Exploring the Life Cycle of Interstellar Matter

The HEAT telescope is exploiting the exceptional weather at Ridge A to construct a spectroscopic map of the Milky Way at terahertz frequencies (far-infrared wavelengths), where the critical spectral emission features of the dominant forms of carbon are located (carbon atoms at 492 and 809 GHz, carbon ions at 1900 GHz, and the prominent carbon-bearing molecule CO, seen at regularly-spaced intervals starting at 115 GHz). HEAT will construct a carbon map of the Southern Milky Way, and make the first ground-based measurements of ionized carbon, the most luminous spectral line in the Milky Way, which can otherwise only be observed from airborne or space observatories. By seeing carbon cycling through all of its principal forms, we will follow the full life cycle of interstellar material and witness for the first time molecular clouds being born, evolving, forming stars, and dissolving, in a Galactic context.

The life cycle of interstellar matter stems from the hot diffuse gas that permeates much of the volume of the Galaxy (top of cycle). Through thermal instability, it can cool to form denser "clouds" of atomic gas that may be the building blocks of the still colder and denser molecular clouds that follow. As far as we know, all stars are formed from molecular clouds. Either through their formation or their eventual death, stars frequently destroy their natal or neighboring clouds (mechanically, or through their radiation), dispersing it back into the diffuse phase again, this time enriched with the heavy elements fused by the star during its lifetime.
Sadly, there are gaping holes in our knowledge about exactly how this life cycle works. One astonishing question is "how do clouds form?" We can observe clouds in visible light, when they are illuminated by nearby stars, or when they are seen in silhouette against a backdrop of stars or gas (as at left). But where did they come from?
A large part of this problem stems from the limited way in which we observe clouds. Most of what we know about interstellar matter comes from cm-wave radio observations of atomic hydrogen (top) and mm-wave observations of carbon monoxide (CO), which acts as a placeholder for molecular hydrogen (the main constituent of dark clouds but which has no emission line spectrum at cold temperatures). These two views are difficult to interpret, as they miss the formative atomic clouds and small molecular (hydrogen) clouds that have no CO yet.
If one were to fly into a molecular cloud (from left to right in this plot), the forms of hydrogen and carbon would be constantly changing. Hydrogen, initially ionized, becomes neutral and then molecular. The same happens to carbon, but these transitions happen much deeper in the cloud, where hydrogen is already molecular. Thus, ionized carbon and atomic carbon play a big role in understanding the life cycle of interstellar matter. They let us see natal molecular clouds, and their building blocks -- whose carbon is ionized or atomic, and not yet formed CO.

This is the goal of the HEAT telescope: make global maps of the Milky Way in neutral carbon, and ionized carbon, to construct maps of the "missing dark gas" in the Galaxy. This missing phase of Galactic matter should contain the material that acts as building blocks for clouds, and will also show their dispersion, or destruction, back into a dilute phase. Thus, we will see the entire life cycle of clouds and understand better how this process sculpts galaxies (and stars, planets, and life) through cosmic time.