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的照相底片过渡到空间望远镜的ccd探测器。这使我们探测小而暗的星系的能力提高了约7等(超过600倍)。在这段时间里，我们探测宇宙中大型但微弱(低表面亮度)结构的能力并没有提高。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，而是对系统误差的控制，例如光的散射和天空的可变性。最近，我与耶鲁大学的一位同事合作，开发了一种新型的新望远镜--蜻蜓电视相片阵列，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上揭示了宇宙的组成(Abraham&van Dokkum 2014)。这是一个几乎完全未经探索的天文学领域，在过去的两年里，我们使用了这台望远镜(由一系列高端商业长焦镜头建造)，将宇宙探测到了迄今为止令人难以置信的微弱表面亮度水平。已经有了许多值得注意的发现，最引人注目的是存在着一批新的巨型但昏暗的星系，它们优先居住在丰富的星系团中。这些与银河系一样大但质量只有银河系的千分之一的超扩散星系，如何解释它们如何在星系团恶劣的环境中生存，是过去一年里高度活跃的研究领域的一大谜题。但蜻蜓的这些早期结果只是冰山一角。现在很明显，低表面亮度宇宙是一个几乎完全未被探索的天体物理现象的宝库，现在正在进行研究这些现象的竞赛。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探测，测试星系组装模型的基本预测；(3)确定蜻蜓概念的运行极限，以便看看我们是否能够将系统发展到能够直接探索“宇宙网”的地步--一个被预测的由暗物质丝状物组成的网络，据信将星系相互连接起来。

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.