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.

Age/Redshift of the Universe

This figure shows a timeline of the universe, which relates the age of the universe to the redshift. Redshift is a quantity astronomers use to measure how much the light from distant galaxies has shifted towards redder wavelengths due to the expansion of the universe. Galaxies which emit light at very early times have very high redshifts. As a result of our detailed knowledge of how the universe expands, we are able to obtain an approximate relationship between the age of the universe and the redshift a galaxy would have if it emitted light at that time.

Credit:Rychard Bouwens

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 500 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 four to seven percent of its current age.

The Drop-Out Method

The Drop-Out Method

A short movie showing how the observed spectrum of star-forming galaxies (thick black line) changes as we observe it at higher and higher redshift. Redshift is denoted here in this movie as "z". Note how the break in the spectrum shifts to redder and redder wavelengths as a result of this redshifting effect. To be able to identify galaxies at the highest redshifts (and thus near the beginning of the universe), it is necessary to be able to measure the fluxes of sources at near-infrared wavelengths (>1000 nm). High-redshift galaxies are frequently found by noting a significant break in the spectrum as seen through a set of discrete filters (shown here in terms of their wavelength sensitivities as a set of colored lines).

Credit:Rychard Bouwens

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).

z=7.0 Drop-Out Galaxy

This figure presents one of the most important techniques for finding galaxies at very high redshifts. This technique has been called the "Drop Out" Technique, or Lyman Break Technique. It takes advantage of the significant break that occurs in the spectrum of high-redshift galaxies due to absorption by neutral hydrogen. One possible spectrum of a star-forming galaxy at a redshift of 7 is shown in the top panel. The presence of neutral hydrogen has a rather dramatic effect on the spectrum of this galaxy -- creating a rather abrupt drop off in flux blueward of 970 nm. Astronomers often look for sources which show this abrupt drop off in flux by taking images of the sky using many different filters. Each filter has sensitivity at different wavelengths. The sensitivities of several of the more useful filters on HST are shown in the middle panel and has been used in the acquisition of data for several of the deepest HST images ever obtained (e.g., HUDF). These filters (shown from left to right) have central wavelengths of 591 nm, 776 nm, 944 nm, 1119 nm, and 1604 nm, respectively, and frequently known by the names "V", "i", "z", "J", and "H" bands, respectively. The bottom panel shows images of the redshift 7 source from the top panel, as seen through these filters. This source clearly shows up in the two longest wavelength filters "J" and "H," but completely disappears in the three bluest wavelength filters "V," "i," and "z." The presence of such a distinct break is a clear indication that we have found a galaxy at very high redshift which emitted its light at very early times.

Credit:Rychard Bouwens

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

next: the latest results

Learn about the latest results from our search for the first galaxies at the edge of the universe. Read More...