哈勃望远镜看到了一个巨大的星系邻居

ESA/Hubble & NASA, J. Dalcanton, Dark Energy Survey/DOE/FNAL/NOIRLab/NSF/AURA; Acknowledgment: L. Shatz

This image from the NASA/ESA Hubble Space Telescope features a richness of spiral galaxies: the large, prominent spiral galaxy on the right side of the image is NGC 1356; the two apparently smaller spiral galaxies flanking it are LEDA 467699 (above it) and LEDA 95415 (very close at its left) respectively; and finally, IC 1947 sits along the left side of the image.

This image is a really interesting example of how challenging it can be to tell whether two galaxies are actually close together, or just seem to be from our perspective here on Earth. A quick glance at this image would likely lead you to think that NGC 1356, LEDA 467699, and LEDA 95415 were all close companions, while IC 1947 was more remote. However, we have to remember that two-dimensional images such as this one only give an indication of angular separation: that is, how objects are spread across the sphere of the night sky. What they cannot represent is the distance objects are from Earth.

For instance, while NGC 1356 and LEDA 95415 appear to be so close that they must surely be interacting, the former is about 550 million light-years from Earth and the latter is roughly 840 million light-years away, so there is nearly a whopping 300 million light-year separation between them. That also means that LEDA 95415 is likely nowhere near as much smaller than NGC 1356 as it appears to be.

On the other hand, while NGC 1356 and IC 1947 seem to be separated by a relative gulf in this image, IC 1947 is only about 500 million light-years from Earth. The angular distance apparent between them in this image only works out to less than 400,000 light-years, so they are actually much closer neighbors in three-dimensional space than NGC 1356 and LEDA 95415!

Text credit: European Space Agency

影像来源：ESA/Hubble & NASA, J. Dalcanton, Dark Energy Survey/DOE/FNAL/NOIRLab/NSF/AURA; 致谢：L. Shatz

这张来自NASA/ESA哈勃太空望远镜的图像展示了大量的螺旋星系：图像右侧突出的大螺旋星系是NGC 1356；它两侧的两个明显较小的螺旋星系分别是LEDA 467699（在它上面）和LEDA 95415（在它左边很近的地方）；最后IC 1947位于图像的左侧。

这张图片是一个非常有趣的例子，它说明了判断两个星系是否真的靠得很近，或者只是从我们地球上的角度来看，这是多么具有挑战性。快速浏览一下这张照片，你可能会认为NGC 1356、LEDA 467699和LEDA 95415都是亲密的伙伴，而IC 1947则更加遥远。然而，我们必须记住，像这样的二维图像只给出了一个角度分离的指示：物体是如何在夜空的球体上分布的。它们不能表示的是物体离地球的距离。

例如，虽然NGC 1356和LEDA 95415看起来如此接近，它们肯定在相互作用，但前者距离地球约5.5亿光年，后者距离地球约8.4亿光年，因此它们之间的距离接近3亿光年。这也意味着LEDA 95415可能远没有看起来的那么小。

另一方面，虽然NGC 1356和IC 1947在这张图像中似乎被一个相对的鸿沟分开，但IC 1947距离地球只有大约5亿光年。在这张图像中，它们之间的角距离只有不到40万光年，所以在三维空间中，它们实际上比NGC 1356和LEDA 95415要近得多!

文字来源：European Space Agency