衝壓力剝離
Ram Pressure Stripping
衝壓壓力其實是我們生活中經常會經歷的一種影響。當一個物體穿過一個介質時，比如空氣，物體運動越快，介質中越多的粒子就會與那個物體發生碰撞。就像我們跑得越快，我們的臉上就會感受到更大的壓力一樣。
Ram-pressure is an effect we experience every day. It’s just the idea when an object moves through a medium, such as air, the faster the motion the more particles in that medium will hit the object. Like as we run faster, we would feel a stronger pressure on our face.
所以，如果衝壓壓力作用於星系，就會產生剝離的效果。當一個星系穿過一個介質時，就是星系團之間的等離子體，由於介質的阻力，星系裡鬆散的氣體會被落下，通常留下一條長長的尾巴。就像主文章中的水母星系一樣。
So, if ram-pressure applied to galaxies, it would cause a stripping effect. As a galaxy move through a medium, in this case the intra-cluster plasma, its loosely bounded gas would be fall behind due to the drag forces from the medium, often leaving a long tail behind.
圖為星系與水母的形態對比圖：左邊為哈勃望遠鏡所拍到的合成圖像，D100正在從左到右移動，身後留下了被遺棄的氣體；右邊是用來比較的水母圖片
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
想像一下，把一袋麵粉倒在一個人的頭上，然後把頭探出行駛汽車的窗外，衝壓力剝離對星系的影響就是這樣。(示範目的，千萬不要嘗試哦！)
Imagen pouring a bag of flour over someone's head and sticking it out of the window of moving car. (Demonstration purpose, DO NOT TRY!)
長按關注 勿裡補說 公眾號，
了解物理點滴
星系圖片來自美國、日本、捷克和英國學者合作， 由 W.J. Cramer 帶領的團隊的 「Spectacular HST observations of the Coma galaxy D100 and star formation in its ram pressure stripped tail" 論文
水母圖片來自 Alexander Semenov
中英文內容為原創
Galaxy picture 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
Jellyfish image from Alexander Semenov
All Chinese and English contents are original