Astronomy Picture of the Day

Explanation: What are those strange blue objects? Many are images of a single, unusual, beaded, blue, ring-like galaxy which just happens to line-up behind a giant cluster of galaxies. Cluster galaxies here appear yellow and -- together with the cluster's dark matter -- act as a gravitational lens. A gravitational lens can create several images of background galaxies, analogous to the many points of light one would see while looking through a wine glass at a distant street light. The distinctive shape of this background galaxy -- which is probably just forming -- has allowed astronomers to deduce that it has separate images at 4, 8, 9 and 10 o'clock, from the center of the cluster. Possibly even the blue smudge just left of center is yet another image! This spectacular photo from HST was taken in October 1994. The first cluster lens was found unexpectedly by Roger Lynds (NOAO) and Vahe Petrosian (Stanford) in 1986 while testing a new type of imaging device. Lensed arcs around this cluster, CL0024+1654, were first discovered from the ground by David Koo (UCO Lick) in 1988.

這些泛藍的“物體“是什麼？ 大部分是單一奇特、帶念珠結構的環狀星系之影像，而這個星系恰好位在一個龐大星系團的後方。 而這個泛黃的星系團及星系團的暗物質，具有重力透鏡的特質。 重力透鏡可產生背景星系的多重影像，酷似透過酒杯觀看遠處的一盞街燈時，可見到多重光點。 這個具有特殊外觀、可能正在形成的背景星系，讓天文學家得以認出在星系團4、8、9和10點鐘方向的都是它的身影。 甚至連星系團中心左方的泛藍的暈斑，可能也是這個星系的蜃影。 這幅精采的影像，是由哈伯望遠鏡攝於1994年10月。 第一幅星系團重力透鏡影像，是由Roger Lynds (NOAO) 和Vahe Petrosian (史坦福大學) 在1986年測試新型的成像設備時意外發現的。 而此星系團CL0024+1654周圍的重力透鏡弧，則是在1988年由David Koo (UCO Lick) 於地面觀測時所發現。