The Galactic distribution of molecular hydrogen was first deduced from CO observations some 30years ago. Surprisingly, the CO had a radial distribution distinct from that of atomic hydrogen. The most spectacular discovery was a large peak in the molecular gas column density halfway between the Sun and the Galactic Center. Subsequent maps of the face-on distribution of CO showed that this feature, dubbed the 5 kpc ring, dominates the Galaxy's structure. If viewed from the Andromeda galaxy, the 5 kpc ring would be the most prominent, distinct feature of our Milky Way Galaxy.

Right: Face-on view of the Milky Way in molecular hydrogen column density. The Galactic ring is the prominent feature at R=5 kpc. 

Most of the Galaxy's Star-formation activity takes place in the ring. With a mass of 2x10⁹ solar masses, the 5 kpc ring contains about 70% of all the molecular gas inside the solar circle. The ring is thus an enormous reservoir of material for the formation of new stars and clusters. Indeed, most of the Galactic Giant H II regions, far-infrared luminosity, diffuse ionized gas, and supernove remnants are associated with the ring.

Because the 5 kpc ring dominates both the molecular interstellar medium and the star-formation activity in the Milky Way, it surely plays a crucial role in the dynamics, structure, and evolution of our Galaxy. Yet, despite its obvious importance, the 5 kpc ring remains largely unexplored. This lack stems from two key obstacles. First, it is challenging to image at high angular resolution such a large expanse of the sky (several tens of square degrees). Second, it is difficult to estimate distances to Galactic star-forming regions. More than 25 years after its discovery, a number of fundamental questions about the 5 kpc ring still wait to be answered.

The Milky Way Galactic Ring Survey (GRS) is being performed by a Boston University (BU) and Five College Radio Astronomy Observatory (FCRAO) collaboration.

The FCRAO observatory. The telescope is located inside the radome.

The GRS exploits recent technological advances in ground-based millimeter wave receiver systems to probe the star-forming dense gas in the Milky Way with unprecedented spectral sensitivity and angular resolution. We are using the 16 pixel (soon to be 32) receiver SEQUOIA on the FCRAO 14m telescope to map the distribution of the ¹³CO molecule in the inner Milky Way.

Right: The SEQUOIA receiver.

The GRS will exploit the full angular resolution of the FCRAO 14m telescope for the first time. The new mm wave array receiver technology enables us to make advances on two fronts. First, we are now detecting a statistically significant sample of both star-forming clouds and their embedded young stars and clusters at good angular resolution over wide areas of the Galactic plane. Second, we are determining their distances accurately, and with these distances, we are for the first time establishing their sizes, masses, luminosities, and spatial distributions.