Hunting the oxygen doublet in distant galaxies

In its quest to uncover the mysteries of dark energy, DESI will measure precise redshifts for more than 15 million emission-line galaxies or “ELGs.” Although they are incredibly distant and faint, DESI will take advantage of a distinctive feature in the light emitted by these galaxies—a feature called the “oxygen two doublet”, represented by the symbol “[OII]”.

But what is this so-called doublet and where does it come from? And why oxygen? To answer these questions we need to dig into some astrophysics!

Did you know that after hydrogen and helium, oxygen is by far the most abundant element in the universe? For example, in our solar system there are one-and-a-half times more oxygen atoms than carbon atoms and nearly twenty times more oxygen atoms than iron atoms! Because it is so common, oxygen is also an incredibly important element in galaxies, especially in the gas between stars, what astronomers call the “interstellar medium.” In fact, oxygen is one of the primary ways for galaxies to “cool down.”

Let’s take a quick look at what this means. In galaxies, new stars generally form at the centers of cold, dense clouds of gas and dust. Occasionally, a very massive star will be born—a hot, bright star which can be anywhere from two to one hundred times more massive than the Sun. These monster stars pump enormous amounts of high-energy photons (i.e., light) into the surrounding gas, stripping away the electrons which are normally bound to atoms and creating a spectacular “soup” of fast-moving electrons, protons, and positively charged heavier atoms like oxygen surrounding the new star. Astronomers call these vibrant, short-lived sites of activity in galaxies “HII regions” or “star-forming regions”.

Now, the typical temperature of these star-forming regions is a balmy 10,000 degrees Kelvin, hotter than the surface of the Sun! So how does the region get rid of this extra energy and cool down? Well, as atoms like oxygen whip around in random directions, they will occasionally crash into one another. When this happens, some of the kinetic energy of motion goes into “exciting” one of the atom’s electrons into a higher energy state. But the electron doesn’t stay excited very long!

After about a second, the excited electron spontaneously “jumps down” to a lower energy level, simultaneously emitting a photon of energy (i.e., light) in the process. Subsequently, this photon escapes from the star-forming region, robbing it of the original energy pumped in by the hot young star and helping it cool down. In the actively star-forming emission-line galaxies which DESI is hunting, we observe oxygen “shining” in this way at two close but distinct energies or wavelengths, 372.71 and 372.98 nanometers. (One nanometer is one billionth of a meter, so the wavelength of this light is about 170 times smaller than the width of human hair!)

This pair of lines is called the “oxygen two doublet” (the “two” confusingly means that the oxygen atom is missing one electron) and it is written using the symbol “[O II].” In December 2020, DESI began observations as part of its “Survey Validation” phase, and one important question this phase aims to answer is, “How efficiently will DESI be able to pre-select (or “target”) emission-line galaxies?” So far, the answer to this question has been a resounding, “Really well!” To illustrate the kind of tremendous data DESI is obtaining, Dr. Julien Guy of Lawrence Berkeley National Lab has created an animation of the [OII] doublet in a sample of roughly 3400 emission-line galaxies which DESI observed in December, a tiny fraction of the more than 35 million galaxies DESI will observe during its 5-year survey.

Credit: DESI Collaboration

This movie shows the strikingly clear [OII] doublet in these galaxies, which brings DESI one step closer to being able to uncover the mystery of dark energy.