迄今最遥远的星系EGS8p7

A team of Caltech researchers that has spent years searching for the earliest objects in the universe now reports the detection of what may be the most distant galaxy¹ ever found. In an article published August 28, 2015 in Astrophysical Journal Letters, Adi Zitrin, a NASA Hubble Postdoctoral Scholar in Astronomy, and Richard Ellis--who recently retired² after 15 years on the Caltech faculty³ and is now a professor of astrophysics at University College, London--describe evidence for a galaxy called EGS8p7 that is more than 13.2 billion years old. The universe itself is about 13.8 billion years old. Earlier this year, EGS8p7 had been identified as a candidate for further investigation⁴ based on data gathered by NASA's Hubble Space Telescope and the Spitzer Space Telescope. Using the multi-object spectrometer for infrared⁵ exploration (MOSFIRE) at the W.M. Keck Observatory⁶ in Hawaii, the researchers performed a spectrographic analysis of the galaxy to determine its redshift. Redshift results from the Doppler effect, the same phenomenon that causes the siren on a fire truck to drop in pitch as the truck passes. With celestial⁷ objects, however, it is light that is being "stretched" rather than sound; instead of an audible drop in tone, there is a shift from the actual color to redder wavelengths⁸.

 

Redshift is traditionally used to measure distance to galaxies⁹, but is difficult to determine when looking at the universe's most distant--and thus earliest--objects. Immediately after the Big Bang, the universe was a soup of charged particles--electrons and protons--and light (photons). Because these photons were scattered¹⁰ by free electrons, the early universe could not transmit light. By 380,000 years after the Big Bang, the universe had cooled enough for free electrons and protons to combine into neutral hydrogen atoms that filled the universe, allowing light to travel through the cosmos¹¹. Then, when the universe was just a half-billion to a billion years old, the first galaxies turned on and reionized the neutral gas. The universe remains¹² ionized today.

 

Prior to reionization, however, clouds of neutral hydrogen atoms would have absorbed certain radiation emitted by young, newly forming galaxies--including the so-called Lyman-alpha line, the spectral¹³ signature of hot hydrogen gas that has been heated by ultraviolet emission¹⁴ from new stars, and a commonly used indicator¹⁵ of star formation.

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