https://familystar.org.tw/index.php?option=com_smf&Itemid=45&topic=26991.0之前的模型說可觀測宇宙中有9成的星系 太暗了 連HST的深空影像也拍不到
所以可觀測宇宙中的星系數量應該是2兆個
現在用新視野號的研究說 可觀測宇宙中只有5成的星系 太暗了 連HST的深空影像也拍不到
所以可觀測宇宙中的星系數量應該是4千億個
https://familystar.org.tw/index.php?option=com_smf&Itemid=45&topic=34572.0http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20210113https://www.nasa.gov/feature/new-horizons-spacecraft-answers-question-how-dark-is-spaceJan. 14, 2021
New Horizons Spacecraft Answers Question: How Dark Is Space?
How dark does space get? If you get away from city lights and look up, the sky between the stars appears very dark indeed. Above the Earth’s atmosphere, outer space dims even further, fading to an inky pitch-black. And yet even there, space isn’t absolutely black. The universe has a suffused feeble glimmer from innumerable distant stars and galaxies.
New Horizons
This artist’s illustration shows NASA’s New Horizons spacecraft in the outer solar system. In the background lies the Sun and a glowing band representing zodiacal light, caused by sunlight reflecting off of dust.
Credits: Joe Olmsted/STScI
New measurements of that weak background glow show that the unseen galaxies are less plentiful than some theoretical studies suggested, numbering only in the hundreds of billions rather than the previously reported two trillion galaxies.
“It’s an important number to know – how many galaxies are there?” said Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author on the study. “We simply don’t see the light from two trillion galaxies.”
The earlier estimate was extrapolated from very deep sky observations by NASA’s Hubble Space Telescope. It relied on mathematical models to estimate how many galaxies were too small and faint for Hubble to see. That team concluded that 90% of the galaxies in the universe were beyond Hubble’s ability to detect in visible light. The new findings, which relied on measurements from NASA’s distant New Horizons mission, suggest a much more modest number, consistent with older Hubble data.
“Take all the galaxies Hubble can see, double that number, and that’s what we see – but nothing more,” said Tod Lauer of NSF’s NOIRLab, a lead author on the study.
These results were presented on Wednesday, Jan. 13, at the 237th meeting of the American Astronomical Society, which is open to registered participants.
The cosmic optical background that the team sought to measure is the visible-light equivalent of the more well-known cosmic microwave background – the weak afterglow of the big bang itself, before stars ever existed.
“While the cosmic microwave background tells us about the first 450,000 years after the big bang, the cosmic optical background tells us something about the sum total of all the stars that have ever formed since then,” explained Postman. “It puts a constraint on the total number of galaxies that have been created, and where they might be in time.”
As powerful as Hubble is, the team couldn’t use it to make these observations. Although located in space, Hubble orbits Earth and still suffers from light pollution. The inner solar system is filled with tiny dust particles from disintegrated asteroids and comets. Sunlight reflects off those particles, creating a glow called the zodiacal light that can be observed even by skywatchers on the ground.
To escape the zodiacal light, the team had to use an observatory that has escaped the inner solar system. Fortunately,the New Horizons spacecraft, which has delivered the closest ever images of Pluto and the Kuiper Belt object Arrokoth, is far enough to make these measurements. At its distance (more than 4 billion miles away when these observations were taken), New Horizons experiences an ambient sky 10 times darker than the darkest sky accessible to Hubble.
“These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time,” said Lauer. “New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it.”
The team analyzed existing images from the New Horizons archives. To tease out the feeble background glow, they had to correct for a number of other factors. For example, they subtracted the light from the galaxies expected to exist that are too faint to be identifiable. The most challenging correction was removing light from Milky Way stars that was reflected off interstellar dust and into the camera.
The remaining signal, though extremely faint, was still measurable. Postman compared it to living in a remote area far from city lights, lying in your bedroom at night with the curtains open. If a neighbor a mile down the road opened their refrigerator looking for a midnight snack, and the light from their refrigerator reflected off the bedroom walls, it would be as bright as the background New Horizons detected.
So, what could be the source of this leftover glow? It’s possible that an abundance of dwarf galaxies in the relatively nearby universe lie just beyond detectability. Or the diffuse halos of stars that surround galaxies might be brighter than expected. There might be a population of rogue, intergalactic stars spread throughout the cosmos. Perhaps most intriguing, there may be many more faint, distant galaxies than theories suggest. This would mean that the smooth distribution of galaxy sizes measured to date rises steeply just beyond the faintest systems we can see – just as there are many more pebbles on a beach than rocks.
NASA’s upcoming James Webb Space Telescope may be able to help solve the mystery. If faint, individual galaxies are the cause, then Webb ultra-deep field observations should be able to detect them.
This study is accepted for publication in the Astrophysical Journal.
https://www.cnbeta.com/articles/science/1077641.htm美国宇航局新视野号任务正帮助天文学家弄清宇宙黑暗空间的星系规模
2021年01月13日 22:48 205 次阅读 稿源：cnBeta.COM 0 条评论
科学探索
帮助科学家们弄清宇宙中有多少星系的一个关键答案是黑暗空间有多大。天文学家已经能够通过计算哈勃深场中所有可见的东西然后再计入天空的总面积来估计宇宙中星系的总数。这个过程的问题是，有一些星系太暗太远，无法直接探测到。
虽然那些星系太远太微弱，无法被探测到，但它们的光还是会让漆黑的太空发光。要测量宇宙中的光量，卫星必须在内太阳系之外，以及消除太阳光反射尘埃时产生的光污染。位于冥王星和柯伊伯带的 "新视野号"任务正被用来帮助弄清太空到底有多黑。
我们抬头看夜空，它看起来非常黑暗，但美国宇航局说太空并不是绝对的黑。对微弱背景光的新测量结果显示，看不见的星系比一些理论认为的要少。根据新的研究，看不见的星系数量 "只有"数千亿个，而不是之前理论上的两万亿。
之前较大的估计值是根据哈勃进行的极深空观测推算出来的。它使用数学模型来估计有多少星系太小太暗，哈勃看不到它。在那项研究中，研究人员得出结论，宇宙中90%的星系是哈勃无法观测到的。新的、更温和的数字依赖于新视野号任务的测量结果。
该项目的一位研究人员说，如果我们把哈勃能看到的所有星系都拿出来，然后把这个数字翻倍，那就是新视野团队认为的外面的星系数量。该团队试图测量恒星存在之前大爆炸本身的微弱余晖。目前，"新地平线"号飞船距离地球超过40亿英里，所经历的环境天空比哈勃所能接触到的最黑暗的天空要暗10倍。
觀星論壇 
