Violent outbursts: The unruly youth of the universe

As far as galaxies are concerned, things just aren't what they used to be. When we look around in the local universe, both the Milky Way and its neighbours seem quite quiescent (quiet). If they were people, they would be in a happy relaxed middle-aged phase. However, like any person they started small and had to grow up. Unfortunately, we don't have a complete record of how this happened, but we can look back to a time when the universe was still in its lively and somewhat unruly youth.

Galaxies have grown from small fluctuations in the early Universe to the massive groups of stars we see today, either by the gradual assembly of stars from dust and gas, or through merging with other galaxies. As far as we know, the vast majority of galaxies form stars at a roughly fixed rate relative to their mass, known as the specific star-formation rate. Put simply, this means the more massive a galaxy is, the more rapidly it will be forming stars. Over the vast stretches of cosmic time, this gradual accumulation of mass is enough to build up galaxies that are 100 billion times more massive than our Sun. However, things are rarely as simple as they seem, and interestingly, some galaxies appear to have taken this to an extreme. The rate at which some of the biggest galaxies in the universe form their stars has not been constant, but was actually higher in the past. It seems that, like people, some galaxies did most of their growing in their youth and have since stayed at about the same mass, forming only a few more stars.

Over the past few decades astronomers have opened up a new window on the universe, through the advent of submillimeter astronomy. These detectors are sensitive to light with wavelengths that are a 1,000 times longer than we can see with our eyes and is typically emitted by dust. As stars form they emit ultraviolet light, and as stars form inside clouds of gas and dust, much of that ultraviolet light is absorbed by this cosmic dust which is then heated to a few tens of degrees above absolute zero (-273 degrees Celsius). The "warmed" (but still rather "cool") dust then re-emits the absorbed energy at far-infrared wavelengths, which is then further redshifted to longer sub-millimeter wavelengths en-route to the Earth by the expansion of the Universe, where it can be collected and measured.

Submillimeter astronomy truly entered the academic scene in the 1990s with the introduction of the Submillimetre Common-User Bolometer Array (SCUBA), so important it can now be seen on display at the national museum of Scotland. While our understanding has developed over time with additional ground-based surveys (using, e.g., AzTEC and LABOCA), the study of sub-millimeter sources is poised to undergo a revolution stemming from a new generation of instruments. Starting with the Balloon-borne Large-Aperture Sub-millimeter Telescope (BLAST), and now with the Herschel Space Observatory, the quality and volume of sub-millimeter data has increased at a tremendous rate. These key projects will soon be joined by the Atacama Large Millimeter Array (ALMA -- see recent post), which will for the first time provide submillimeter imaging at a similar resolution to that of the famous Hubble Space Telescope. However, many mysteries remain regarding the nature of the currently known sub-millimeter sources, and many more are sure to be uncovered with the new generation of instruments and telescopes.

Example of two-dimensional modelling from Targett et al. (2012) for a single component clumpy disk (top), multi-component disk-dominated system (middle), and apparently irregular system (bottom). The first left-hand panels show false-color HST I, J, H-band images. The center left-hand panels show the H-band CANDELS postage-stamp images centered on the sub-millimeter galaxies. The center panels show the best-fitting two-dimensional models. The right-hand panels show the residual images after subtraction of the models from the data.

One of the most interesting and important discoveries in this new field was that some galaxies weren't simply forming more stars in the past, but that they were forming stars at a rate up to 1000 times more rapidly than expected. As these sources were detected at submillimeter wavelengths, they are rather originally named submillimeter galaxies (SMGs). Clearly, understanding the nature of these violently star-forming galaxies is important given how abnormal their growth seems when compared to that of 'normal' galaxies. Specifically, many scientists are interested in whether sub-millimeter sources are large disk-like galaxies (such as our Milky Way) or merging systems (where two or more galaxies have collided and coalesced). Knowing this would help to determine which of the theoretical models currently used to explain them are correct, and therefore enhance our understanding of what role they play in galaxy evolution. Using galaxy modelling software we were able to measure the sizes and morphologies (shape) of submillimeter galaxies in a field known as GOODS-South. The exquisite depth and resolution of the CANDELS data available in this field has provided some of the most detailed morphological analysis of galaxies to date (see above picture). As with many results in science the picture is not always clear, and many of our sources show evidence of merging or interaction, but overall these submillimeter galaxies were well-described by either a single component exponential disk, or a multiple component system in which the dominant constituent is disk-like. Taken together with other results, these data seem consistent with the view that most submillimeter-selected galaxies are simply the most extreme examples of normal star-forming galaxies at that era, and could therefor be critical to the formation and evolution of the most massive galaxies.