質(zhì)量無(wú)差別：圍繞在銀河系周圍的三個(gè)矮星系盡管在恒星數(shù)量上大相徑庭，卻擁有相同的質(zhì)量。
No Slimming Down for Dwarf Galaxies
| Imagine putting 100 people of various sizes on a scale and finding they all weigh the same. Astronomers have discovered something equally puzzling while surveying a group of tiny galaxies that neighbor the Milky Way: No matter how many stars they contain, every galaxy has the same mass. The results may reveal something profound about how galaxies form--or even about the very nature of the mysterious substance known as dark matter. | 試想象一下：把100個(gè)塊頭各不相同的人放到秤上，卻發(fā)現(xiàn)他們體重一模一樣！在觀測(cè)與銀河系相鄰的一群微型星系時(shí)，天文學(xué)家們發(fā)現(xiàn)了跟上述情況同樣莫名其妙的現(xiàn)象：不管各星系包含多少恒星，每個(gè)星系的質(zhì)量都是相同的。有關(guān)星系的形成，甚至有關(guān)被稱為“暗物質(zhì)”的那種神秘物質(zhì)的特性，該發(fā)現(xiàn)或許可以揭示其中的深?yuàn)W道理。 |
| The Milky Way is surrounded by 23 so-called dwarf galaxies that are the least luminous galaxies known. Hoping to learn more about these dim collections of stars, cosmologist Manoj Kaplinghat of the University of California, Irvine, and colleagues measured the mass of 18 of them--the most complete accounting yet. The team estimated the mass by observing the speed of a representative sample of stars in each galaxy. A bigger spread in velocity implies a higher mass. | 銀河系由23個(gè)所謂的矮星系包圍著，這些矮星系在已知星系中是最為暗淡的。加州大學(xué)爾灣分校的宇宙學(xué)家馬諾·卡普林哈特及同事希望更多地了解一下這些由恒星構(gòu)成的暗淡群體，他們測(cè)量了其中18個(gè)星系的質(zhì)量，到目前為止這是一套最完善的計(jì)算資料。研究小組通過(guò)觀測(cè)每個(gè)星系中一個(gè)代表性恒星的旋轉(zhuǎn)速度，來(lái)估算星系的質(zhì)量。該恒星的旋轉(zhuǎn)速度越快，意味著其所在星系的質(zhì)量就越大。 |
| In spite of the fact that the dwarfs contain very different amounts of stars--ranging from a few thousand to more than 10 million--the total masses in the central regions were all equivalent to about 10 million suns, the team reports tomorrow in Nature. This implies that the seemingly smallest galaxies have a lot more invisible dark matter than do their more luminous neighbors. | 研究小組在明天出版的《自然》雜志上報(bào)道：這些矮星系所包含的恒星在數(shù)量上差別很大，從幾千個(gè)到一千多萬(wàn)個(gè)不等，盡管如此，它們中心區(qū)域的總質(zhì)量都大約相當(dāng)于一千萬(wàn)個(gè)太陽(yáng)。這意味著這些看起來(lái)最小的星系比更為明亮的相鄰星系所包含的不可見(jiàn)暗物質(zhì)要多得多。 |
| Why such a disparity exists is hard to explain, Kaplinghat says. Also strange is the lack of galaxies below this mass. "It seems to be telling us there is a threshold when galaxies form," he says. This could be because smaller clumps of matter have too little gravity to form stars and therefore never become visible to us. | 卡普林哈特說(shuō)，很難解釋為何存在這樣的差異。另外一個(gè)奇怪的現(xiàn)象是，沒(méi)有比這個(gè)質(zhì)量更小的星系?！斑@好像是在告訴我們，當(dāng)星系形成的時(shí)候有一個(gè)最低質(zhì)量限度，”他說(shuō)。這可能是由于小塊物質(zhì)引力太小，不足以形成恒星，因此對(duì)于我們來(lái)說(shuō)永遠(yuǎn)都不可見(jiàn)。 |
| A more speculative possibility is that small clumps of matter never form in the first place. This would impact the current favorite model of dark matter, which assumes that massive particles condense into clumps that end up being the gravitational seeds for galaxies. Because these heavy particles are "cold"--meaning they move slowly--they can condense to form very small clumps, as well as very big ones. However, a lighter, "warm" dark-matter particle would make only big clumps and therefore might explain the lack of galaxies at the low-mass end. If the dark matter were warm, it might require different search methods than those currently being applied in underground lab experiments and at the Large Hadron Collider in Geneva, Switzerland, Kaplinghat says. | 一種更富于推測(cè)性質(zhì)的可能是：原本就連小塊物質(zhì)也從未形成。這與目前受歡迎的暗物質(zhì)理論模型是互相沖突的。目前的理論認(rèn)為，重粒子凝結(jié)成物質(zhì)塊，物質(zhì)塊最終成為星系的引力種源。由于這些重粒子是“冷”的——意味著它們移動(dòng)很慢，所以它們可以凝結(jié)，形成大型物質(zhì)塊，也可以形成很小的物質(zhì)塊。然而，較輕的“溫”暗物質(zhì)粒子只會(huì)形成大型物質(zhì)塊，因此可以說(shuō)明超出這個(gè)最低質(zhì)量限度時(shí)就不存在星系?？ㄆ樟止刂赋?，如果暗物質(zhì)是溫的，也許就需要運(yùn)用不同的方法去研究，而不宜運(yùn)用目前地下實(shí)驗(yàn)室的試驗(yàn)以及瑞士日內(nèi)瓦的大型強(qiáng)子對(duì)撞機(jī)中正在使用的方法。 |
| "This is an intriguing result," says cosmologist Piero Madau of the University of California, Santa Cruz. As to what this might mean, he says, "I have a slight preference for not modifying the properties of the dark-matter particle." | “這是一項(xiàng)有趣的發(fā)現(xiàn)，”加州大學(xué)圣克魯茲分校的宇宙學(xué)家皮埃羅·馬導(dǎo)說(shuō)。至于這項(xiàng)發(fā)現(xiàn)可能意味著什么，馬導(dǎo)說(shuō)：“我稍微傾向于不改變暗物質(zhì)粒子的特性?！?/span> |
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