，对多环芳烃分子的性质或星系性质(特别是ISM的性质)是否以及如何影响多环芳烃发射没有太多的了解。例如，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)做出反应，或它们如何受到紫外线光处理和冲击的影响，目前尚不清楚。因此，活动星系核对PAH排放的确切影响，或者这种影响可能是如何由活动星系核周围核区的分子氢(H2)的存在所介导的，目前尚不清楚。此外，缺乏适用于星系恶劣环境的理论模型并不是很普遍。以前对星系中多环芳烃发射的研究使用的是ISO和Spitzer的光谱，并且受到较低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，特别是对低亮度区域，詹姆斯·韦伯太空望远镜有望给PAH研究带来革命性的变化。这些新的尖端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。DPHIL的焦点将集中在以下问题上：1)活动星系核对核和核周区域多环芳烃发射的作用是什么？2)多环芳烃对通过光电加热调节星系的恒星形成效率有什么作用，它是否会随着红移而变化？3)我们能否利用遥远星系的多环芳烃发射来收集有关红移时ISM条件的信息？为了回答这些问题，DPhil的学生将使用从JWST获得的新观察结果。这些观察结果将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的计算。此外，DPhil的学生将探索一种新的方法来模拟星系的PAH发射，方法是根据理论PAH模型和实际星系连续体的组合来开发“合成的”MIR光谱。如果成功，这项技术有可能在未来彻底改变对MIR光谱的解释方式。这项研究的结果肯定会将多环芳烃确立为稳健的恒星形成率指标，不仅适用于本地星系，而且一直适用于红移为~7的遥远星系，这将可以通过JWST获得。

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.