标题： 红外观测0.55处椭圆星系的远紫外辐射 显示英文标题

标题： Far-UV Emission from Elliptical Galaxies at z=0.55

摘要： 紫外至光学波段流量比，用以表征“紫外转变”现象，可能是椭圆星系中年龄最敏感的示踪剂；模型预测它在几十亿年的时间内可能会变化几个量级。 为了追踪红移条件下紫外转变的演化，我们利用空间望远镜成像光谱仪上的远紫外相机对红移 z=0.55 的星团 CL0016+16 进行了成像。 我们的 25''x25'' 视场包括四颗亮椭圆星系，通过光谱确认它们是被动演化的星团成员。 图像中星系微弱的紫外发射表明，与 z=0 和 z=0.375 时星团中 E 型和 S0 型星系的紫外转变测量值相比，在回溯时间比我们自身早 5.6 Gyr 时，紫外转变较弱。 这些图像是首次具有足够深度来展示预期在中等红移下的紫外转变消退现象。 我们讨论了这些观测结果及其对星系形成历史的影响。 显示英文摘要

摘要： The restframe UV-to-optical flux ratio, characterizing the ``UV upturn'' phenomenon, is potentially the most sensitive tracer of age in elliptical galaxies; models predict that it may change by orders of magnitude over the course of a few Gyr. In order to trace the evolution of the UV upturn as a function of redshift, we have used the far-UV camera on the Space Telescope Imaging Spectrograph to image the galaxy cluster CL0016+16 at z=0.55. Our 25''x25'' field includes four bright elliptical galaxies, spectroscopically confirmed to be passively evolving cluster members. The weak UV emission from the galaxies in our image demonstrates that the UV upturn is weaker at a lookback time 5.6 Gyr earlier than our own, as compared to measurements of the UV upturn in cluster E and S0 galaxies at z=0 and z=0.375. These images are the first with sufficient depth to demonstrate the fading of the UV upturn expected at moderate redshifts. We discuss these observations and the implications for the formation history of galaxies.