October 10, 2012
New Hubble Data Reveals Dusty Galaxies At Ancient Epoch
April Flowers for redOrbit.com - Your Universe Online
Dust can be the bane of any good housekeeper or computer technician. However, it is an important building block for stars and planets, and astronomers need to understand how cosmic dust forms over time. This understanding is an integral step in figuring out the evolution of galaxies and the stars and planets that they´re made of.
Assistant professor Steven Finkelstein, of the University of Texas at Austin, and his colleagues are conducting one of the largest Hubble Space Telescope projects to date called CANDELS: the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. They are studying dust in thousands of galaxies over a wide range of cosmic time. Preliminary results have been published in a recent issue of The Astrophysical Journal.
"We don't yet understand how galaxies build up their dust reservoirs," Finkelstein said. "We know that dust builds up through time, but exactly when the formation of dust begins is unknown."
The research team is working to bridge that knowledge gap. The astronomers are studying nearly 3,000 galaxies seen between 500 million to 1,500 million years after the Big Bang — a cosmic "drop in the bucket" compared to the 13.7-billion-year age of the universe.
The team is combining the data from their own Hubble observations — over 900 orbits studying distant galaxies with the Wide Field Camera 3 (WFC3) — and data from several other large Hubble galaxy surveys, including Great Observatories Origins Deep Survey (GOODS) and the Hubble Ultra-Deep Field survey.
"We found something we wouldn't expect," Finkelstein said. "Although dust can form quickly, I don't think many people expected galaxies at only 800 million years after the Big Bang to have a lot of dust. These observations caused us to change our thinking."
Even for the Hubble, studying such ancient, faint galaxies is tricky. Though only a miniscule amount of information is available through the tiny stream of photons they send our way, the color can be determined and this is what the team was after. Galaxy colors offer clues as to how much dust a galaxy contains. A blue galaxy has less dust, while a redder galaxy has more.
The significant levels of heavy metal dust found in these early massive galaxies means they must have been forming stars for a while. Heavy elements were not created in the Big Bang itself, they are built up over time inside stars as they fuse together lighter elements into heavier ones through nuclear fusion. As a massive star runs through all of its nuclear fuel, it explodes as a supernova, spewing these heavy elements into the galaxy and creating the building blocks for the dust which CANDELS is looking for.
"These results are very interesting because they tell us that dust does form at early times," he said. "This is important because the same elements that compose the dust grains are necessary for the formation of planets. Also, we think that dust is a key component in allowing hydrogen gas to form molecules, which is necessary for star formation."
The team also found that between 800 million and 1.5 billion years after the Big Bang all galaxies appear to get dusty, not just the massive ones.
"The presence of dust means that a previous generation of stars has lived and died. So, when we can peer back to even farther later this decade with JWST [the James Webb Space Telescope], there should be a lot for us to see!" Finkelstein said.