Dust That Formed Early Stars Seen By Astronomers

Astronomers have observed, for the first time, dust contents of galaxies as seen just 1 billion years after the Big Bang.

Galactic dust is created by the elements released during the formation and collapse of stars. Although hydrogen and helium, the most plentiful elements in the universe, were created by the Big Bang, stars are responsible for making all of the heavier elements in the universe, such as carbon, oxygen, nitrogen, and iron.

Previous observations had suggested that since distant, young galaxies have had less time to make stars, these galaxies should theoretically have less dust. Up until now it was impossible to directly measure the dust in these faraway galaxies.

First author Peter Capak, staff scientist at the Infrared Processing and Analysis Center (IPAC) at Caltech, said:

“Before we started this study, we knew that stars formed out of these clouds of gas and dust, and we knew that star formation was probably somehow different in the early universe, where dust is likely less common. But the previous information only really hinted that the properties of the gas and the dust in earlier galaxies were different than in galaxies we see around us today. We wanted to find data that showed that.”

Capak and colleagues aimed to do a direct analysis of the dust in these very early galaxies using the amazingly sensitive telescope at the Atacama Large Millimeter Array (ALMA) in Chile.

Previous observations of these young galaxies, which formed just 1 billion years after the Big Bang, a time period known as redshift 5-6, were made with the Hubble Space Telescope and the W. M. Keck Observatory. Both detect light in the near-infrared and visible bands of the electromagnetic spectrum.

To make inferences about the dust, the color of these galaxies at these wavelengths can be used. For example, galaxies that appear bluer in color tend to have less dust, while those that are red have more dust.

But other effects like the age of the stars and our distance from the galaxy can mimic the effects of dust, making it hard to determine just what the color means.

Capak and colleagues initially analyzed these early galaxies with the Keck Observatory. Keck confirmed the distance from the galaxies as redshift greater than 5, verifying that the galaxies were at least as young as they previously had been thought to be.

The researchers then observed the same galaxies using ALMA to detect light at the longer millimeter and submillimeter wavelengths of light. The ALMA readings provided a wealth of information that could not be seen with visible-light telescopes, including details about the dust and gas content of these very early galaxies.

“In galaxies like our Milky Way or nearby Andromeda, all of the stars form in very dusty environments, so more than half of the light that is observed from young stars is absorbed by the dust,” Capak says. “But in these faraway galaxies we observed with ALMA, less than 20 percent of the light is being absorbed.

“In the local universe, only very young galaxies and very odd ones look like that. So what we’re showing is that the normal galaxy at these very high redshifts doesn’t look like the normal galaxy today. Clearly there is something different going on.”

COSMOS field

The ALMA data for four of these galaxies is show in relation to objects in the COSMOS field taken with the Hubble Space Telescope. (Credit: ALMA (NRAO/ESO/NAOJ), P. Capak; B. Saxton (NRAO/AUI/NSF), NASA/ESA Hubble)

That X factor gives astronomers like Capak a peek into the lifecycle of galaxies. Galaxies form because gas and dust are present and eventually turn into stars—which then die, creating even more gas and dust, and releasing energy.

“This result is really exciting. It’s the first time that we’re seeing the gas that the stars are forming out of in the early universe. We are starting to see the transition from just gas to the first generation of galaxies to more mature systems like those around us today.

“Furthermore, because the carbon line is so bright, we can now easily find even more distant galaxies that formed even longer ago, sooner after the Big Bang,” Capak says.

Lin Yan, a staff scientist at IPAC and coauthor on the paper, says:

“This is just an initial observation, and we’ve only just started to peek into this really distant universe at redshift of a little over 5. An astronomer’s dream is basically to go as far distant as we can. And when it’s complete, we should be able to see all the distant galaxies that we’ve only ever dreamed of seeing.”

P. L. Capak, C. Carilli, G. Jones, C. M. Casey, D. Riechers, K. Sheth, C. M. Carollo, O. Ilbert, A. Karim, O. LeFevre, S. Lilly, N. Scoville, V. Smolcic & L. Yan
Galaxies at redshifts 5 to 6 with systematically low dust content and high [C ii] emission
Nature 522, 455–458 (25 June 2015) doi:10.1038/nature14500

Photo: ESO/ALMA (ESO/NAOJ/NRAO)/H. Arce. Acknowledgements: Bo Reipurth