Viewing Early Galaxies through Gravitational Lenses
A color HST image of the giant galaxy cluster Abell 383. The lensed background galaxy appears as a tiny dot—twice because of the way the cluster’s mass deflects the galaxy light in multiple directions. Credit: NASA, ESA, J. Richard (Centre de Recherche Astronomique/Observatoire de Lyon, France) and J.-P. Kneib (Le Laboratoire d'Astrophysique de Marseille, France)
Distant and faint galaxies that would normally be invisible even to the largest present-day telescopes are sometimes visible thanks to the effect of gravitational lensing, the deflection and amplification of light from a distant source by a concentration of mass (the lens) between the source and the terrestrial observer. Recently, IfA astronomers took part in two studies in which gravitational lensing played a significant role.
In the first case, the fortuitously placed lens is the giant galaxy cluster Abell 383. A camera aboard the Hubble Space Telescope (HST) took an image in which a lensed background galaxy appears as a tiny dot twice because of the way the cluster’s mass deflects the galaxy light in multiple directions. Tentatively identified as a very distant galaxy by Johan Richard (Centre de Recherche Astronomique, Lyon, France), the two dots were confirmed to be multiple images of the same object by comparing their spectra. The spectra also yielded its distance, which is so large that we are seeing the system as it appeared a mere 950 million years after the Big Bang. (Looking at distant objects enables us to see the Universe as it was early in its history. If we look at a galaxy one billion light-years away, we are seeing it the way it looked one billion years ago, when the light now reaching Earth left that galaxy.)
IfA astronomer Harald Ebeling, who obtained the galaxy’s spectrum using the Keck II telescope on Mauna Kea, explains, “Paradoxically, the value of this faint galaxy lies in the fact that it is most likely very average. This is important because it makes this object much more representative of its era than the few, much brighter, systems at similar distance that are detectable without lensing amplification.” If the newly discovered galaxy is indeed representative of the infant Universe, its age raises important questions. An analysis of the galaxy’s light, as recorded by the Hubble and Spitzer Space Telescopes, reveals that the galaxy harbors stars that could be as old as 750 million years. This means they may have formed a mere 200 million years after the birth of the Universe, several hundred million years earlier than previously thought.
“This challenges theories of how soon galaxies formed and evolved in the first years of the Universe,” says Richard. “It could even help solve the mystery of how the hydrogen fog that filled the early Universe was cleared.” If the properties of the newfound galaxy are typical, light from stars born in huge numbers in these galaxies may have flooded the young Universe with ultraviolet radiation—a critical ingredient needed to create the conditions from which all structure in today’s Universe originated. Ebeling is optimistic that additional discoveries of similarly faint and distant galaxies will allow this hypothesis to be tested soon.
The gravity of a gigantic cluster of galaxies has bent and magnified the light of the distant spiral galaxy Sp1149, making its spiral arms visible and available for study by astronomers. Normally, gravitational lensing distorts the structures of distant galaxies beyond recognition. The inset labeled "galaxy" shows how Sp1149 would look without lensing.
Ebeling figures in our second story, too. Graduate student Tiantian Yuan and IfA astronomer Lisa Kewley also used a gravitational lens and the Keck II telescope to study a small spiral galaxy named Sp1149 located 9.3 billion light-years away. The galaxy’s image was magnified 22 times by a different galaxy cluster acting as a gravitational lens. The system was discovered by Ebeling as part of his Massive Cluster Survey, and he also acquired the HST image that shows the spectacularly lensed spiral galaxy not just once, but four times.
While, in previous work, Ebeling and his team used this multiply-imaged galaxy to measure the mass distribution in the lensing cluster, the more recent study led by Yuan and Kewley focuses on unveiling the properties of the galaxy itself. Viewed almost exactly face-on, Sp1149 is magnified such that its core and spiral arms can be easily separated and studied individually—a usually impossible undertaking for a galaxy so distant that we see it as it was when the Universe was only a third of its present age. This makes the system a great specimen for testing different models of galaxy formation and evolution.
The Keck II telescope gave the team an unprecedented look at the distributions of elements in Sp1149. Oxygen, in particular, is very revealing because the element accumulates more in the parts of galaxies where stars have lived and died more. “The oxygen in the spiral galaxy was much more concentrated at the center,” said Kewley. This sharp oxygen gradient, from core to outer disk, suggests that stars in the cores of galaxies formed first and created the oldest stellar neighborhoods in Sp1149, followed later by the disk and arms. That supports what is called the inside-out model of galactic evolution, she said.
“This is an idea that has been out there,” explained Kewley. “Some models predict the opposite.” What has been needed was a way to study a nonlocal galaxy to see how the oxygen gradients looked much earlier in a galaxy’s history. Without that, astronomers would have nothing but middle-aged galaxies to judge from, like a biologist studying the lives of frogs without ever having seen a tadpole. “This is the first time anyone has done such a detailed and precise oxygen gradient that wasn’t on a local galaxy,” said Kewley.
The team is now hoping to study galaxies that are midway between the ages of our local galaxies and Sp1149. With samples from different ages, they should be able to piece together a much clearer life history of galaxies like our own. Fortunately, the massive clusters that Ebeling continues to discover are all extremely powerful gravitational lenses, and several very promising cases of magnified background galaxies have already been selected for future, detailed study.