BYR Achieve · 镜像论坛

观测到的最远天体记录再次刷新

2009/4/28镜像同步12 回复

之前在这个版里探讨过这个话题很有意思。当时大家对红移的数值的理解有些偏差。现在读了这篇文章，仍然迷惑。上次讨论的是一个通过引力透镜发现的遥远星系，红移显著超过了7（http://forum.byr.edu.cn/wForum/disparticle.php?boardName=Astronomy&ID=2535&pos=1）。这次则是更“直接”的观测。NASA的Swift在上周四（4月23日）记录到了一个很弱的伽马射线暴，其红移达到了8.2. 随后夏威夷的双子座望远镜和智利的甚大望远镜都进行了跟踪观测。该红移值意味着，这束被观测到的伽马射线是在宇宙诞生后6亿3千万年时发出来的，旅行了131亿年才到达地球。链接是《Sky and Telescope》和NASA新闻对该事件的报道，并附上《Sky and Telescope》的全文： http://www.skyandtelescope.com/community/skyblog/newsblog/43700627.html http://www.nasa.gov/home/hqnews/2009/apr/HQ_09-088_Swift_Gamma-ray_Burst.html The fading infrared afterglow of GRB 090423 is circled in this infrared image taken with the Gemini North Telescope in Hawaii. The burst is the farthest, earliest cosmic explosion yet seen. Gemini Observatory / NSF / AURA / D. Fox / A. Cucchiara / E. Berger This artist's illustration depicts two opposite jets shooting out from the center of a massive collapsing star. If the right kind of star collapses the right way, it produces narrow jets at next to light-speed. Turbulence within the jets, according to current models, produces gamma rays streaming forward. If a jet happens to be aimed at Earth, we see a brief but powerful gamma-ray burst. NASA / Swift / Cruz deWilde The Farthest Thing Ever Seen A faint gamma-ray burst (GRB) captured last Thursday by NASA’s Swift satellite has smashed the record for the earliest, most-distant known object in the universe — with a redshift of about 8.2. The burst, named GRB 090423 for its discovery date, went off in Leo and was seen to last for 10 seconds. Several teams, including a group using the Gemini-North telescope in Hawaii and a European group using the Very Large Telescope in Chile, followed up the Swift detection by observing the burst’s fading infrared afterglow. Based on how much the afterglow’s light was redshifted (stretched) by cosmic expansion since the era when the burst happened, the group determined that it went off about 630 million years after the Big Bang. This means that the GRB's gamma rays traveled for a mind-boggling 13.1 billion years before reaching Earth. That's so far back in time that it's meaningless to assign a specific "distance," since large distances in the universe have themselves expanded by a factor of 9.2 since that time. From the burst's perspective, Earth's formation lay 8.5 billion years in the future. The redshift of 8.2 also means that the observed duration of the burst was stretched out by 9.2 times. In other words, what Swift saw lasting for 10 seconds actually took just over 1 second to happen in its own "rest frame." But this does not necessarily mean that GRB 090423 was a "short burst," a type of event that seems to have a different cause from the usual "long bursts." It could have emitted gamma rays for much longer than 1 second but with only a brief peak that was strong enough for Swift to see. The GRB's measured redshift beats the previous GRB record holder, a burst seen last year with a redshift of 6.7. The most distant galaxy with a well-measured redshift is at 6.96. "This is a fantastic burst," says Swift lead scientist Neil Gehrels (NASA/Goddard Space Flight Center). "We have been waiting for 4.5 years since the Swift launch for such a distant GRB." "This burst breaks all the records for galaxies, quasars, anything," adds Edo Berger (Harvard-Smithsonian Center for Astrophysics), who led the Gemini observations. "This is more than just a breaking a record. This is a demonstration that the best way to study the young universe and the first generation of stars and galaxies is to use GRBs." A GRB comes from the cataclysmic explosion of a massive star. Since the Swift satellite was launched in 2004, it has undoubtedly seen GRBs with even higher redshifts, but most bursts at this immense distance have afterglows so faint that astronomers are unable to see the afterglows, which are necessary for measuring a redshift. Nevertheless, this detection demonstrates that until NASA launches the James Webb Space Telescope next decade, GRBs offer the most promising method for probing the universe at redshifts of 8 and higher. The discovery also proves massive stars capable of exploding as GRBs existed when the universe was only 630 million years old. This is not a surprise; cosmologists think the very first stars formed when the universe was between 200 and 400 million years old — that is, between redshifts 12 and 20. See the press releases put out by NASA, the European Southern Observatory, the U.K.'s Science and Technology Facilities Council, and the Gemini Observatory.

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