ビッグバン後の暗黒時代、従来説より短い可能性　ウェッブ観測

【ワシントンＡＦＰ＝時事】宇宙で最初の銀河は、これまで考えられていたよりもはるかに早期に形成された可能性があるとする研究が発表された。ジェームズ・ウェッブ宇宙望遠鏡の観測結果に基づくもので、初期宇宙に関する天文学者らの理解を塗り替える可能性がある。（写真は、観測された最遠方級の銀河「ＧＬＡＳＳ－ｚ１０」〈中央上の写真〉と「ＧＬＡＳＳ－ｚ１２」〈同下〉）
　英学術誌「アストロフィジカル・ジャーナル・レターズ」に発表された２本の研究論文によると、７月のウェッブ運用開始後数日間に収集されたデータに基づき、非常に明るく、非常に遠方にある銀河が二つ記録されていたことを確認した。
　これらの銀河が極めて大きな光度を持つことは、二つの興味深い可能性を示している。
　一つは、現在の銀河と同様、低質量星を多数有する非常に大質量の銀河である可能性だ。そうすると銀河の形成は、１３８億年前に宇宙が誕生したビッグバンから１億年後に始まったと考えられる。
　これは、「宇宙の暗黒時代（星や銀河などの光を放つ天体が存在しない時代）」が終わった時期に関して、現在支持されている説よりも１億年早い。
　もう一つは、これらの銀河が「種族ＩＩＩ」の星で構成されている可能性だ。種族ＩＩＩ星は、まだ観測されたことのない仮説上の天体で、水素とヘリウムだけでできており、それより重い元素が存在する前の時代の恒星として存在すると推定されている。
　種族ＩＩＩ星は極めて高温で非常に明るく燃焼するため、種族ＩＩＩ星で構成される銀河は、ウェッブで観測された明るさを説明するのに、それほど大質量である必要はなく、形成の始まりはそれほど早くなかった可能性があると考えられる。
　米カリフォルニア大学サンタクルーズ校のガース・イリングワース氏は、「われわれが目の当たりにしているのは、これほど初期の時代の、非常に明るく、非常に大光度の銀河で、ここで何が起きているのかについては全く不明だ」と記者会見で述べた。
　今回発見された二つの銀河は、ビッグバンから約４億５０００万年後と約３億５０００万年後に間違いなく存在していたことが明らかになった。後者の銀河「ＧＬＡＳＳ－ｚ１２」は、観測史上最も遠方の星光を示している可能性がある。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2022/12/02-16:04）

The first galaxies may have formed far earlier than previously thought, according to observations from the James Webb Space Telescope that are reshaping astronomers' understanding of the early universe.
Researchers using the powerful observatory have now published papers in the journal Astrophysical Journal Letters, documenting two exceptionally bright, exceptionally distant galaxies, based on data gathered within the first few days of Webb going operational in July.
Their extreme luminosity points to two intriguing possibilities, astronomers on a NASA press call said Thursday.
The first is that these galaxies are very massive, with lots of low-mass stars like galaxies today, and had to start forming 100 million years after the Big Bang which occurred 13.8 billion years ago.
That is 100 million years earlier than the currently held end of the so-called cosmic dark age, when the universe contained only gas and dark matter.
A second possibility is that they are made up of Population III stars, which have never been observed but are theorized to have been made of only helium and hydrogen, before heavier elements existed.
Because these stars burned so brightly at extreme temperatures, galaxies made of them would not need to be as massive to account for the brightness seen by Webb, and could have started forming later.
We are seeing such bright, such luminous galaxies at this early time, that we're really uncertain about what is happening here, Garth Illingworth of the University of California at Santa Cruz told reporters.
The galaxies' rapid discovery also defied expectations that Webb would need to survey a much larger volume of space to find such galaxies.
It's sort of a bit of a surprise that there are so many that formed so early, added astrophysicist Jeyhan Kartaltepe of the Rochester Institute of Technology.
- Most distant starlight -
The two galaxies were found to have definitely existed approximately 450 and 350 million years after the Big Bang.
The second of these, called GLASS-z12, now represents the most distant starlight ever seen.
The more distant objects are from us, the longer it takes for their light to reach us, and so to gaze at the distant universe is to see into the deep past.
As these galaxies are so distant from Earth, by the time their light reaches us, it has been stretched by the expansion of the universe and shifted to the infrared region of the light spectrum.
Webb can detect infrared light at a far higher resolution than any instrument before it.
Illingworth, who co-authored the paper on GLASS-z12, told AFP disentangling the two competing hypotheses would be a real challenge, though the Population III idea was more appealing to him, as it would not require upending existing cosmological models.
Teams are hoping to soon use Webb's powerful spectrograph instruments -- which analyze the light from objects to reveal their detailed properties -- to confirm the galaxies' distance, and better understand their composition.
The Atacama Large Millimeter/submillimeter Array (ALMA), a ground telescope in northern Chile, might also be able to help in weighing the mass of the two galaxies, which would help decide between the two hypotheses.
JWST has opened up a new frontier, bringing us closer to understanding how it all began, summed up Tommaso Treu of the University of California at Los Angeles, principal investigator on one of the Webb programs.