the first galaxies

Searching for the First Galaxies

To locate galaxies which were forming near the beginning of the universe, astronomers look for sources which have very high redshifts.

Cosmic Redshift

As a result of the expansion of the universe, light from distant galaxies shifts towards longer wavelengths. The process is called redshifting and is fairly analogous to the well-known Doppler effect where an increase in pitch is noted for approaching sounds and a decrease is noted for receding sounds.

The redshift of a source tells us how much light from that source has been shifted in wavelength since it was originally emitted. For example, a redshift of 2 means that light from a source has tripled in wavelength since it was emitted, a redshift of 3 means that light from a source has quadrupled in wavelength since it was emitted, and a redshift of 0 means that there was no change in the wavelength of light since emission. Redshift is usually abbreviated as "z." Since the redshift of a source tells astronomers how much smaller the universe was when the source emitted its light, astronomers are able to use the redshift of sources to determine the age of the universe at that time.

At present, current searches for the first galaxies are taking place at redshifts between 6 and 10, which corresponds to between 400 and 900 million years after the Big Bang. Since we know from current WMAP measurements that the universe is 13.6 billion years old, we are looking back to a time, when the universe was just three to seven percent of its current age.

The Drop-Out Method

Astronomers employ a number of different strategies to find sources at the highest redshifts. One of the most popular and useful of these strategies is the "dropout" technique. The "dropout" technique relies upon the fact that the universe is filled with a large amount of neutral hydrogen and this hydrogen absorbs light at wavelengths bluer than 121.6 nm. As a result of this absorption, we see a very distinct break in the spectrum of an object. The position of this break allows us to determine how much the light from a source has been redshifted. For objects with a redshift of 0, there will be no change in the wavelength of this break, and it will occur at 122 nm. However, for objects at redshifts of 6, this break will occur at a much redder wavelength (851 nm).

Astronomers often search for galaxies that emitted their light at specific epochs by searching for this spectral break. They obtain images of the sky at a number of different wavelengths and then look for the sources that disappear or "drop-out" at a specific wavelength. An illustration of what one of the candidate high redshift objects might look like is shown in the figure to the left.

Some Recent Results

Using approaches like the dropout technique described above, we now know of more than 600 galaxies which have redshifts of 6, which means that these objects emitted their light when the universe was just 900 million years old. The large number of galaxies found at such early times allows us to make great progress in understanding their properties. For example, we have been able to estimate the volume density of these galaxies as a function of their luminosity. We have been able to measure their sizes: they are about ten times smaller than our own Milky Galaxy. We have also been able to weigh the stars in these objects (some contain masses equivalent to more than ten billion suns).

However, when you consider searches for galaxies which emitted their light at <750 million years after the Big Bang (redshift 7 or higher), considerably less is known. As of the present time, we know of only ~10-20 sources which appear to have originated from such early times. We show images of 4 such galaxies in the figure to the right that were presented in a recent Nature paper. Each of these sources were found in the ultra-deep near-infrared images taken with Hubble Space Telescope (HST) NICMOS (Near-Infrared Camera and Multi-Object Spectrograph). These images are exciting since they provide us with examples of what galaxies looked like at these early times. All of these galaxies are very compact, but still quite luminous. From our searches, we infer that luminous sources were much rarer at these epochs than they were just 200 million years later.

Identifying galaxies at such early times (redshift 7 and greater) is challenging because light from these objects shifts into the infrared. To be able to detect these objects and measure a break, it is necessary to obtain very deep images at both optical and near-infrared wavelengths, and this is not easy to do using ground-based telescopes since there is a significant amount of background light that comes from our own atmosphere. In fact, this background light is some 100,000 times larger than the very faint high-redshift sources we are looking for.

Background light is much less of a problem, if we use telescopes in space to make the observations. The good news is that we already have a very powerful telescope in space called the Hubble Space Telescope (HST). HST has cameras to obtain images at both optical (visible) and infrared wavelengths. The most efficient optical camera on the Hubble is the Advanced Camera for Surveys (ACS) and there is an infrared camera on HST called NICMOS (Near-Infrared Camera and Multi-Object Spectrograph). NICMOS is efficient enough that with a 10-20 hour exposure, we can probe faint enough to begin finding very high redshift objects.

The NICMOS instrument on HST has one significant limitation though. The NICMOS camera can only view one very small patch (0.8 square arcminutes) of the sky at a time. This area is equivalent to just 0.1% of the surface area of the full moon. As a result, it can require significant amounts of telescope time to survey any sizeable area on the sky with NICMOS. This situation will likely soon change once the WFC3 camera is installed on HST. next...