Galaxy (I Zw 18) as seen by the Hubble Space Telescope. Its striking blue colors are a result of both active star formation and itslack of dust. Credit: NASA/ESA/A. Aloisi (ESA and Space Telescope Science Institute, Baltimore)
Herschel Space Telescope observations of I Zwicky 18 (I Zw 18), a ragtag irregular galaxy in the relatively nearby universe, has enabled an international team of astronomers to make new assumptions about dust in galaxies very near the dawn of time.
In a paper to be published in the journal Nature, the team details comparative observations of I Zw 18 to conclude that Himiko, a highly-redshifted blob of a galaxy thought to have formed only 850 million years after the Big Bang, has a factor of 100 times less dust than previously expected.
Although Himiko and other early such galaxies are assumed to be undergoing star formation, the question is how?
Interstellar dust is known to be an essential component in the complicated process of star formation and should normally be key to any sort of galactic starbursts. But in Himiko and other galaxies like it, Fisher, an astronomer at Australia’s Swinburne University of Technology in Melbourne, and the paper’s lead author says such dust simply isn’t there.
“We knew there was a dust problem,” said Fisher. “But our result is the first to put a lower than expected number on it.”
Fisher says when trying to characterize terribly faint or distant objects, it’s standard astronomical procedure to look for relatively, more easily observable analogues. Astronomers then extrapolate the nearby objects’ characteristics to more far-flung objects which are inherently problematic to observe.
“Based on the fact that both I Zw 18 has very little dust and few metals and is actively star forming,” said Fisher, “we make the assumption that it is like highly-redshifted galaxies.”
The team concluded that Himiko has a “dust to stellar mass” of less than 0.05 percent.
What’s causing this dearth of dust?
Fisher says supernovae both produce and destroy dust. But he says, in this particular scenario, supernovae are like destroying more than they are creating mainly via extreme radiation bombardment of individual dust particles and follow-on shockwave fronts that basically sweep these nascent galaxies clean.
Fisher says this creates a whole new problem for theorists who will have to rework their galaxy models to include their paper’s results. That’s because, as Fisher notes, dust remains crucial to the star-forming process. It allows the molecular clouds to cool, fragment, and break into smaller star forming cores.
But if there’s no dust to seed the collapse of its Giant Molecular Clouds, how can such high-redshift galaxies form stars?
“This is the big open question,” said Fisher. “We don’t understand how the cosmos made stars before it had a sufficient supply of heavy elements, dust and molecules, [particularly], during the universe’s first one billion years.”
This lack of dust will also cause observational astronomers to continue to have a real struggle in finding these extremely ancient galaxies.
A Molecular Cloud (Credit: Wikipedia)
“We really want to know is how much gas and dust is in these galaxies and how they made their stars,” said Fisher.
But Fisher says that if their paper’s assumptions are correct, normal galaxies’ gas and star formation within the cosmos’ first 500 million years will be a factor of a 100 times harder to observe. He says even the newly operational Atacama Large Millimeter Array (ALMA) will find such galaxies too faint to measure.
“Galaxies we will manage to observe in the early universe are going to be almost exclusively the super bright, one-in-a-billion freaks,” said Fisher. “They’re interesting, but we really want to know what’s going on in galaxies that will end up looking like our own.”
Follow me on Facebook, Twitter and Google +.
from Forbes.com: Most popular stories http://www.forbes.com/sites/brucedorminey/2013/10/27/lack-of-dust-in-earliest-galaxies-baffles-astronomers/
ليست هناك تعليقات:
إرسال تعليق