Molecules in Space--the story continues:

The next major event in our story takes place in 1970, when Arno Penzias, Robert Wilson, and Keith Jefferts, at Bell Labs, detected carbon monoxide (CO) in space for the first time. Hydrogen is the simplest and most abundant element in the universe, and molecular hydrogen (H₂) is by far the most abundant molecule. Unfortunately, under typical interstellar conditions H₂ does not emit at radio or millimeter wavelengths. CO, however, the second most abundant ingredient in molecular clouds, has a rich and strong millimeter-wave spectrum and it seems to maintain a fairly constant ratio with H₂ of about 1:100,000. For this reason, CO has become the standard tracer or "stain" for the invisible H₂ which constitues most of the molecular mass. Observations of CO soon reveled that molecular gas in space was much more extensive than ever suspected.

Mapping the Giant Molecular Clouds

Initially, Pat Thaddeus and his colleagues, Ken Tucker and Marc Kutner, began mapping the CO using the sixteen-foot radio telescope at the McDonald Observatory in western Texas. The plan was to keep mapping outward from the clouds they were observing (the Orion Nebula and the Horsehead Nebula) until they found a place where there was no more CO. They soon discovered that there was so much to be mapped that to do it with that size telescope would take many years. That large telescope could look at only a very small area of the sky with each observation. Such high resolution is usually an asset, but when trying to map large areas of the sky, it becomes a severe liability. Pat Thaddeus compares it to looking at an elephant with a magnifying glass, or painting a barn with a quarter-inch brush. Obviously, a larger "brush" was needed.

The "Mini" is born!

Pat Thaddeus and his colleagues designed a radio telescope custom-built for the task of mapping the entire Galaxy in CO. The "Mini" was designed with a relatively small dish and consequently a relatively large beamwidth of about 1/8 degree, which can be likened to a wide-angle lens. With this new instrument, it suddenly became possible to map large stretches of sky in relatively small amounts of time. The Mini began its life in a rather unlikely spot for a major research telescope--the 15 story high roof of Pupin Physics Laboratories on the Columbia campus, just off of upper Broadway in Manhattan. As Sam Palmer, the electrical engineer who built most of the telescope, says, "It's best to set up your astronomical operations in the 'center of the universe'." Although light and human-produced radio interference renders most optical or radio telescopes useless in a major city like New York, at 2.6 mm, the optimum wavelength for observing CO, the earth's atmosphere, light, and human noise do not interfere at all.

Using the Superbeam

Over the course of the next several years, a remarkable network of molecular clouds and filaments was uncovered, extending much further away from the Orion Nebula than expected. So large was the area covered, in fact, that Pat Thaddeus and Tom Dame, who had since joined the Columbia group, wished that they had an even smaller telescope, one which could quickly show them the big picture. Instead of building a smaller telescope, however, they decided to make a relatively simple change in the mini's control program. Rather than pointing at a single spot on the sky, they had the telescope antenna step through a square array of sixteen points on a 4 x 4 grid (a very cool test of the superbeam software can be viewed here). In effect, this allowed the mini to mimic a smaller antenna with a half-degree beam. Because it is impossible to view the entire Galaxy from New York, they also built an identical twin of the mini, which was shipped to Cerro Tololo, Chile to observe the southern sky.

Molecular Clouds in the Spotlight

Molecular clouds contain about half of the Galaxy's interstellar gas and give birth to all of its stars. These clouds are typically tens of light-years across and contain as much gas as a thousand Suns. The largest molecular clouds, appropriately called Giant Molecular Clouds (GMCs), measure hundreds of light-years across and weigh as much a million Suns. After a decade of mapping using the superbeam technique, Tom Dame and Pat Thaddeus had created the first complete map of the Galaxy in CO, covering more than 7,700 square degrees (nearly one-fifth of the sky) and representing more than 31,000 individual observations. The mapping revealed the distribution of molecular gas not only on the plane of the sky, but also in radial velocity. The large spread of observed velocities result mainly from the differential rotation of the Galaxy.

The Great Move and Beyond

In 1986, the mini was moved from New York to the Harvard-Smithsonian Center for Astrophysics. Since then, it has resided on the roof of the historic "Building D" here at the Harvard College Observatory. Pat Thaddeus, who occupies a joint position as a Professor of Astronomy and Applied Physics at Harvard University and Senior Space Scientist at the Smithsonian Astrophysical Observatory, continues to lead the millimeter-wave group. Tom Dame, who also came to Harvard from Columbia at that time, has coordinated telescope observations over the last decade, resulting in a second, updated map of the Milky Way, published in 2001. Sam Palmer continues to maintain the telescope hardware while also working on equipment at the Spectroscopy Lab. The telescope observations are finally nearing the conclusion of what was called by Pat Thaddeus "the zeroth order experiment"--the initial mapping of the Galaxy in CO--and is searching for ever weaker and more distant signals. It is currently in operation from October to May each year, working on mapping various sections of the Milky Way in greater detail and at greater sensitivity, among other projects.