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 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
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
z~7.0 Drop-Out Galaxies
This panel shows four candidate galaxies that are likely to have redshifts of 7 and thus have emitted their light whe n the universe was just 750 million years old. Each of the four candidate high-redshift galaxies are presented in a distinct row. All four candidate galaxies are shown using images at each of five different wavelengths (591 nm, 776 nm, 944 nm, 1119, and 1604 nm). These galaxies are all clearly detected at wavelengths redder than 1000 nm, but remain completely undetected at wavelengths bluer than 800 nm. This abrupt drop-off in the flux is strongly characteristic of star-forming galaxies at high redshifts and occurs due to the absorption of light by the large amounts of neutral hydrogen in the universe at early times. Astronomers use the presence of this break to find high-redshift galaxies. The present sources were found over the Hubble Ultra Deep Field and the Great Observatory Origins Deep Survey Fields. The search is described in a recently accepted paper to Nature.
Credit:Rychard Bouwens
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.