The Radial Dependence of the Stellar Initial Mass Function in Massive Galaxies

恒星初始质量函数（IMF）描述了恒星在星系中新“诞生”时的质量分布;例如，在银河系中，IMF可以通过直接计算不同质量恒星的频率来确定，低质量和高质量恒星相对较少。IMF提供了一种在我们用望远镜观察到的恒星光和我们在模拟星系形成时模拟的恒星质量之间进行转换的方法，因此它对于用观测来测试理论模型至关重要。人们经常假设IMF在所有星系中都是一样的，并且通过将银河系IMF应用于更遥远的星系已经取得了很大进展。然而，最近的证据表明，IMF在不同类型的星系之间存在系统性差异，这些研究的含义是天文学家一直在为一些物体分配错误的质量，可能低估了最大质量星系的恒星含量。这样做的一个后果是，这种“缺失”的质量被认为是星系暗物质晕的一部分，因此这些星系的中心暗物质含量被高估了，目前的努力集中在量化IMF如何从星系到星系变化，试图最终确定它为什么变化。然而，这些研究仍然假设IMF是“普遍的”，即使在一个单一的星系，尽管有一些理论依据可以预期，它应该从星系的中心系统地变化到它们的外围。如果能直接测量星系的真实质量分布，IMF的模糊性就可以克服。不幸的是，大多数测量宇宙质量的方法（例如，动力学测量或引力透镜）对包括恒星和暗物质在内的总质量敏感;因此，恒星质量/IMF和暗物质质量之间存在简并。该研究项目旨在使用类星体微透镜-一种对恒星质量唯一敏感的技术-来打破这种简并性，从而提供对遥远的大质量星系最可靠和精确的恒星质量估计。特别是，通过以自洽的方式组合大量星系的结果（例如，“按比例放大”较小的星系，使它们能够与质量较大的天体进行比较），就可以确定“真正的”典型恒星质量分布-从而确定国际货币基金组织分布-的估计。本提案将使用新发起的调查（例如，暗能量调查）和设施（例如，盖亚）发现新的类星体透镜，获得这些系统的额外数据，并开发将应用于这些数据的新模型，以便首次测量IMF是否在星系群中发生空间变化。

The stellar initial mass function (IMF) describes how the masses of stars are distributed when they are newly `born' in galaxies; in the Milky Way, where the IMF can be determined by directly counting the frequency of stars of different masses, there are relatively few low-mass and high-mass stars, for example. The IMF provides a means of translating between the stellar light that we observe with our telescopes and the stellar mass that we model in simulations of galaxy formation, and it is therefore critically important for testing theoretical models with observations. It has often been assumed that the IMF is the same in all galaxies, and much progress has been made by applying the Milky Way IMF to more distant galaxies. However, recent evidence suggests that the IMF systematically varies between different types of galaxies, and the implication of these studies is that astronomers have been assigning the wrong mass to some objects, potentially under-estimating the stellar content of the most massive galaxies by a factor of two. One consequence of this is that this `missing' mass is instead assumed to be part of the galaxies' dark matter halos, and hence the central dark matter content of these galaxies is over-estimated, and current efforts are focusing on quantifying how the IMF varies from galaxy to galaxy in attempt to eventually determine why it varies.However, these studies still assume that the IMF is `universal' even within a single galaxy, although there are some theoretical grounds to expect that it should systematically change from the centres of galaxies to their outskirts. The ambiguity of the IMF could be overcome if the true stellar mass profiles of galaxies could be directly measured. Unfortunately, most ways of measuring mass in the Universe (e.g., dynamical measurements or gravitational lensing) are sensitive to the total mass, including stars and dark matter; there is thus a degeneracy between the stellar mass/IMF and the dark matter mass. This research project aims to use quasar microlensing -- a technique that is uniquely sensitive to just the stellar mass -- to break this degeneracy and therefore provide the most robust and precise stellar mass estimates of distant, massive galaxies. In particular, by combining results for a large number of galaxies in a self-consistent way (e.g., `scaling up' smaller galaxies so that they can be compared with more massive objects) an estimate of the `true' typical stellar mass profile -- and hence IMF profile -- can be determined. This proposal will use newly-initiated surveys (e.g., the Dark Energy Survey) and facilities (e.g., Gaia) to discover new quasar lenses, obtain additional data for these systems, and develop new models that will be applied to these data in order to make the first measurement of whether or not the IMF varies spatially within a population of galaxies.