The Curious History of Luminous Infrared Galaxies

A selection of luminous infrared galaxies from the Great Observatories All-sky
LIRG Suvey (GOALS). Credit: NASA/STScI/NRAO/A.Evans et al.

Our understanding of galaxy evolution changed dramatically after the launch of IRAS (the Infrared All-Sky Satellite) in 1983 when the first all-sky infrared survey was conducted. Upon analyzing this new data, taken in a portion of the electromagnetic spectrum that up until then had been little studied, a new class of galaxies was discovered. These galaxies contained more energy in the infrared portion of the spectrum, beyond what the human eye could see, then they did at all other wavelengths combined. The total amount of energy emitted by these galaxies is comparable to quasars, which had previously been known as the most energetic objects in the universe.

These unique objects were given a name, LIRGs (an acronym of course), which stands for Luminous Infrared Galaxies, precisely because these objects are so luminous in the infrared. These galaxies have infrared luminosities of over 100 billion times that of the sun. Even more luminous classes were named in a similar fashion, ULIRGs (Ultraluminous -- more than a trillion times more luminous than the sun), and HyLIRGs (Hyperluminous -- greater than 10 trillion solar luminosities). These objects are very rare in the nearby universe, but a number of them were identified because the IRAS survey covered the entire sky.

What makes these galaxies so bright in the infrared? Most of the light that we see from galaxies comes from the stars, but some galaxies also contain large amounts of gas and dust (we call these galaxies 'gas-rich'). This material absorbs a lot of the light from stars and re-emits it in the infrared. So this light still comes from stars but it is processed by the dust in the galaxy first. Most of the light we see from stars is dominated by young, very massive stars. For these galaxies to have so much energy emitted in the infrared means that they are made up of a lot of young stars. In fact, they are forming stars at incredible rates, up to 1000 times the rate that the Milky Way forms stars. Not all of the light in these galaxies comes from star formation - some comes from AGN activity. In fact, a large fraction of galaxies with these extreme luminosities are known to host an active black hole.

After luminous infrared galaxies were first discovered, astronomers turned to optical telescopes to find out what these galaxies looked like. The answer was stunning. Almost all of the objects looked at turned out to be merging or interacting galaxies (such as those shown in the image above). Such a large fraction, that it had to be more than a coincidence. From this, they deduced that the reason these galaxies were so luminous was that the merger of two gas rich spiral galaxies had induced huge bursts of star formation (explaining the very high star formation rates observed) and in many cases, drove gas toward the central black hole. This was held up by the realization that the more luminous systems tended to be at the most advanced merger stages.

All of these pieces lead to the evolutionary scenario, first suggested in 1988 by David Sanders, currently at the University of Hawaii. In this scenario, the merger of two gas rich spiral systems forms a luminous infrared galaxy that gets more luminous as the merger progresses. At the same time, the central black hole is being fed by infalling gas and obscured by the surrounding dust. Over time, the fuel gets used up and possibly expelled from the system at which point the infrared luminosity decreases and the central active nucleus is exposed. We observe such a system as a quasar. Eventually, the black hole runs out of fuel and is no longer active, and the merger remnant relaxes into an elliptical galaxy.

Diagram illustrating the merger scenario, where the merger of two gas rich spirals forms a luminous infrared galaxy, which evolves into a ULIRG, then a Quasar, and eventually an elliptical galaxy. All images are from Hubble.

This picture of galaxy evolution appears to explain many observables in the local universe, but the question of whether this was also the case for galaxies in the early universe remained a mystery. Samples of infrared galaxies at higher redshifts were found through various other surveys, but the pace really began to pick up in 2003 after the launch of the Spitzer Space Telescope (and accelerated with the launch of Herschel in 2009). All of these facilities look at light in the infrared, and each has been more sensitive and had better resolution than its predecessor. What was realized early on, and confirmed by initial results from Spitzer, was that luminous infrared galaxies are much more common in the distant universe. In fact, even though they are rare oddities locally, at one point in time they were the norm. If luminous infrared galaxies were so common in the universe's history, clearly they played an important role in galaxy evolution and in shaping the universe that we observe nearby. 
 
The immediate question that comes to mind is whether or not these distant luminous infrared galaxies have the same properties as the nearby ones. Are they all the result of galaxy interactions, as has been shown locally, or are their incredible luminosities caused by something else? Do they also harbor obscured accreting black holes? There are certainly reasons to believe that different processes may be at work in the early universe. Many initial studies have found a mix of galaxy shapes and properties using deep Hubble images - some of them are clearly mergers while others are not. Other observations have shown that galaxies in the early universe may have had a lot more gas than is present in galaxies today, providing a much larger reservoir from which to form stars - with or without a merger. Certainly, the picture is a lot more complicated!

Deep imaging in the near-infrared from CANDELS provides the ideal window into the properties of these systems because at high redshift, the observed light was originally emitted in the optical, just like we observe in nearby galaxies. The combination of rest-frame optical light and the exquisite resolution of images from Hubble allow us to study galaxy morphologies at these distances in greater detail than has ever been possible. In a future post, I will discuss the results of our recent paper analyzing a sample of these dramatic systems in the distant universe.