Science Motivation for the Surveys

There are three primary, inter-related science investigations were are undertaking with the Mopra-Nanten2-STO surveys:

• The Formation of Molecular Clouds
• The Dark Molecular Gas
• Gamma Rays, Cosmic Rays and the Dark Gas

We discuss these below.

The Formation of Molecular Clouds.

The basic activity of a spiral galaxy like our own is the continual collection of diffuse and fragmented gas and dust clouds into large “Giant Molecular Clouds” (we will refer to these as “giant clouds” for simplicity though the normal abbreviation is GMC). These contain from 10⁵ to 3 x 10⁶ solar masses of molecular gas and are somehow formed from smaller atomic or molecular clouds. Once formed, they are the main reservoirs for molecular gas in the Galaxy. They also produce most of its star formation. To some degree this process is a cycle, as stellar death not only partially replenishes the interstellar gas and dust (some gas is also added by infall of material onto the spiral disk), but the stars also destroy the giant clouds and provide turbulence to the interstellar medium, thereby affecting the collection of the diffuse and fragmented small clouds in this medium back into giant clouds. One of the largest unsolved mysteries in galactic astronomy is how these giant clouds are formed.

Observations of external spiral galaxies show that giant clouds tend to form in the compressed regions of spiral arms, behind the spiral density wave shock. If the region of the galaxy is primarily atomic, then the atomic gas must somehow be collected to form giant clouds, as is seen in the galaxy M33. If the region is mainly molecular, such as in the “molecular ring” which extends from about 3000 to 5000 pc from the centre of our Galaxy, then the collection into giant clouds may involve small molecular clouds (“fragments”) bound by pressure rather than by self-gravity. These clouds may also be mostly “dark” as they are too small to shield molecules, other than hydrogen, from dissociating radiation, as discussed further below. They do, however, always have a small surface region of atomic hydrogen gas (which is not “dark”).

The manner in which gas is gathered into the giant clouds has yet to be observed, but four principal mechanisms have been proposed:

1. the self gravitational collapse of an ensemble of small clouds, possibly along magnetic field lines as in the Parker instability,
2. the random collisional agglomeration of small clouds,
3. the accumulation of material within high pressure environments such as shells driven by the winds and supernovae from high mass stars, and
4. the compression and coalescence of gas in the converging flows of a turbulent medium.

These four mechanisms provide quite different pictures for the structure and evolution of the giant clouds. For instance, under mechanism (i) the gravitational collapse of a cluster of clouds produces molecular clouds that are long-lived and stable, supported against gravity by internal turbulence and magnetic fields. This can be regarded as the classical view of a giant cloud. This contrasts strongly with the picture given by mechanism (iv), of compression in converging flows, where gravity plays little role. It produces molecular clouds that are transient features. These different scenarios produce different signatures in the emitting gas, which allow them to be distinguished.

The Dark Molecular Gas

It has been clear for many years, from a variety of observations (e.g. of gamma rays and infrared extinction, that a substantial amount of the molecular gas in the interstellar medium must exist in a form where not only is the hydrogen molecule (H₂) dominant but where there is little or no carbon monoxide (CO), the next most abundant molecule in space. This is called the “dark” molecular gas, for CO line emission is the normal feature used to signal the presence of molecular gas; H₂ itself cannot be directly seen as the gas is usually too cold to excite it appreciably. Such dark gas is expected from theoretical arguments to reside either in the surface layers of the giant clouds or in “translucent” clouds, which are smaller and have a lower column density. In both cases the gas is exposed to far-UV radiation which can dissociate the molecules, however the much greater abundance of H₂ to CO allows it to self-shield, and so H₂ can exist where CO is absent. Models for the dark molecular gas suggest that it could comprise one-third of the molecular mass of the giant clouds and nearly all the mass in the translucent clouds. Although its presence is inferred, it remains unobserved through direct emission. However these models also indicate that ionized carbon (C⁺) and atomic carbon (C) exist in detectable amounts in the dark gas. The C⁺ ion has a strong emission feature in the terahertz spectrum. It will be accessible with the forthcoming Stratospheric Terahertz Observatory. STO will be able to discern where the dark gas is in the Galaxy, in conjunction with measurements of the neutral (C) and molecular (CO) forms of carbon, made using two other telescopes our team has developed – Nanten2 and Mopra – and combined with archival observations of the atomic hydrogen (H).

Gamma Rays, Cosmic Rays and the Dark Gas

Cosmic rays are extremely energetic particles that are accelerated in violent events such as supernovae. They pervade the interstellar medium. Gamma rays can be produced when the cosmic rays interact with the nuclei of gas atoms or molecules they encounter. Hence maps of gamma rays can be interpreted as the total amount of gas, regardless of the physical state of the gas (i.e. whether molecular or atomic, including the dark gas). With the advent of the HESS and Fermi telescopes, the first true images of gamma ray sources have been obtained, revealing a rich variety of sources. Many can be identified, e.g. with supernova remnants, with pulsars and perhaps star formation regions. However a new mystery is that over 30% remain unidentified, with no counterparts seen in other wavebands. Moreover, even for the many gamma ray sources associated with atomic or molecular clouds, the morphologies only partially correspond. The key to resolving this mystery, and thus the high energy astrophysics behind these gamma ray sources, is to determine the distribution of all forms of gas, so as to be able to compare the total gas to that of the distribution of gamma rays. The missing ingredient here is the dark gas, which we will at last be able to measure through this project, as we describe in the next section.