Astronomers have discovered the new record-holder for the most distant developing cluster of galaxies. Images of the cluster from the Hubble Space Telescope offer a glimpse of the universe as it was a mere 600 million years after the big bang. (The extremely faint galaxies are identified in the composite image above by the circles labeled a-e.)
Galaxies are the building blocks of the large-scale structure of the universe, and most of them reside in groups or clusters. Our own Milky Way is part of what’s known as the Local Group, a collection of about 50 galaxies, which is itself a part of the Virgo Supercluster of galaxies.
In order to understand the development of the universe over cosmic time, astronomers try to observe galaxies over a wide range of cosmic history, stretching back to the earliest stages of formation. It’s important to find extremely distant galaxies, such as the newly-discovered members of the protocluster above, but their extreme faintness render these galaxies extraordinarily difficult to detect.
The favored method of detection, and the one that revealed the presence of the protocluster galaxies, is a process called the Lyman-break method. “Lyman” refers to a particular series of absorption or emission lines for neutral hydrogen, which is by far the most abundant element in the universe. The Lyman limit is the shortest wavelength possible in this series, and it corresponds to the energy required to strip the electron from a hydrogen atom1. A Lyman-break galaxy is a galaxy whose spectrum shows a steep drop-off at the Lyman limit wavelength. This drop-off occurs because most stars do not emit very much light at shorter wavelengths, and the neutral hydrogen surrounding star forming regions in galaxies tends to absorb what little there is.
Since the stretching of the wavelength of light from a distant object is proportional to its distance from us, all astronomers have to do is measure where in the spectrum this drop-off occurs to estimate how far away the object is. The problem is, when you’re fishing around an enormous cosmos for very distant galaxies, it’s far too impractical to find them with spectra, which require pin-point accuracy. This is why the Lyman-break method uses images, which can capture comparatively large swaths of the sky.
To see how this works, look at this spectrum of a distant Lyman-break galaxy followed by some images at different wavelengths:
The wavelength of light from this distant galaxy has been stretched by the expansion of the universe. The Lyman limit, which would appear at 91.2 nm if you lived inside the galaxy, has been stretched to about 400 nm. Instead of using a spectrum, astronomers can simultaneously detect the presence of a Lyman-break galaxy and get a rough estimate of its distance by observing at what wavelength the Lyman break occurs photometrically — that is, at what point it disappears from images. The galaxy above was imaged in three different “bands” — astronomers often use filters to block all light from an object except for a narrow range (or band) of wavelengths — corresponding to ultraviolet, green, and red. The above galaxy is apparent in the G-band image but disappears in the U-band image. So, from the images alone, astronomers would know that the Lyman break must occur somewhere between 350 and 500 nm, and could estimate a range for its distance accordingly.
Since the Lyman-break method provides only a rough estimate of the distance, astronomers usually follow-up such detections by observing a spectrum — now that the locations of the galaxies are known — which will not only give a more precise distance, but will tell astronomers other things about the galaxies in the protocluster, such as chemical composition and the types of stars developing in the galaxies.
1 To be precise, the Lyman limit corresponds to the energy required to eject an electron from its lowest-energy state.