Hubble Survey Finds Largest-Ever Sample of Ancient Galaxies

Author:  Jessica Kloss
Institution:  Princeton University
Date:  October 2008

Astronomers say that looking at the stars is like looking back in time: the star-light we see has often spent millions of years traveling to Earth. The farther away an object is, the farther the light has to travel to reach Earth, and the older the image of the object. So when we say a galaxy is a million light-years away, we literally mean that the image we see is actually a million years old. This ability to look back in time is a useful tool for astronomers studying the history of the universe. And now, by combining Einstein's insights with observations with the NASA/ESA Hubble Space Telescope, Dr. Johan Richard from the California Institute of Technology and a team of scientists have come up with the largest sample yet of incredibly distant galaxies – so far away that we observe them as they were 13 billion years ago.

These recent observations of galaxies were taken by combining both visible and near-infrared observations from Hubble's Advanced Camera for Surveys and Near Infrared Camera and Mutli-Object Spectrometer. Then the galaxies only visible in the near-infrared, the farther-away objects, were selected. With this technique, ten galaxies were selected, observed as they were when the universe was still quite young , a mere 700 million years old.

"The challenge for astronomers is that galaxies beyond a distance of 13 billion light-years. are exceedingly faint and are only visible in the near-infrared , just at the limit of what Hubble can observe," explained Jean-Paul Kneib from the Laboratoire d'Astrophysique de Marseille. But the scientists knew to take advantage of an effect from Einstein's famous equations called "gravitational lensing," which can magnify the light of far away objects. With gravitational lensing it becomes possible to observe these distant, faint galaxies.

Gravitational lenses work remarkably like the lenses we use every day: by causing the path of light to bend, they warp, focus, or magnify objects. But the mechanism through which gravitational lenses bend light is drastically different from ordinary curved glass. It has to do with Einstein's theory of General Relativity, which says that heavy objects bend the fabric of space and time.

So how exactly does this work? Picture a bowling ball on a mattress: its weight would make a dent in the mattress. If one were to roll something very light, such as a ping pong ball, on a path that passed near the bowling ball, the path of the ping pong ball would bend toward the dent made by the bowling ball as it rolled by. The effect of massive objects on the fabric of spacetime is very similar: the more massive the object, the bigger the "dent" in spacetime, and the greater the effect it has on nearby objects.

Astronomers have discovered that galaxy clusters can make excellent lenses, bending light coming from beyond them very effectively (of course, only if the angle is right). Why? Galaxies are massive objects, and galaxy clusters consist of many galaxies grouped very closely together , sort of like having a whole group of bowling balls in the center of a very large mattress. Indeed, it was by studying six likely galaxy clusters that Johan Richard, and a team of scientists, were able to push the limits of Hubble's observing power and find magnified images of such distant galaxies.

"These candidates could well explain one of the big puzzles plaguing astronomy today. We know that the Universe was re-ionised within the first 5-600 million years after the Big Bang, but we don't know if the ionising energy came from a smaller number of big galaxies or a more plentiful population of tiny ones," said Johan Richard. The relatively high number of distant galaxies found in this survey seems to point to a large number of dim and abundant galaxies at that time.

Re-ionising the universe in this context effectively means turning on the lights again – the theory goes that for several billion years, the universe was in the midst of what astronomers call the "dark ages." An ion is an atom or group of atoms with a positive or negative electrical charge. During the dark ages, the universe was neutral, the opposite of ionized. It turns out that a neutral, non-ionized universe is an opaque universe, because photons are easily absorbed into stable, neutral atoms instead of moving freely. In this way, light was stifled, and the universe was dark.

But at the same time, in this early universe, objects were beginning to form – objects that could produce high-energy photons. These energetic photons could knock electrons off atoms, thereby making once-neutral atoms become ions. Since ions don't have the same ability as neutral atoms to absorb photons, light was able to move freely once more.

Scientists like Johan Richard are exploring just how this incredible switch from dark to light happened. What objects re-ionised the universe? Based on the survey, it might just be a large number of small galaxies. Either way, Johan Richard and his team await the construction of more powerful, ground-based telescopes to confirm that the galaxies they observed are really as far away as they seem and may even observe the fascinating, outer reaches of our universe.

Written by: Jess Kloss

Edited and reviewed by: Jeff Kost and Nira Datta

Published by: Hoi See Tsao