Galactic Cosmology: Dark Matter and the Faint End of Galaxy Formation

我们对宇宙学的理解建立在“暗物质”和“暗能量”这两个基本难题之上，它们的解决方案将重新定义我们理解物理学的方式。特别是暗物质的性质，是扩展粒子物理标准模型和理解宇宙结构形成的分层性质的关键。对宇宙微波背景辐射和星系团的研究导致了一个真正的范例，即暗物质的形式是从早期宇宙中以可以忽略的热速度出现的弱相互作用的基本粒子。这种兰姆达-冷-暗物质(LCDM)假设现在已经成熟为一个完整的、可证伪的理论，其中详细的理论预测可能直接面对观测和实验。 LCDM的一个基石预测是暗物质晕(星系形成的非线性结构)大量存在，质量范围非常大。然而，LCDM晕的质量函数与星系质量函数明显不同：因此，星系质量和晕质量必须由一个包含支配星系形成的天体物理过程特征的强非线性函数联系在一起。这种关系在暗(矮)星系体系中有着特别耐人寻味的结果。由于低质量晕的数量远远多于矮星，要使LCDM与观测结果保持一致，就需要发光星系只能在一小部分低质量晕中形成。因此，大多数低质量的晕一定是“暗的”，矮星系的形成必须有效地切断在大约109个太阳质量的特征晕质量以下。这种“门槛”光晕质量应该会在最微弱的矮星上留下清晰的观测印记，如果得到适当的解释，这可能有助于验证或证伪冷暗物质范式的一些基本原理。一个例子是光晕，它刚好在阈值以下，虽然不能形成恒星，但应该仍然保留了它们原始气体的一部分。探测到这种充满气体、没有恒星的“迷你光环”将是LCDM在小规模上的一项重大发现和巨大成功。 最暗的矮星最好在局部星系群中进行研究，因为它们只能被探测到几个MPC。此外，一些矮星可能在早期融入了银河系，在银河系恒星的运动学、金属性和丰度模式中留下了它们起源的线索。我提出了一个雄心勃勃的计划，将银河系(“银河系”)变成一个强大的宇宙学探测器，结合超级计算机模拟、数据挖掘和直接观测调查，以破译隐藏在银河系及其邻近星系中的宇宙学线索。这门“银河系宇宙学”的目的是学习如何使用最微弱、最令人费解的星系样本来撬开一扇独特的天文窗口，以了解我们这个时代的决定性物理挑战之一--暗物质的性质。

Our understanding of Cosmology rests on two fundamental puzzles“dark matter” and “dark energy” whose solution will redefine the way we understand Physics. The nature of dark matter, in particular, is key to extending the Standard Model of Particle Physics and to understanding the hierarchical nature of structure formation in the Universe. Studies of the Cosmic Microwave Background radiation and of galaxy clustering have led to a true paradigm, where dark matter takes the form of weakly-interacting elementary particles that emerge from the early Universe with negligible thermal velocities. This Lambda-Cold-Dark-Matter (LCDM) scenario has now matured into a full, falsifiable theory where detailed theoretical predictions may be directly confronted with observation and experiment. A cornerstone prediction of LCDM is that dark matter halos (non-linear structures where galaxies form) exist in vast numbers and span a very large range in mass. The mass function of LCDM halos, however, differs markedly from the galaxy mass function: galaxy masses and halo masses must thus be related by a strongly non-linear function that contains signatures of the astrophysical processes that govern galaxy formation. This relation has particularly intriguing consequences in the regime of faint (dwarf) galaxies. Since low-mass halos vastly outnumber dwarfs, reconciling LCDM with observation requires that luminous galaxies have only been able to form in a small fraction of low-mass halos. Most low-mass halos must thus be “dark”, and dwarf galaxy formation must effectively be cut off below a characteristic halo mass of order ~109 solar masses. This “threshold” halo mass should leave clear observational imprints in the faintest dwarfs which, if properly elucidated, could serve to validate or falsify some of the basic tenets of the cold dark matter paradigm. One example are halos just below the threshold, which, although unable to form stars, should still retain part of their primordial gas content. Detecting this population of gas-filled, star-less “mini-halos” filled would be a major discovery and a resounding success for LCDM on small scales. The faintest dwarfs are best studied in the Local Group of Galaxies, since they can only be detected out to a few Mpc. Some dwarfs, in addition, may have melded into the Galaxy at early times, leaving clues to their origin in the kinematics, metallicities, and abundance patterns of Galactic stars. I propose an ambitious plan to turn the Milky Way (the "Galaxy") into a powerful cosmological probe, combining supercomputer simulations, data mining, and direct observational surveys to decipher the cosmological clues buried in the Galaxy and its immediate neighbours. The aim of this “Galactic Cosmology” is to learn how to use the faintest, and most puzzling, specimens of the galaxy population to pry open a unique astronomical window into one of the defining Physics challenges of our erathe nature of dark matter.