🎓星系成年礼🎓Becoming an Adult
从最小的存在 —— 夸克和轻子，到最大的结构 —— 星系团，宇宙中的一切都在不断地相遇和分开。
From the smallest possible things - quarks and leptons to largest possible structures – galaxy clusters, everything is constantly meeting and dispersing.
尽管整个宇宙在不断的膨胀，但在这个大环境里一切事物又在不断的邂逅。那是命中注定？还是机缘巧合？还是只是因为我们太孤独，潜意识驱使着我们抱团取暖?
Though the universe as a whole is constantly expanding, everything somehow still gathers together. By fate? By luck? Or just because we are too lonely?
但无论如何，这场‘聚力’游戏的王者是星系团们——宇宙中最大的引力束缚结构。
So, at the top of the ‘gathering’ game would the galaxy clusters – largest gravity bound structures in the Universe.
图为哈勃望远镜拍到星空，左下角是一个遥远的漩涡星系，而它背后的则是一整个离我们更远的SDSS J0333+0651星系团
An image from the Hubble Space Telescope, the bottom left shows a distant spiral galaxy, and its background shows an even more distant galaxy cluster, named SDSS J0333+0651
现实的社会
The Realistic Society
在引力的神奇作用下，星团可以由成百上千个星系组成，大小可以达到几百万光年。他们远处看一来是安静的美人，但了解后会发现是其实暴躁的野兽。不仅内含超高温的x射线等离子体，还有着足以撕裂星系的强大引力潮汐力。
Clusters can contain anything from hundreds to thousands of galaxies, with the magic of gravity, and can reach to several megaparsecs (millions of light year) in size. Though they look pretty from far away, they are not gentle beasts. Not only contains extremely hot X-ray emitting plasmas, but they can also have gravitational tidal forces strong enough to tare apart galaxies.
由于这恶劣的环境，星系团中的星系和其他地方的星系（场星系）可能有巨大的差异。进入星团社会的星系通常是椭圆星系，而且他们的恒星形成率较低，气体含量也很低。
Because of these tough environments, galaxies in the clusters and the one elsewhere (field galaxies) can have huge differences. The ones that have entered the cluster society are often elliptical galaxies that have low star formation rate and very low gas content.
虽然说我们已经知晓这些星系之间形态与密度的关系几十年了，但是他们形成的原因仍然不清楚。
Although we have known these morphology-density relations for decades, the cause of these patterns is still unclear.
因此，一组来自美国、日本、捷克和英国的好奇研究人员们开始了对星系进入集群星系过程中的演化的研究。尤其关注在一种称为冲压力剥离的机制上（附属文章中可以找到更多），因为这也许可以解释了为什么星系会在进入集群的时候变得气体贫穷。
Therefore, a group of curious researchers from USA, Japan, Czech Republic and UK started their research on how galaxies evolves as they enter clusters, in particular focusing on a mechanism called ram-pressure stripping (more explanations can be found in attached article), which explains why galaxies go from gas-rich to gas poor as they enter the clusters.
图为一幅艺术插图，图中一个螺旋星系正穿过一个巨大的星系团，过程中明亮的气体被它所经过的星系团拖到了太空中，形成了条纹状
An artist's illustration of a spiral galaxy moving through a massive cluster of galaxies and undergoing a process where streaks of bright gas are being dragged out into space by the cluster it is passing through
畅游群星的水母
Jellyfish Through Stars
由W.J. Cramer领导的研究小组在研究中主要关注了一个“水母星系”——D100，一个彗发星系团附近的棒旋星系。他是星系中受到冲压力剥离影响的一个极端例子，剥离的气体在星系后面留下了一条超长的尾巴，创造了一种独特的水母外观。
The team lead by W.J. Cramer focused on a ‘jellyfish galaxy’ – D100, a barred spiral galaxy near the Coma Cluster. Here D100 represents an extreme example of ram-pressure stripping, where the stripped gas leaves a super long tail behind the galaxy, creating a unique jellyfish look.
图为星系与水母的形态对比图：左边为哈勃望远镜所拍到的合成图像，D100正在从左到右移动，身后留下了被遗弃的气体；右边是用来比较的水母图片
The comparison between the galaxy and jellyfish: the left shows the composition image from HST, where D100 is moving from left to right leaving the gas behind as a tail; the right is an image of a jellyfish for comparison
使用了斯巴鲁望远镜（位于夏威夷的日本国家天文台）的数据，小组观察到D100 和他的尾巴都有Hα发射线，表明了活跃氢气的存在。根据2016年使用哈勃太空望远镜（HST）对D100的观测，可以看到尾部由于恒星稀少，看起来比母星系盘暗得多，但仍然有恒星形成的可能性。
Using the Subaru Telescope (the National Astronomical Observatory of Japan in Hawaii), Hα emission lines were observed in the tail, indicating the presence of exited hydrogen gas. Based on observation from the Hubble Space Telescope (HST) in 2016, it is also seen that due to lack of stars, they appear much fainter than the mother galaxy disc, however still possibilities of star formation.
虽然Hα发射线不一定来自恒星的形成，但HST观测结果显示了多个观测波段F814W（红色/近红外波段）、F475W（蓝色波段）和F275W（近紫外波段），表明了不同区域的不同恒星形成率及其分布。
Though Hα emission lines may not necessary have come from star formation, the HST observations showed multiple observation bands – F814W (red/near-IR wavelengths), F475W (blue) and F275W (near-UV), which would indicate different rates of star formation as well as their distribution if there are any.
在研究小组的颜色分析之后，结果表明，D100星系外围结束恒星形成的时间要比相对内部的区域早得多，而星系核中仍有恒星形成中。这意味着形成恒星的那些气体首先是从星系外围被移走的，从而由外而内的阻止了恒星形成。
After the team’s color analysis, the results show that star formation in the galaxy’s outskirt stopped much earlier in time than the inner sections, and it is still an ongoing process in the galaxy core. This means that the star-forming gas was removed from the galaxy outskirts first, stopping star formation outside-in.
而那些被主力部队拉下的气体并没有就此消散到太空中，渐渐他们开始自己形成恒星。在使用HST的观测中，研究小组在尾部发现了37个明亮的斑块。对它们的逐个进行了颜色分析后发现，其中10个是恒星形成的团块，另外27光团可能来自背景源，比如更远的一些星系。
Those gas left behind the main troops did not just disperse into space, in fact, they started to form stars by themselves. Using the HST observations again, 37 bright patches were found by the team, and analysis of their color showed that 10 of which are star forming clumps. The 27 are likely to be from background sources, such as distant galaxies.
图为D100星系周围的一张‘地图’，图中标记所有37个发光点，蓝色标记为恒星形成的团块，最右边的黑色斑块则是D100星系本身
A map the area around D100, where all 37 bright sources are labeled. The blue labels are the 10 star forming clusters, while the right most black cluster is D100 itself
成年人的世界
The Adult World
所以此次研究的主要结论是，当场星系被吸引加入星系团时，它们通常必须经历一个剥离洗礼，在过程中放弃体内的气体。就像当我们踏入社会后，逐渐失去了当初的稚气和天真，在向中心的努力奋斗的过程中变得沉稳。
The main conclusion the analyses is that when field galaxies get attracted to join the galaxy clusters, they would often have to go through a stripping process, leaving a trail of gas. Like when we enter the society, slowly leaving the childishness and naiveness behind, becoming the more composed self as we work towards the centre.
但在这些被遗忘的气体中，新的生命和无限的潜力将会绽放。形成恒星，再次形成团块，不均匀地分布在整个星系尾中，团块的大小可达100秒差距（几百光年），也许最终成为新的星系。
In these gases, new lives and potentials blossom. Stars form, distributed unevenly throughout the tail as clumps up to 100 parsecs (few hundreds light years) in size, which maybe become new galaxies eventually.
图为D100的HST图像，箭头指向的是一个恒星形成团块，它被包裹在一个黑暗的尘埃区域中也正在被剥离
The HST image of D100. Arrow is pointing to a star-forming clump embedded in a dark region of dust that is also being stripped
正如前面提到的，检测到的Hα发射线也许说明了恒星的形成，但是尾部区域的亮度远远不足以解释所有检测到的亮度总量。所以一定有其他机制也会产生Hα发射线。最有可能便是粒子相互作用或等离子体不稳定性造成的气体冲击。但遗憾的是，这些机制到底是怎样组合而形成这些现象的确切原因暂时还是一个未解之谜。
As mentioned earlier, detected Hα emission lines indicates star formation, however the brightness of the region is not high enough to explain the total amount seen. Therefore, there must also be other mechanisms responsible for this. The most likely being gas shocks from particle interactions or plasma instabilities, however, the precise mechanism overall is a unsolved mystery.
尽管现在我们了解到，当星系进入星系团时，冲压力剥离会影响星系的结构，但这并不能解释星系在不同环境中的所有差异。例如，为什么螺旋星系在星团中很少出现?
Even though it is now seen that ram-pressure stripping would affect the galaxy’s structure as it enters the cluster, it does not explain all the differences between galaxies in the different environments. For example, why spiral galaxies are rare in clusters?
解释星系与其环境之间关系的完整理论无疑将会是复杂的，但在我们不断的探索下，答案一定会浮出水面。
The full theory for explaining the relations between galaxies and their environment is no doubt going to be a complex one, but who’s there to stop us keep exploring.
虽然冲压力剥离单独来说
只在整体星系变化中承担了的一小部分，
但没有它星系的成长便不会完整，
每个小小的变数对一个星系的一生来说
都是至关重要的！
alone may only play a small part in
the overall development of the galaxy,
each contribution is significant in
completing the whole puzzle.
图为哈勃望远镜拍到恒星出生时的喷射景象。上图为HH 47, 左下为HH 34，右下为HH 2，它们也被称为赫比格—哈罗天体
Images of stellar jets from HST showing the scenes of star birth. Top panel shows HH47, bottom left HH34 and bottom right HH2, they also called Herbig-Haro objects
天体图片来自 NASA, Hubble, ESA 官网以及以下论文
水母图片来自 Alexander Semenov
文中部分英文信息参考来自美国、日本、捷克和英国学者合作， 由 W.J. Cramer 带领的团队的 “Spectacular HST observations of the Coma galaxy D100 and star formation in its ram pressure stripped tail" 论文
其余中英文内容为原创
Stellar images from official website of NASA, Hubble, ESA and the below article
Jellyfish image from Alexander Semenov
Parts are from “Spectacular HST observations of the Coma galaxy D100 and star formation in its ram pressure stripped tail" by the scientist team lead by W.J. Cramer from USA, Japan, Czech Republic and UK
The rest of the Chinese and English content is original
