What was the topic of your 1983 PhD, and who was your advisor?
For my thesis work I was fortunate to be in the right place at the right time. Just after passing my qualifying exams, I was contacted by Pat Thaddeus who told me about the little radio telescope that he was building on the roof of the physics building on the Columbia campus. Pat was a brilliant and inspiring scientist who sold me on the importance of the work right away. Several of his graduate students, including Gordon Chin, Hong-Ih Cong, and Richard Cohen, had been working hard for the previous few years on building the 1.2 m telescope, so by the time I joined the group it was ready to begin large-scale CO surveys of the Galaxy. My thesis was entitled “Molecular Clouds and Galactic Spiral Structure” and dealt with some subjects which at the time were very controversial: the confinement of molecular clouds to the spiral arms and their lifetimes. Based on modeling of the spiral features evident in my CO survey of the first Galactic quadrant, I argued that molecular clouds were tightly confined to the arms and so couldn't live longer than a spiral arm crossing time of a few 10 Myr. Since then, interferometric CO surveys of many external spirals have shown that molecular clouds are indeed excellent tracers of spiral structure.
Your first paper with Pat Thaddeus presented a CO survey of the first Galactic quadrant with 1 degree angular resolution. How and why did you carry out such a survey?
That 1985 paper presented the first of our so-called “superbeam” surveys for which we degraded the telescope's angular resolution by offsetting the pointing through a square raster of points separated by a beam width (~ ⅛ deg) during an observation; it bore some resemblance to modern day on-the-fly mapping except that our Data General Nova minicomputer was not up to the task of saving every data sample. It might seem crazy to degrade the resolution of the world's smallest radio telescope, but the technique allowed us to well sample large areas of the sky very rapidly, and the angular resolution was still more than adequate for comparison with the high-energy gamma ray surveys that were being obtained around the same time by the SAS-II and COS-B satellites. Pat and I collaborated with the COS-B group on such a comparison (Lebrun et al. 1983) that provided the first gamma-ray calibration of the CO-to-H2 conversion factor - what we unimaginatively named the “X factor” in the paper.
A few years later you and a team of collaborators presented a composite survey of the entire Milky Way. This was something of a revolution in millimeter astronomy, and the paper has been cited over 800 times. What were the key conclusions?
Even though that 1987 paper was cited so many times, I suspect that most older researchers today have forgotten about it and most younger ones have never heard of it. That's because the '87 whole-Galaxy survey was superseded by a higher-resolution version that we published in 2001; the former survey was a composite of 0.5o “superbeam” surveys while the latter was largely beam width sampled. Both journal papers included unique 5-page foldouts displaying the whole-Galaxy lb and lv maps and in both cases we put those same maps on wall posters that were widely distributed. Because our research group -- along with the northern 1.2 m telescope -- moved from Columbia to CfA in 1986, the former survey is generally called the Columbia CO survey and the latter one the CfA survey. Together these two surveys incorporate the data from most of the 24 people who have obtained their PhDs with the northern and southern 1.2 m telescopes.
Thanks to the superbeam technique, we were able to get our first look at the molecular Milky Way 15 years earlier than would otherwise have been possible. Owing to its wide latitude coverage, the Columbia survey was especially valuable for studying local molecular clouds. We cataloged 23 within 1 kpc of the Sun, plotted their locations on the Galactic plane, and used them to determine the mean molecular surface density and scale height in the solar neighborhood. It was reassuring that these direct measurements agreed well with values obtained for the inner Galaxy using kinematic distances and axisymmetric modeling.
In 1986 you and your colleagues published a paper on the largest molecular clouds in the first Galactic quadrant, aiming to characterize their distribution and properties. What did you learn?
This was an early attempt to decompose the CO emission in the first quadrant into its component clouds. We focused on the few dozen largest and brightest objects, what we called “complexes”, because they were the best defined and I knew from my thesis work that such objects contained most of the molecular mass. Unlike the many subsequent attempts that relied on programs such as clumpfind, this analysis was largely done by hand, particularly when it came to resolving the kinematic distance ambiguity. We had an appendix which gave a “lawyer's brief” of evidence for each cloud supporting either the near or far distance. I think this work was important because it broke a long-standing stalemate as to whether the apparent spiral structure in lv diagrams was due to velocity or gas density variations along the line of sight. Very influential work by W. Butler Burton in the early 1970s had shown that in the case of 21 cm lv diagrams, the apparent spiral structure might be just “kinematic illusions” produced by density wave streaming motions. In our case we definitely had real, well-defined objects, and we were able to locate them one by one on the Galactic plane, and they lined up nicely along the spiral arms. We also confirmed and extended the radius-linewidth relation of Larson to much higher cloud masses and showed that the clouds were in approximate virial equilibrium.
Besides the Milky Way, the 1.2 m telescopes also obtained the first complete CO surveys of the LMC and M31. Can you tell us about those surveys?
The Magellanic Clouds, along with the Galactic center, were prime targets for the second 1.2 m telescope, which was built and tested on the roof of the Goddard Institute for Space Studies in New York City before being transported to CTIO in Chile in 1982. Certainly the most spectacular discovery of the LMC survey was the enormous concentration of molecular clouds extending south from the giant HII region 30 Doradus for nearly 2400 pc and containing about 60 million solar masses of molecular gas. There is really nothing comparable to that structure in the Milky Way. Perhaps it is the solid body rotation of the LMC that allows such structures to form. We found that individual GMCs in the LMC were similar to those in the Milky Way, except that they were about 6 times fainter in CO, suggesting that the CO luminosity in a galaxy scales roughly with its metallicity.
Our complete CO survey of M31 was the most difficult ever undertaken with either of the 1.2 m telescopes. Beam dilution and velocity gradients across our 1.7 kpc beam resulted in extremely weak (~0.01 K) and broad (50-100 km/s) lines that required exceptionally flat baselines for detection. Only mm-wave observing aficionados will appreciate this, but we developed a unique azimuth-switching technique in which we slightly adjusted the elevation of the OFF after every switching cycle to cancel out any accumulated ON-OFF power to that point in the scan. This active feedback technique balanced the ON-OFF powers to one part in 105 and yielded spectra that almost did not require baseline removal. Although we could not resolve individual clouds, we obtained robust results on the molecular radial distribution. We found that beyond a radius of ~8 kpc M31 and the Milky Way have comparable amounts of gas, both atomic and molecular, and both have central gas holes. We suggested that the essential difference between the two galaxies may be the larger size of the hole in M31 (~8 kpc) compared with that in the Milky Way (~3 kpc).
In 2001 you published, together with Dap Hartmann and Pat Thaddeus, your magnum opus, a new complete CO survey of the molecular clouds in the Milky Way. With almost 1500 citations, this study evidently has been highly influential. Please tell us about the survey.
This was our second whole-Galaxy CO survey, already mentioned earlier. The many citations to this paper are largely owing to the fact that molecular clouds, along with their associated dust and star formation, influence the large-scale Galactic emission in every major wavelength band. In many cases, our CO survey provides the crucial third dimension of velocity or kinematic distance that is lacking in continuum surveys such as those in gamma rays and the infrared. Much of my work in recent years has been in collaboration with astronomers studying the Galaxy at other wavelengths. I'm proud of the fact that our small telescopes, largely operated by students for about $25K per year, have provided data that were essential for the interpretation of satellite surveys costing tens to hundreds of millions of dollars.
In the past decade or so you and Pat have discovered two new spiral arm structures in the Milky Way. How did these discoveries come about and what are their significance?
During my thesis work I developed a great familiarity with and, some would say, bizarre fondness for 21 cm and CO longitude-velocity diagrams. My feeling is that any gaseous spiral feature must be evident in the lv diagram--essentially the raw data of a spectral line survey--otherwise it's crazy to think that it is going to suddenly appear after a complex transformation into the plane of the Galaxy. I spotted the far-side counterpart of the long-known “Expanding 3-kpc Arm” while preparing a review talk on CO spiral structure for the June 2008 AAS meeting. I was trying to make lv diagrams that showed the CO spiral structure most clearly when the so-called “Far 3-kpc Arm'” revealed itself. I was a bit skeptical at first, but subsequently calculated that all of its physical characteristics closely matched those of the near-side Expanding Arm. My review talk turned into a report on this new spiral feature, which was very exciting and fun. Scott Tremaine called it the first direct evidence for a two-fold symmetry in the Galaxy.
Oddly enough, I came upon the Outer Scutum-Centaurus Arm in a similar manner -- while preparing a review talk for an ESO conference in Chile. In this case the arm was first visible only in 21 cm emission. Since it was located a few degrees above the plane in the distant outer Galaxy, no CO survey had covered the region. Concerned that the arm might be considered a “kinematic illusion” of the sort discussed earlier, Pat and I began very sensitive CO observations of the structure with the CfA telescope. We concentrated on HI peaks and ultimately found molecular clouds at about a dozen locations along the arm. Molecular clouds are so rare in the distant outer Galaxy that these detections strongly supported the idea that this was a major spiral arm structure. In our 2011 paper, we argued that it was most likely the far end of the mighty Sct-Cen arm, which originates at the end of the central bar before spiraling through the first and fourth quadrants and then back around to the first quadrant again in the outer Galaxy. This result too argued for a degree of Galactic symmetry, since the Outer Sct-Cen arm appears to be the distant counterpart of the Perseus spiral arm in the third quadrant.
What are you currently focused on?
Both 1.2 m telescopes are still in operation, the northern one just a few years shy of its 50th anniversary. The CfA telescope recently completed a very long-term project to map the entire northern sky in CO. This was a background observing project for about a decade, and most of the data were obtained at night and on weekends by several generations of Harvard undergraduate observers. I'm working now on the final reduction and publication of this massive survey. I think Pat, who passed away in 2017, would have agreed that completion of that survey is an appropriate endpoint to what he called the “zeroth-order experiment”, to determine the overall distribution of molecular gas in the Milky Way. Accordingly, over the past year or so we have been tuning the telescope to other molecules of interest, most notably HCN at 89 GHz. Extragalactic observers have long favored this high dipole moment molecule for tracing dense gas (Gao & Solomon 2004), but interpretation of their data has been stymied by the lack of nearby Galactic clouds that have been mapped in HCN. Completing such maps is an ideal project for our telescope, so we are currently planning modifications to our receiver that will double its sensitivity at the HCN line while maintaining its current sensitivity at CO.
Besides such observing projects and other collaborations involving our surveys, I've been very excited to be involved in the BeSSeL maser survey led by Mark Reid at the CfA. This project has recently completed a five-year run on the Very Long Baseline Array during which highly accurate parallaxes and proper motions were measured for about 200 molecular masers associated with very young high-mass stars (Reid et al. 2019). One of my main roles in the project has been to pick out maser targets based on their locations in the lv diagram and their potential for clarifying the structure of particular spiral arms. Having struggled throughout my career to locate the Galactic spiral arms using kinematic distances, it's thrilling to finally be pinning down the arms with such high precision. I can hardly wait to see results start to come in from the southern counterpart of BeSSeL, which is scheduled to start soon using an array of telescopes in Australia and New Zealand.
