Two studies offer new clues to how galaxies form and emerge from 'dark ages'
The results, unveiled this week, provide fresh revelations about the formation and evolution of galaxies early in the universe's history and their impact on the evolution of the cosmos.
A team of astronomers has taken the measure of the most distant galaxy yet. It shines with unexpected brightness – an observation that could yield new insights into a period when the universe was emerging from its "dark ages."
The results come from one of two studies unveiled this week that each provide fresh revelations about the formation and evolution of galaxies early in the universe's history and their impact on the evolution of the cosmos.
The dark ages lasted for some 400 million years after the universe cooled following the Big Bang. During this period, a fog of neutral hydrogen gas permeated the cosmos between infant galaxies. Over time, however, the collective radiation from the enormous, hot stars that filled these growing galaxies slowly burned off the fog by ionizing the hydrogen. The gradual clearing allowed radiation to traverse the cosmos.
NASA's Hubble and Spitzer Space Telescopes initially spotted the galaxy. Their data hinted that it was filled with what some team members termed unusually hot stars. Using the 10-meter Keck telescope in Hawaii, the research team detected the galaxy, designated EGSY8p7, via ultraviolet light coming from its young, massive, hydrogen-burning stars.
The detection of this light was a surprise, noted Richard Ellis, an astronomer at the California Institute of Technology and a member of the international team, in a statement. The galaxy's record-breaking distance translates into a look-back time during which the cosmos should have been filled with clouds of neutral hydrogen.
Based on redshift, a measure of the expansion rate of the universe that can be used as a yardstick, the galaxy sits at a redshift of 8.68, or some 13.2 billion light-years away.
The previous record-holder appears at a redshift of 7.73. Astronomers have noted that around redshift 6, ultraviolet light from galaxies grows increasingly hard to spot because the hydrogen fog is too dense at longer redshifts to allow as much ultraviolet light through.
Galaxies like EGSY8p7 will offer new insights into how re-ionization lifted the cosmos out of its dark ages, noted Adi Zitrin, another CalTech astronomer and the lead author of a paper describing the observation and set for publication in the Astrophysical Journal Letters.
The re-ionization process could be "very lumpy, so that we happen to see an object in a more re-ionized line of sight," he writes in an e-mail. The general environment could be more uniformly foggy, but the object itself was energetic enough to form a bubble of re-ionized gas around it, rendering it visible. Or "the current, yet uncertain timeline for re-ionization may be somewhat off, and in this case this object will help pin it down better."
The second study speaks to the question of how galaxies form.
Another team, led by CalTech astrophysicist Christopher Martin, detected a rotating disk of gas some 400,000 light-years across and some 10 billion miles away taking up cold hydrogen gas via what could be termed a strand of cosmic pasta.
Gravity from the protogalaxy and its halo of dark matter is drawing a filament of hydrogen gas toward the disk, which gathers it up as the disk rotates. The filament is part of a larger, more extensive filament of tenuous matter. These larger filaments form webs interlinked throughout the cosmos and along which galaxies appear. The filaments typically are separated by vast voids of virtually galaxy-free space.
For a long time, researchers had held that galaxies formed within halos of dark matter – a type of matter that so far can only be detected through its gravitational influence on matter that researchers can detect directly. As the dark-matter halos collapse, they compress the hydrogen gas they harbor and heat it significantly. Only after the gas slowly cools to about 10,000 degrees Celsius (18,000 degrees Fahrenheit) can clouds of this "cold" gas collapse to form stars, and at a steady pace.
This explanation held up until the late 1990s, when astronomers found that it couldn't explains the enormous bursts of star formation that they were observing in galaxies that appeared at distances corresponding to about 2 billion years after the Big Bang.
That led researchers to develop a "cold flow" model, in which hydrogen embedded in the cosmos's larger filaments cools and collapses into thin strands. If strands are thinner than the size of the dark matter halo surrounding the protogalaxies, they can readily be drawn in and "will connect directly to galaxies or protogalaxies, often by forming disks," explains Dr. Martin in an e-mail.
Once part of the protogalaxy, the gas can quickly begin the gravitational collapse that forms stars.
But slurping the cosmic pasta has its limits, Martin suggests. As the universe expands, the dimensions of the filaments and their strands grow. Once the strand of hydrogen grows larger than the protogalaxy's dark-matter halo, the flow of gas into the galaxy becomes more inefficient. Other factors may slow or vary the supply as well, he notes.
"Most of this is speculation at this point," he acknowledges, adding that it's an active area of research. The results appear online, published by the journal Nature.