An interview with Harvey Liszt

Interview by Bo Reipurth, SFN #340 - April 2021

What was your thesis about and who was your advisor?

My thesis was a compendium of small maps of 2.6mm CO emission around giant HII regions, with data from the UT mm-wave antenna at McDonald Observatory. My advisor, Arno Penzias, had an adjunct position at Princeton where I had been a grad student for two years before dropping out to avoid being sent to Vietnam. The Vietnam war wreaked havoc with graduate school enrollment. Things eventually got so bad that the graduate school began to admit women on a regular basis, a small glimmer of light at a difficult time.

Anyway, interstellar CO was discovered around the time that I could reconsider returning to grad school and I convinced Arno to take me on as his first PhD student. The Bell Labs group knew they would need help with the observing that was in store for them at Kitt Peak and McDonald, and I had experience working in the spectroscopy labs at Princeton that provided a safe harbor for me during the War. The labs were created largely to measure the spectrum of H₂ in preparation for the launch of Copernicus but they also measured oscillator strengths in small molecules whose optical spectra are diagnostics when nose cones ablate. I had this job thanks to Wm. Hayden Smith for whom I had worked after he took over the labs from Jim Hesser (yes, the Jim Hesser). There's no way to repay a debt like that.

Working at Bell Labs with Arno and Bob Wilson at Crawford Hill and Keith Jefferts at Murray Hill was simply marvelous. We spent months at McDonald Observatory in west Texas getting the telescope ready to observe. Paul Vanden Bout had just taken over as director of the radio telescope at McDonald and later I would work for him at NRAO.

I also benefited enormously from discussions with Peter Goldreich who was on sabbatical in Princeton. Our discussions provided me with an understanding of the line profiles I was acquiring, and he and John Kwan wrote the famous 1974 paper that introduced the LVG radiative transfer approximation. Unfortunately, Peter also beat me in the annual astronomy department ping-pong tournament.

In the late 1970s, you and Butler Burton published three highly cited papers on the gas distribution in the central region of the Galaxy. What were the main results within the context of the time?

Butler was on the scientific staff at NRAO when I arrived there as a post-doc and we met when he and Bob Sanders (whom I knew from Princeton) recruited me to help with some Kitt Peak 12m CO observations of the Galactic center. I had learned from Keith Jefferts and Bob Wilson how to tweak that system and system temperatures of 5000K were still better than 10000K. I spent the year 1975-1976 on the faculty at the University of Pittsburgh with Bob Sanders and Artie Wolfe, then returned to NRAO to be project scientist of a 25m telescope on Mauna Kea that was never built. Butler and I continued our discussions of the gas in the Galactic center and did a coarse, large-scale HI survey at the 140-ft telescope in Green Bank that we complemented with CO from Kitt Peak. Butler had noticed that the HI wasn't aligned with the Galactic equator: there was emission far from the Galactic equator in two opposite sky quadrants that supposedly represented episodic nuclear gas ejections like those which at the time were invoked by Harry van der Laan to explain double-lobed radio galaxies. Near the Galactic equator, there was a rotating ring of HI from which the inner Galaxy rotation curve was derived, coexisting with the ``”expanding molecular ring” of Norio Kaifu and, separately, Phil Solomon and Nick Scoville.

When Butler and I considered our CO and HI maps, there were previously unrecognized symmetries and similarities suggesting the unification of disparate phenomena. The expanding molecular ring and the nuclear rotating ring gas were tilted in just the same way as the HI that supposedly had been ejected. So I derived the velocity transformation equations, Butler put them into his PL1 language modelling program on the IBM360/65 mainframe, and with surprisingly few iterations we had red CalComp pen plots clearly showing that all the gas shared a common explanation. The same behavior was present in HI and CO, and both the circular and non-circular (outward-directed) motions were geometric projections of a common large-scale velocity field. The framework provided its own ultimate justification. Once it had been fit to the features we knew about, we discovered other features elsewhere that were explained for free.

This unification allowed an explanation in terms of gas motion in a strong bar of the sort that is observed and modeled in other galaxies. This was really brought home to me when reading the mid-90's papers modelling gas flow in other barred galaxies by Peter Teuben, Kartik Sheth, and Mike Regan. In physical terms, the Expanding Molecular Ring is the spray of inflowing material off the inner end of the near-side dust lane, meeting gas in a more nearly circular motion in the Milky Way's circumnuclear star-forming ring that is usually described as the Central Molecular Zone. But there is one really weird aspect to all this. The tilts of the inner-galaxy gas distribution, which provided the insight for everything Butler and I achieved, have never been explained. In any case, I continued to work with Butler and with Xiang Delin and Tom Bania on Butler's program of mapping the Galactic molecular cloud ensemble through the early-mid 80's. And I worked sporadically with Butler (and Thijs van der Hulst and Bob Brown) on the Galactic center, until Butler's retirement from Leiden into ApJ editorship in 2000. Butler was and is my teacher in all matters concerning HI emission and Galactic structure.

You have had a long-term collaboration with Robert Lucas, and in some of your papers together you have compared the chemistry in different diffuse clouds. What did you learn?

Robert and I met at the 1991 astrochemistry meeting in Brazil and I later wrote to him proposing that he do an experiment at the IRAM Plateau de Bure Interferometer to observe ¹³CO absorption toward BL Lac. He suggested that we collaborate and, while on duty at the Plateau, Robert saw that phase calibration data taken against BL Lac unexpectedly showed strong HCO⁺ absorption. The IRAM Director gave us a bit of discretionary time and with that and some observing proposals, we quickly discovered very widespread mm-wave HCO⁺, HCN, HNC, C₂H, and C₃H₂ absorption. In retrospect, we might have known better that Pierre Cox had seen widespread C₃H₂ absorption at 18.3 GHz in 1989, but our motivation hadn't been chemistry at all.

Robert retired in 2014 and I've continued this work at IRAM and the JVLA with Jérôome Pety and Maryvonne Gerin. We found that molecules as complicated as CH₃CN are detectable in absorption in clouds with A_V lesser or equal to 1 mag (like toward BL Lac), at number densities much too low to allow them to be seen in emission. This allows very accurate abundance determinations. What we learned is that the chemistry of diffuse gas is much more complex than was foreseen, that the chemistry is dominated by turbulent processes, so that fully molecular clouds born out of diffuse gas have a large complement of complex molecules at t=0.

Ten years ago you led a study of the CO to H₂ conversion factor in diffuse clouds. What are the implications for our understanding of the interstellar medium?

In the fall of 1981 I was on sabbatical at the University of Toronto teaching the graduate course in radiative processes in the ISM for Ernie Seaquist. When it came time to discuss CO and GMCs I shook my head at the bewildering mass of ``information'' that would be required as background. To provide a framework, I wondered what happens when you compare the brightness of CO emission and N(H₂) in the two most disparate environments: Toward Orion A at A_V > 100 mag and toward zeta Oph at A_V lesser or about 1 mag. To my shock, there was only one CO/H₂ ratio in either direction and, looking at the Burton and Gordon (1978) and other galactic plane CO surveys, I saw that they had implicitly used the same number to derive N(H₂) from CO emission.

So the idea was around but there was no recognition of the applicability of one number, under different conditions. When I returned to NRAO I wrote an ApJ paper that I think was the first to propose a universal CO-H₂ conversion. But the factor I derived was too high, owing to various inaccuracies that existed at that time. Credit for defining the current value belongs to Tom Dame who compared accurately calibrated CO data from Pat Thaddeus's mini-telescope with H₂ column densities from gamma-ray data.

It turns out that the conversion factor in diffuse gas is nearly canonical because small N(CO)/N(H₂) ratios are compensated by weak excitation that causes more energy to escape in the J=1-0 line. But the more important point is that the CO chemistry of rapid HCO⁺ formation and recombination has serious consequences for observing CO emission almost anywhere. Most of the large scale CO emission we see on the sky arises in gas at modest reddening, even in dark and molecular cloud complexes, and at low galactic latitude, and must be treated as such. The ¹²CO/¹³CO ratio is often artificially low due to fractionation and most of the gas-phase carbon is in C⁺. Paul Goldsmith's papers on Taurus provide an important point of reference here.

You've spent much of your career doing things that are removed from research.

In the late 1980's NRAO moved from a focus on single dish astronomy and spectroscopy at Green Bank and Kitt Peak to bring the VLA fully online while starting the VLBA.

NRAO's head of engineering, Hein Hvatum, bought me an 8 MHz AT&T MSDOS computer with a math coprocessor. I bought a Turbo Pascal compiler and the same computer for home and labored day and night to write a program called drawspec, forgetting about research and not reemerging from that rabbit hole until early 1991. I put drawspec on an ftp server and it saw some use but very few of its adopters ever contacted me and I learned only much later that it was being used for undergraduate education in Sweden and with grad students at Jodrell Bank. That said, one enthusiast was Butler's PhD student Dap Hartmann, who was an avid Pascal programmer and used drawspec to reduce the Dwingeloo data that became the northern part of the Leiden-Dwingeloo all-sky HI survey. Drawspec was also used in Argentina for the southern portion. I still use drawspec code for my own spectral line work and I do mathematical modelling in Pascal using the fast assembly language vector routines I wrote so long ago.

I was Project Scientist for the Green Bank Telescope in the mid-90's when the contractor filed for arbitration to recover an additional $30,000,000 beyond the contracted $55,000,000. I shifted over to become the technical advisor to the lawyers who defended NRAO, spending the next several years flying to paper document discoveries and witness depositions and living in hotels for weeks and months on end. Working with the very dedicated Seattle-based legal crew that defended NRAO was a humbling experience.

What are your current research interests?

Most of what I do now is related to diffuse interstellar atomic and molecular gas in one way or another. I have continued the search for new molecules in diffuse gas with Jérôme Pety and Maryvonne Gerin. We filled out the roster of small hydrocarbons and showed that linear C₃H₂ couldn't be a DIB-carrier, as had been proposed. We detected methyl cyanide CH₃CN toward BL Lac and in other clouds with less than 1 magnitude extinction, which is mind-boggling. I'm also loosely affiliated with their large Orion-B molecular cloud mapping project on the IRAM 30m telescope - star-formation!

My most enjoyable solo paper ever was the one from 2001 on radiative equilibrium in HI showing that the 21cm spin temperature can't be assumed to be thermalized in the warm neutral ISM. HI has been on my mind ever since and recently I've written papers relating HI emission and absorption to the famous all-sky maps of optical reddening E(B-V) of Schlegel, Finkbeiner, and Davis (1998). In 2014 I showed that the N(H)/E(B-V) ratio had to be revised upward and this was very pleasing because it used Butler's HI survey data. More recently I wrote a couple of papers showing and exploiting the fact that 21cm HI absorption, although it samples only a small portion of the ISM, is so tightly correlated with E(B-V) that the absorption by itself allows a determination of N(H), perhaps even toward obscured AGN in radio galaxies. This is partly in response to a question asked of me long ago by Barry Clark, whose own much more profound insight into HI forms the basis of the notion of phase equilibrium in the ISM. ``What is HI absorption really good for, anyway?'' he said.

You spend a lot of time on spectrum management now. What does that entail?

I am NRAO's spectrum manager dealing with protection and regulation of the radio spectrum as it affects our telescopes, and I am Chair of IUCAF (the Scientific Committee on Allocation of Frequencies for Radio Astronomy and Space Science) that is chartered by the International Science Council and has members from IAU, URSI, and COSPAR. IUCAF was created in 1960 to protect the spectrum around the 21cm line of atomic hydrogen. That band and others like it that are internationally forbidden to transmitters are also used by satellite remote sensing that is a vital component of weather forecasting and climate change assessment.

Access to the electromagnetic spectrum is eroding for all of science as new radiocommunication systems proliferate. Issues of spectrum have recently achieved notoriety with the coming of 5G and the launch of satellite mega-constellations whose light trails carelessly appear across the dark night sky. HST images are now regularly ``”photobombed” and the Rubin Telescope stands to be especially impacted. We are in the perverse situation that radiofrequency spectrum regulators like the Federal Communication Commission in Washington and the International Telecommunication Union in Geneva are gatekeepers to the sky and space, while their main mission remains the relatively narrow one of preventing harmful interference among commercial radio operators. This is a huge and necessary task but is mostly blind to the needs of science.

Interested readers can follow recent efforts by the scientific community to defend access to the sky and spectrum in the output of the June 2020 AAS-NSF SATCON1 workshop and that of the October 2020 Dark & Quiet Skies workshop organized jointly by the IAU, the UN Office of Outer Space Affairs and the Instituto de Astrofisica de Canarias