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 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.
Milky Way Molecular Line Surveys
|Sensitivity (K)||0.4 (0.2)||0.4||0.18||0.1|
|Vel. resolution (km s-1)||0.2||1.0||0.65||0.68|
|l extent (degrees)||18 to 55.7||8 to 90||10 to 70||-5 to 117|
|b extent (degrees)||-1 to 1||-1.05 to 1.05||-6 to 6||-1 to 1|
|Vel. extent (km s-1)||-5 to 135||-100 to 200||-140 to 140||-250 to 250|
|Total survey region (sq. degrees)||75.4||172.2||660||244|
The ~2 million GRS spectra will have sufficiently high spectral sensitivity to resolve all detected lines. GRS images will reveal the structure and kinematics of the molecular gas in the inner Milky Way with unprecedented detail.
In particular we expect GRS images to identify dense gas associated with deeply embedded infrared sources and clusters across the full disk of the Galaxy. Once a gas-IR source identification is established, GRS data can provide the velocity and distance to the IR source. Because of this ability to identify and probe active star forming gas and to establish distances to and hence luminosities of the IR sources, GRS data will play a crucial role in the astrophysical analysis of the new generation of Galactic infrared surveys (e.g., SOFIA, FIRST, SIRTF).
A comparison of GRS 13CO and 8 micron infrared emission obtained with the Midcourse Space Experiment (MSX) towards l = 45 degrees. Move over the GRS image to switch to the infrared image (allow some time for loading the images). Both images show emission from all velocities along the line of sight. Clearly, molecular line emission can be associated with infrared emission, demonstrating that with the GRS data it is possible to assign radial velocities (using channel maps of 13CO) to the infrared sources. The MSX data are available at IPAC.
If you click on the image, you will be transfered to a larger GRS image from which you can select regions to do the same comparison. Keep in mind that the images are large and may take a while to load!