Star formation and mass assembly at the highest redshifts

最新的望远镜和仪器使我们能够看到星系在令人难以置信的距离。寻找这些遥远物体的动机是通过时间来回顾-光在数十亿年前离开这些星系。我们可以追溯宇宙的演化，从最早的结构到今天的星系。我们已经开发出了使用哈勃太空望远镜和地面上最大的望远镜来发现最遥远的物体的技术：这些物体大约在120亿年之外-所以光已经旅行了地球年龄的两倍多到达我们。我们可以通过分析它们的光谱（随波长扩展）来确定这些物体的距离，并寻找氢原子的特征发射线指纹。在很远的距离，波长将通过宇宙的膨胀被拉伸到更长（更红）的波长-我们感兴趣的是红移大于1，自从光子离开我们的目标星系以来，宇宙的大小增加了一倍以上。我们可以超越在敏感图像中选择候选的高红移星系，并用光谱学确认它们的距离;我们可以通过测量年轻的、大质量的、短命的恒星发出的紫外线光量来确定恒星诞生的速率。我们还可以研究在一定波长范围内的亮度，以估计恒星的总数和它们的平均年龄。最后，我们可以通过研究这些星系中的恒星和气体绕中心运行的速度来“称量”这些遥远的星系--星系的质量越大，速度的扩散就越大（通过光谱中发射线和吸收线的波长扩散来测量）。星系的总质量包括恒星和气体，以及神秘的“暗物质”。星系的恒星质量与总质量的关系，以及这在宇宙历史上是如何演变的，是星系形成中重要的未解问题。

The latest telescopes and instruments enable us to see galaxies at incredible distances. The motivation for searching for these distant objects is to look back through time - light left these galaxies many thousands of millions of years ago. We can trace the evolution of our Universe from the earliest structures to galaxies today. We have developed techniques using the Hubble Space Telescope and the largest telescopes on the ground to discover the most distant objects yet: these are about 12 thousand million years away - so light has been travelling more than twice the Earth's age to reach us. We can confirm the distance to these objects by analysing the spectrum of their light (spreading out with wavelength), and looking for the characteristic emission line fingerprints of atoms such as hydrogen. At great distances, the wavelengths will be stretched to longer (redder) wavelengths through the expansion of the Universe - we are interested in redshifts greater than one, where the Universe has more than doubled in size since the photons left our target galaxies. We can go beyond selecting candidate high-redshift galaxies in sensitive images and confirming their distance with spectroscopy; we can determine the rate at which stars are being born by measuring the amount of light emitted in the ultra violet by young, massive, short-lived stars. We can also study the brightness over a range of wavelengths to estimate the total number of stars and their average age. Finally, we can `weigh' these distant galaxies by studying the speed at which the stars and gas within these galaxies orbit the centre - the greater the mass of a galaxy, the higher the spread in speeds (measured by the spread in wavelength of the emission and absorption lines in the spectrum). The total mass of a galaxy comprises both the stars and gas, and the mysterious `dark matter'. How the stellar mass of a galaxy is related to the total mass, and how this evolves over the history of the Universe, are important unanswered questions in galaxy formation.