"Long before it's in the papers"
August 14, 2012

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Findings could sharpen view of first stars

Aug. 14, 2012
Courtesy of Harvard University
and World Science staff

As­tro­no­mers could peer a good deal fur­ther in­to space—and fur­ther back in­to the his­to­ry of time—than they can now, by us­ing a new ap­proach, re­search sug­gests.

By look­ing fur­ther in­to space with tele­scopes, as­tro­no­mers are al­so look­ing fur­ther back in time, be­cause it takes time for the light from the ob­served ob­jects to get he­re. If the light takes a year to reach us, we see the ob­ject ap­prox­i­mately as it looked a year ago.

As­tro­no­mers can cur­rently see ob­jects formed at about 800 mil­lion years af­ter the es­ti­mat­ed time the uni­verse was born. But sim­ula­t­ions used in the new study sug­gest as­tro­no­mers should be able to study stars formed when the uni­verse was just 180 mil­lion years old. That could be a crit­i­cal step in a wid­er un­der­stand­ing of the uni­verse both then and to­day, in our 14.6-billion-year-old cos­mos, re­search­ers say.

In the stu­dy, pub­lished last month in the re­search jour­nal Na­ture, Eli Vis­bal, a phys­ics grad­u­ate stu­dent at Har­vard Uni­vers­ity, and col­leagues sim­ulated how stars formed in the uni­verse’s in­fan­cy clump to­geth­er in­to mas­sive web-like struc­tures. 

The key to the sim­ula­t­ion, he said, was the in­clu­sion of a 2010 dis­cov­ery that nor­mal mat­ter, such as hy­dro­gen gas, and the mys­terious “dark mat­ter” — which makes up more than 80 per­cent of the uni­verse — move through the uni­verse at dif­fer­ent speeds. Those web-like struc­tures, Vis­bal said, could greatly sim­pli­fy the de­tec­tion of sig­na­tures of the ear­li­est stars.

“What was clear from look­ing at our sim­ula­t­ion was that, based on the large fluctua­t­ions in these ‘web’ forma­t­ions, ob­serv­ing early stars should be far eas­i­er than we pre­vi­ously thought,” Vis­bal said.

As­tro­no­mers hunt­ing for early stars aren’t hunt­ing for the ac­tu­al stars, but sig­na­tures of their ex­ist­ence. Among the best such sig­na­tures, Vis­bal said, is an emis­sion giv­en off by hy­dro­gen gas as it is warmed by the stars. This light is known as the 21-centimeter wave­length emis­sion; the name lit­er­ally char­ac­ter­izes the meas­ured length of the light waves.

The dif­fi­cul­ty is that the radia­t­ion from early stars is of­ten ob­scured by back­ground radia­t­ion pro­duced by our gal­axy and oth­er near­by ones.

But the web-like struc­tures iden­ti­fied in Vis­bal’s sim­ula­t­ions sug­gest as­tro­no­mers should be able to iden­ti­fy early stars by search­ing for fluctua­t­ions in the 21-centimeter emis­sion: by iden­ti­fying ar­eas that emit high amounts of the radia­t­ion, as­tro­no­mers should be able to spot early stars.

One fac­tor that makes it eas­i­er, Vis­bal ex­plained, is the sheer size — as much as 400 mil­lion light-years across — of the “web” of early stars. A light-year is the dis­tance light trav­els in a year. To see a re­gion that size, as­tro­no­mers need a tel­e­scope with a rel­a­tively rough res­o­lu­tion of just two-thirds of a de­gree across the sky.

The dif­fer­ent speeds lead to “huge re­gions where star forma­t­ion is sup­pressed,” Vis­bal ex­plained. “The re­sult is that you don’t need par­tic­u­larly high-res­o­lu­tion tele­scopes to make these ob­serva­t­ions. That’s why it’s more fea­si­ble to see these struc­tures.”

“Struc­ture in the uni­verse is formed hi­er­ar­chic­ally,” he said. “This means that larg­er ob­jects are built from the merg­ers of smaller ones. So, in some sense, if we are able to de­ter­mine how the first stars and ga­lax­ies formed, we can un­der­stand the build­ing blocks which make up large ob­jects in the cur­rent uni­verse.”

Re­search­ers will now turn to mak­ing real-world ob­serva­t­ions to de­ter­mine if the sim­ula­t­ion’s pre­dic­tions are ac­cu­rate, he added. “Ra­dio ob­servatories are be­ing used right now to ob­serve 21-centimeter emis­sion from much lat­er times,” Vis­bal said. “Si­m­i­lar facil­i­ties, de­signed to ob­serve dif­fer­ent fre­quen­cies [colors], will need to be con­structed to de­tect the sig­na­ture of the first stars.”

* * *

Astronomers could peer a good deal further into space—and further back into the history of time—than they can now, by using a new approach, research suggests. By looking further into space with telescopes, astronomers are also looking further back in time, because it takes time for the light from the observed objects to get here. If the light takes a year to reach us, we see the object approximately as it looked a year ago. Astronomers can currently see objects formed at about 800 million years after the estimated time the universe was born. But simulations used in the new study suggest astronomers should be able to study stars formed when the universe was just 180 million years old. That could be a critical step in a wider understanding of the universe both then and today, in our 14.6-billion-year-old cosmos, researchers say. In the study, published last month in the research journal Nature, Eli Visbal, a physics graduate student at Harvard University, and colleagues simulated how stars formed in the universe’s infancy clump together into massive web-like structures. The key to the simulation, he said, was the inclusion of a 2010 discovery that normal matter, such as hydrogen gas, and so-called dark matter — which makes up more than 80 percent of the universe — move through the universe at different speeds. Those web-like structures, Visbal said, could greatly simplify the detection of signatures of the earliest stars. “This is the first simulation of the three-dimensional distribution of stars that includes this relative velocity effect,” Visbal said. “What was clear from looking at our simulation was that, based on the large fluctuations in these ‘web’ formations, observing early stars should be far easier than we previously thought.” Astronomers hunting for early stars aren’t hunting for the actual stars, but signatures of their existence. Among the best such signatures, Visbal said, is an emission given off by hydrogen gas as it is warmed by the stars. This light is known as the 21-centimeter wavelength emission; the name literally characterizes the measured length of the light waves. The difficulty is that the radiation from early stars is often obscured by background radiation produced by our galaxy and other nearby ones. But the web-like structures identified in Visbal’s simulations suggest astronomers should be able to identify early stars by searching for fluctuations in the 21-centimeter emission: by identifying areas that emit high amounts of the radiation, astronomers should be able to spot early stars. One factor that makes it easier, Visbal explained, is the sheer size — as much as 400 million light-years across — of the “web” of early stars. A light-year is the distance light travels in a year. To see a region that size, astronomers need a telescope with a relatively rough resolution of just two-thirds of a degree across the sky. The different speeds lead to “huge regions where star formation is suppressed,” Visbal explained. “The result is that you don’t need particularly high-resolution telescopes to make these observations. That’s why it’s more feasible to see these structures.” “Structure in the universe is formed hierarchically,” he said. “This means that larger objects are built from the mergers of smaller ones. So, in some sense, if we are able to determine how the first stars and galaxies formed, we can understand the building blocks which make up large objects in the current universe.” Researchers will now turn to making real-world observations to determine if the simulation’s predictions are accurate, he added. “Radio observatories are being used right now to observe 21-centimeter emission from much later times,” Visbal said. “Similar facilities, designed to observe different frequencies, will need to be constructed to detect the signature of the first stars.”