An OVERVIEW of SUBMIllimeter Astronomy



This page has been adapted and updated from Craig Kulesa’s original SORAL website.

Attempting to discuss all of the possible kinds of observations our receivers could yield isn't possible -- the submillimeter portion of the electromagnetic spectrum is relatively unexplored, and it will probably be unexpected surprises that will ultimately be the most influential. However, there are many outstanding astrophysical questions that will benefit greatly from information gleaned from the submillimeter band. Here are a few of them:

Molecular Clouds and Star Formation

Among the most pressing of these questions is how stars are formed. Stars are born from the material between other stars. In some regions of space, the density of gas and dust is much higher than the norm, and atoms are sufficiently shielded from destructive high-energy photons to interact with other atoms to form simple molecules. In the highest density central regions of such molecular clouds, material is so well shielded that delicate, complex molecules can form. It is from these molecular cloud cores that new stars (including our own Sun) and planets are born. These are the places of creation; understanding them may help astronomers understand the conditions from which the Earth, and life has evolved.

Because the star forming process occurs behind so much intervening dusty material, visible-light telescopes cannot see what is happening. By moving to the infrared, sub-millimeter and millimeter wavelength regions, where the effects of this obscuration are nearly negligible, astronomers can begin to directly probe regions where stars are actively being born.

Understanding the star forming process has many astronomical consequences. Knowing what physical conditions are needed to form molecular cloud complexes is important in understanding the star-forming evolution of galaxies, both in the current age and, perhaps most importantly, when galaxies were first forming. The understanding of how a single star forms from a molecular cloud, then, has even cosmological implications.

The current working model for star formation may be represented by four conceptually distinct stages of development that are the culmination of the last several decades of theoretical and observational efforts. Within molecular clouds, there are cores of material which are denser than the surrounding cloud. Dense molecular condensations form within these molecular cores as the loss of magnetic support through ambipolar diffusion allows gas and dust to contract gravitationally. Eventually these condensations become sufficiently centrally concentrated to undergo dynamical collapse; the inner regions form an evolving protostar and surrounding disk while the outer regions form an extended infalling envelope of material (the arrows). At some point, a bi-polar wind breaks out along the rotational poles of the system, while material continues flowing inward along the equatorial regions. The visual extinction toward the central protostar is typically tens to thousands of magnitudes at this point, effectively obscuring it from scrutiny at optical wavelengths. Over time, the angle occupied by the wind broadens, removing surrounding material and halting the inward flow of material. At this point, the system becomes detectable at near-infrared and even optical wavelengths as a star plus a disk, commonly recognized as a T Tauri system.

Molecular clouds themselves harbor a number of fundamental myseteries which submillimeter data and analysis is expected to shed light upon. Our understanding of the evolution, and (kinematic/chemical) structure of a giant molecular cloud is incomplete. Submillimeter measurements will uncover the warmer, denser components of a molecular cloud as well as the UV-illuminated "surfaces" of clouds where the delicate interplay and feedback between massive stars and molecular clouds can be explored. This will be crucial to improving our understanding of how molecular clouds form and are disrupted by their environments (i.e. the stars they give rise to).


The chemistry of molecular clouds is instrumental to deciphering their structure and evolution. Through their interaction with radiation and with each other, interstellar molecules partly regulate the dynamical evolution of molecular clouds and star-forming regions. Astrochemistry has also evolved into a subfield of its own. Astrochemists are interested in the composition of matter, and the physical processes that alter it, and what that tells us about the conditions and evolution of (inter)stellar matter. The study of astrochemistry can not only lead to an improved understanding of the physical conditions in, say, a molecular cloud or "stellar nursery", but also can provide clues to the composition of other star-forming (and possibly planet-forming and life-giving) solar systems. Such studies will help us understand how life evolved on Earth, and give us clues as to how pervasive this may be throughout the Galaxy and the Cosmos.

Submillimeter observations can test our understanding of interstellar chemistry in many important ways. Submillimeter observations can help identify heating and cooling processes in molecular clouds (i.e. the thermal balance). Pivotal neutrals and ions will be measured, such as H2D+, NH, NH2, H3O+, and metal hydrides like MgH, NaH and AlH+. The UV-radiated surfaces of molecular clouds harbor a rich photochemistry that serves as an excellent testing grounds for theories of molecule formation and dissociation. Evolved (dying) stars undergo periods of brief but potent mass loss. This ejected material forms dust and molecular gas which allows astronomers to probe dust formation, the enrichment of matter by stellar nucleosynthesis, and stellar winds.

Other Galaxies

Star formation takes place in other galaxies too -- and often on a much larger and more violent scale than ever imagined! Galaxy interactions can funnel large quantities of molecular gas
into the centers of galaxies, triggering giant outbreaks of massive star formation called starbursts. With submillimeter interferometers we will be able to identify single molecular clouds in the nearest galaxies, but even with single antennas we can study the properties of ensembles of star-forming clouds as a whole and thereby determine the global molecular properties of that galaxy. As in the Galaxy, submillimeter observations will extend our diagnosing power to warmer, denser regions that are actively forming stars. Line emission from these galaxies is expected to peak at submillimeter wavelengths, and will allow high-resolution studies of the large scale distribution of dense gas in galaxies. Since galaxies glow in visible light due to stars, and stars owe their existence to molecular gas, these studies will provide information on galaxy evolution, dynamics, and morphology.


At cosmological distances, powerful redshifted emission from singly-ionized carbon (rest frame = 158 microns) moves into the submillimeter waveband. The capability of exploring the starforming properties of galaxies at high redshift via [CII] spectroscopy is tantalizing. Very large bandwidth observations of apparently "blank" regions of sky like the Hubble Deep Field (see image above) should yield many detections of [CII], which allows the determination of redhsifts, distances, and gross star formation rates.

A microwave-sensitive telescope like the HHT can also probe the edge of the observable Universe directly, which is of interest to cosmologists. One of the great success stories of the Big Bang model has been the detection of the Cosmic Microwave Background Radiation (CMBR). This radiation stems from about 1 million years (virtually a blink of an eye, cosmologically speaking) after the Big Bang, when the Universe had expanded and cooled to an extent that atoms could form; leaving light waves to travel freely through space. Until that point, the Universe was ionized, and too dense and hot for light to travel far without interacting with matter. The so-called "last scattering surface" marks the epoch when the Universe became "transparent" and not "opaque". The radiation from this epoch has been shifted in wavelength due to the continued expansion of the Universe, now appearing in the microwave region of the spectrum, and is observable with millimeter and submillimeter antennas/receivers.

WMAP sky map