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|style="text-align: left;"|Stephen S. Hall, Mapping the Next Millennium

In the early 1970s, an astronomer at the Goddard Institute of Space Studies in New York named Patrick Thaddeus shattered centuries of precedent in the field of astronomy and bucked a trend dating back to Galileo when he decided that, in order to proceed on a modest project to map the entire Milky Way, he simply did not need and in fact refused to use a larger telescope made available for his research. He wanted a small one. In an era made conspicuous by bigger, more sophisticated, and more expensive telescopes, Thaddeus insisted on a small and relatively inexpensive instrument, which he and his colleagues proceeded to build from scratch.

Purpose

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|style="text-align: left;"|You can't see a nucleic acid or protein within a cell, so you have to use a drop of dye to bring out the structure. Well, in the densest star-forming regions, we're caught in a similar situation. We can't see the dominant molecule—molecular hydrogen—either.

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|style="text-align: left;"|—Patrick Thaddeus, quoted in Thursday's Universe by Marcia Bartusiak

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Interstellar carbon monoxide is the best general tracer of the largely invisible molecular hydrogen that constitutes most of the mass in molecular clouds. Hydrogen is the simplest and most abundant element in the universe, and molecular hydrogen is by far the most abundant molecule. Unfortunately, under typical interstellar conditions molecular hydrogen does not emit at radio or millimeter wavelengths.

Carbon monoxide, 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 molecular hydrogen of about 1:100,000. For this reason, carbon monoxide has become the standard tracer or "stain" for the invisible molecular hydrogen which constitutes most of the molecular mass.

Achievements

A total of 24 PhD dissertations have so far been written based on observations or instrumental work with these telescopes.

The 1.2 meter telescope has played an important or dominant role in all of the important general findings on molecular clouds (MCs) listed below. Many of these are now considered conventional wisdom but some were originally controversial (e.g., the very existence of giant molecular clouds, their ages, and their confinement to spiral arms).

  • 1977: Carbon monoxide is the best general-purpose tracer of molecular cloud mass.
  • 1977: Galactic carbon monoxide emission peaks in a broad "molecular ring" at R~4 kpc.
  • 1977/1994: Molecular clouds are mainly confined to a thin Gaussian layer ~100 pc wide, but a faint layer ~3 times as wide also exists.
  • 1980/1983: Molecular clouds are excellent tracers of galactic spiral structure.
  • 1980: Molecular clouds are relatively short-lived galactic objects.
  • 1982/1983: The molecular cloud mass spectrum is steep, with most of the mass in the largest clouds.
  • 1983: Intercomparision of carbon monoxide, HI, and diffuse gamma ray emissions provides perhaps the best large-scale calibration of carbon monoxide as a molecular mass tracer. The term X-factor was coined in this paper.
  • 1985/1989/1991: Molecular clouds are dark nebulae both in the optical and the near infrared.
  • 1986: Giant molecular complexes containing more than a million solar masses are not kinematic artifacts—as some had argued—but are well-defined objects that can be readily located throughout the galaxy.
  • 1988: Roughly half of the interstellar gas within the solar circle is molecular.
  • 2008: The enigmatic Expanding 3-kpc Arm has a Far 3 kpc symmetric counterpart on the far side of the Galactic Center.
  • 2011: The Scutum–Centaurus spiral arm extends almost 360 degrees around the galaxy, from the end of the central bar to the warp near its outer edge.

thumb|center|600px|The Milky Way in different tracers. Fourth from the top is the distribution of [[Hydrogen|H<sub>2</sub>, derived from carbon monoxide(1–0) observations made with the CfA 1.2 m Millimeter-Wave Telescope]]

Personnel

Prof. Patrick Thaddeus (the Robert Wheeler Willson Professor of Applied Astronomy, Emeritus, Harvard University; Senior Space Scientist, Smithsonian Astrophysical Observatory), who was leading the Millimeter-Wave group, died on April 28, 2017. Tom Dame (Radio Astronomer, Smithsonian Astrophysical Observatory; Lecturer on Astronomy, Harvard University) has coordinated telescope observations over the last decade. Sam Palmer (Electronics Engineer, Smithsonian Astrophysical Observatory; Lecturer on Astronomy, Harvard University) continues to maintain the telescope hardware.

History

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|style="text-align: left;"|Comparing and combining data from radio telescopes is generally difficult because of differences in resolution, sensitivity, and calibration. But the twin minis provide an unprecedented opportunity to produce uniform superbeam maps of the entire Milky Way, and, eventually, of the entire sky. . . .Without the superbeam technique, the twin minis would have required several decades to map such a large area. Two telescopes with 1-arc-minute beams (like the antenna at Kitt Peak) could barely complete the job in two centuries.

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|style="text-align: left;"|Tom Dame, Sky & Telescope

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Built by Thaddeus and colleagues in 1974, the telescope was operated from a Columbia University rooftop in Manhattan until it was moved to the CfA in 1986. Its twin instrument was constructed at Columbia and shipped to Cerro Tololo Inter-American Observatory, Chile in 1982.

Observations of carbon monoxide had revealed that molecular gas in space was much more extensive than ever suspected. Initially, Thaddeus and his colleagues, Ken Tucker and Marc Kutner, had originally begun mapping the carbon monoxide 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 carbon monoxide. 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 small area of the sky with each observation.

Thaddeus and his colleagues designed a radio telescope custom-built for the task of mapping the entire galaxy in carbon monoxide. 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.

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 Thaddeus and 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. 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.

After a decade of mapping using the superbeam technique, Dame and 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.

In 2011, Dame and Thaddeus found clear evidence in existing 21&nbsp;cm surveys for a large extension of the Scutum-Centaurus Arm, one of the two major spiral arms thought to extend from the ends of the galactic bar. The "Outer Sct-Cen arm" lies well beyond the solar orbit on the far side of the galaxy, roughly 21 kpc from the Sun. The CfA 1.2 m telescope has so far detected 22 distinct giant molecular clouds associated with HI peaks in the arm, and a large, unbiased carbon monoxide survey of the entire arm was begun in the fall of 2013; it is expected to require ~2 years to complete.

References

  • CfA 1.2 m Millimeter-Wave Telescope (CfA_mini) on the internet
  • The Milky Way in Molecular Clouds survey of the galaxy