I arrived in Tucson at 1:30 pm and took a shuttle bus to a motel on the east side of town where my rental car for Kitt Peak was waiting in the parking lot. My first stop would be Safeway to pick up a few snacks like Oreo and chocolate chip cookies, necessities not available on the mountain. Driving south through the Papago Indian reservation my mind wandered a bit, thinking about how I would carry out my program at the 12-Meter Millimeter Wave Observatory . Never having used this facility before, I was hoping that the notes in my lab notebook would help me through the long hours of observing to come. My colleague Lee J. Rickard had given me a 30 minute crash course on how to use the telescope, and I had also read many articles by other astronomers describing how they had used it; I was confident that I knew enough about the system to get the data I needed.
My observing program consisted of a list of twelve spiral galaxies located in a cluster 54 million light years away in the direction of Ursa Major. Other astronomers had long since discovered that these galaxies contained large amounts of atomic hydrogen gas between their constituent stars. The interstellar space of our own galaxy is also filled with atomic hydrogen; I wondered whether these galaxies would contain giant clouds of molecular gas just like our Milky Way. To find out, I planned to use the 12-meter radio telescope to search for the electromagnetic 'fingerprint' of the carbon monoxide molecule which is found in the Milky Way clouds. Because these galaxies were so far away, I needed the largest telescope I could get observing time on, in order to capture enough of the photons to see this molecule. The 12-meter had been used in this fashion many times before, and it was the perfect choice.
By the time I arrived at the observatory it was already well passed sunset. I parked in the visitors area and got out of the car. My first instinct while on top of a mountain at night is to look up at the sky, and my expectations of what I would see were more than satisfied. It would be a perfect night for the lucky optical astronomers. The sky was inky black and the atmosphere was so stable that the stars barely twinkled. I entered the 12- meter dome and was greeted by the giant dish of the radio telescope, illuminated by crimson lights and taking up nearly the entire volume of the observatory dome. Over my head I could hear the rhythmic chirping of a pump straining to preserve the vacuum within a cryogenically cooled dewar. Inside this specially designed, high tech thermos bottle the amplifier for the microwave receiver is kept at a temperature only a few degrees above absolute zero in a liquid helium bath. When in use, the telescope and its microwave electronics are tuned so that only those photons over a narrow range of wavelengths around the carbon monoxide line pass through its circuitry and are recorded on magnetic tape by a computer.
Inside the control room, I walked past a wall filled with electronic equipment which busily kept the telescope pointing in the right direction and tuned to the proper frequency and bandwidth. The Telescope Operator sat before a large console and monitored the instrument's performance while filling out an observing log. After a brief introduction, I found out that there were no problems with the equipment or the weather that would interfere with my observing program which starts tomorrow afternoon. During my visit here, I would be staying in the dormatory next door. This seemed like a good time to get settled into one of the bedrooms, and have a bite to eat at the cafeteria before I started working on the details of my observing plan.
Kitt Peak is an exciting place to be on a clear night. On the summit, all of the prime locations for seeing into the valley down below are occupied by giant observatory domes, the most spectacular of which is the one containing the 4-meter Mayall telescope. Nearly all the observatories were in use this evening, the pale red glow of safety lights pouring out of the narrow slits in each of the domes. At the cafeteria, groups of optical astronomers took time out to eat during 'Night Lunch' and talk about new discoveries they had just made about some distant star or galaxy. Among them was James Liebert, a former Teaching Assistant from my undergraduate days. Small world! The others were anonymous faces I'd never seen before, but in my lifetime as an astronomer I will probably come to recognize them too.
Back at the 12-meter, I continued my discussion with the Telescope Operator and worked out my observing plan for tomorrow. He needed to know the sky coordinates of each galaxy on my list, how long to spend observing each one, and to what frequency to tune the receiver. That last piece of information was a bit tricky. Just as the pitch of a fire engine's siren changes as it moves towards or away from you, each galaxy moves relative to the Milky Way and the frequency of the carbon monoxide line would be shifted. Because I knew the velocity of each of these galaxies from the work that others had done, it was a simple matter to calculate at what frequency each one should be observed. Looking through my notes and several manuals, I began to familiarize myself with the computer programs I would be using to analyze the data, but soon my eyes were beginning to close of their own volition and I decided that 1 AM was a good time to hit the sack.
The first few hours of my session the next day were spent observing calibrator sources in order to get information about the sensitivity and pointing characteristics of the telescope during my observations. It was much easier than I had anticipated, and in 15 minutes I was taking data from the calibration sources Jupiter and DR-21 and performing simple statistical and line-shape-fitting analyses to the spectral line profiles. The immediacy of the experience impressed me the most. Usually you sit by passively and watch the computer tape go round and round on the drive. Only after you are home do you get the satisfaction of seeing what you've got. Here, the data comes to you directly and you can analyze it within minutes after the observation has been made!
The first galaxy I looked at was M 82 whose CO emission at 115 gigacycles had been detected years ago by Lee J Rickard, Mark Morris, and their co-workers. I locked-on to the position of the core of this galaxy for 15 minutes, but I saw nothing at all! INSTANT PANIC ! It took a few minutes to discover that I had requested the wrong filter orientation. The two banks of 256 channels in the receiver had been set up in the 'parallel- parallel' mode so that they both looked at the same frequency range. The CO line from M 82 was just outside this range. I asked the operator to set-up for 'series-series' observing. In a moment, the two rows of 256 channels were arranged to look at two neighboring frequency intervals centered on 115 gigacycles. This did the trick; after another 15-minute integration, I was gratified to watch the CO line begin to appear in the data.
Fortunately this was the only problem I came up against during my observing run. There are stories, however, of far more irritating problems that have befallen 12-meter observers. My collegue Jay Lockman at the National Radio Astronomical Observatory in Charlottesville, Virginia once told me about the time when a 2 millimeter long lady bug decided to go for a stroll on the 3 millimeter receiving horn at the telescope's cassegrain focus. It took the operators a long time to discover that the reason the radio signals were suddenly appearing and disappearing had nothing to do with the million dollar electronics.
My final spectrum of this calibrator, the sum of all the individual observations, was virtually identical to the one published in 1975. With my confidence restored, I continued with my program -- an excursion into the unknown territory of galaxies that had never been studied for CO emission. I knew from the articles I had read that searching for carbon monoxide in other galaxies can be a very discouraging undertaking. Galaxies would often have to be observed for many hours before even a feeble trace of this line could be discerned against the ever-present background noise. My hopes were not very high that the galaxies I had chosen would be CO boomers like M-82. Still, perhaps one of them would show some signs of life on one of the 512 channels of this monster radio receiver. Little did I suspect when I started that before the night was over my vigil would be rewarded.
NGC 4088 is a faint galaxy, barely 11th magnitude, located in the heart of the Ursa Major cluster. It may once have been a galaxy not unlike our own Milky Way. About 100 million years ago a nearby galaxy called NGC 4085 passed by NGC 4088 in its journey through the cluster. This encounter lasted millions of years and when it was over, one of the graceful spiral arms of NGC 4088 was distorted. My first half hour on NGC 4088 convinced me that it was different from the other galaxies I had already studied during the last few hours. In scan after scan, I could see a broad, weak enhancement begin to take shape in the central channels of the receiver, right where I expected the line to fall. I waited impatiently, adding one scan to the next, to accumulate the traces of those precious photons. By the end of the alloted 3 hours observing time for this galaxy it was abundantly clear to me that I was not playing with noise. From a distance of 54 million light years I was certain beyond reasonable doubt that NGC 4088 had sent me a message: "The nucleus of this galaxy contains molecular gas, probably in the form of giant gas clouds like those in our Milky Way". In some distant time millions of years from now, new stars may be born in the clouds I had just detected. I had been the first human ever to see these nurseries intact before the violence of star birth tears them asunder and scatters their molecular contents into interstellar space.
On Wednesday night, I replayed this evening's activity, discovering two other 'CO galaxies' in the Ursa Major cluster. Once the last photons had been recorded, I gathered up my notes and spectral plots and returned to the trailer for some much- needed sleep. My plane would be departing from Tucson tomorrow morning. On the flight back home, I began writing the section of my paper that dealt with how I had performed the observations and the preliminary analysis of the data while the details were still fresh in my mind. From time to time I looked up from the notepad and watched the countryside far below, wondering what things would be like the next time I traveled this way again, and how last night's results would fit in with my future research.