Probing the Low Surface Brightness Universe with Dragonfly

在过去的40年里，天文成像的前沿已经从地面望远镜上的照相板过渡到空间望远镜上的CCD探测器。这将我们探测微弱星系的能力提高了大约7等（超过600倍）。在同一时期，我们探测宇宙中大而暗（低表面亮度）结构的能力没有任何提高。这是因为在这种情况下，限制因素不是光子统计或图像分辨率，而是系统误差的控制，例如光的散射和天空变化。我最近与耶鲁大学的一位同事合作，开发了一种新型望远镜，蜻蜓远摄阵列，它成功地突破了这一技术障碍，并在非常微弱的表面亮度水平下揭示了宇宙的组成（Abraham & van Dokkum 2014）。这是一个几乎完全未被探索过的天文学领域，在过去的两年里，我们使用这台望远镜（由一系列高端商用长焦镜头组成）探测宇宙，直到迄今为止，它的表面亮度都达到了令人难以置信的水平。许多值得注意的发现已经取得，最值得注意的是存在一种新的巨大但微弱的星系，它们优先栖息在丰富的星系团中。这些“超扩散星系”和银河系一样大，但质量只有银河系的1/100 - 1/1000，解释它们是如何在星系团这种非常恶劣的环境中生存下来的是一个主要的谜题，这在去年已经成为一个高度活跃的研究领域。但蜻蜓项目的这些早期结果只是冰山一角。现在很清楚，低表面亮度的宇宙是一个几乎完全未被探索的天体物理现象的宝库，现在人们正在研究这些现象。在这个提议中，我描述了一个研究计划，其目标是：(1)通过确定星系恒星晕中的暗物质质量来探索超扩散星系中暗物质的性质；(2)通过对低表面亮度天空进行雄心勃勃的广域调查，检验星系组合模型的基本预测；(3)确定蜻蜓概念性能的最终极限，以便看看我们是否可以将系统发展到可以直接探索“宇宙网”的地步-一个预测的暗物质细丝网络，被认为连接星系彼此。

Over the past 40 years the cutting edge of astronomical imaging has transitioned from photographic plates on ground based telescopes to CCD detectors on space telescopes. This has improved our ability to detect small faint galaxies by about seven magnitudes (over a factor of 600). Over this same period of time there has been no improvement in our ability to detect large but faint (low surface brightness) structures in the Universe. This is because the limiting factor in this case is not photon statistics or image resolution, it is control of systematic errors, such as the scattering of light and sky variability. Working with a colleague at Yale, I recently developed a novel new telescope, the Dragonfly Telephoto Array, which has succeeded in breaking through this technical barrier and which is revealing the composition of the Universe at very faint surface brightness levels (Abraham & van Dokkum 2014). This is an area of astronomy that is almost totally unexplored, and in the last two years we have used this telescope (constructed from an array of high-end commercial telephoto lenses) to probe the Universe down to hitherto impossibly faint surface brightness levels. A number of remarkable discoveries have been made, most notably the existence of a new population of giant but faint galaxies that preferentially inhabit rich galaxy clusters. These "ultra-diffuse galaxies" are as big as the Milky Way but have only 1/100 – 1/1000 of its mass, and explaining how they survive in the very hostile environment of a galaxy cluster is a major puzzle that has become a highly active area of investigation in the last year. But these early results from Dragonfly are only the tip of the iceberg. It is now clear that the low surface brightness Universe is a treasure trove of almost totally unexplored astrophysical phenomena, and the race is now on to study these. In this proposal I describe a program of investigation whose goals are to: (1) explore the nature of dark matter in ultra-diffuse galaxies by determining their masses in galactic stellar halos; (2) test fundamental predictions of models for galaxy assembly by undertaking an ambitious wide-area survey of the low surface brightness sky; (3) determine the ultimate limits for the performance of the Dragonfly concept, in order to see if we can grow the system to the point that we can directly explore the "cosmic web"– a predicted large-scale network of dark matter filaments believed to connect galaxies to each other.