Abstract
We review the development of submillimetre continuum astronomy on UKIRT between the telescope’s inauguration and the handing over of the baton to the JCMT in 1997.
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Introduction
Submillimetre continuum astronomy means getting above most of the atmospheric water vapour and that translates to high altitudes for ground-based sites. The Queen Mary College group had spent a number of years sampling telescopes and sites, such as the Pic du Midi at around 10,000 ft and Mount Evans at 14,260 ft in the Colorado Rockies. However, the best combination of telescope size and altitude by far was the University of Hawaii’s 88-inch telescope on Mauna Kea, at almost 14,000 ft above sea-level. Although our experience of using the telescope and weather had not been wonderful (Eve et~al. 1977), nevertheless, it was clear that for truly submillimetre (as opposed to near-millimetre) astronomy, Mauna Kea was the best developed site at the time. Therefore, when I joined the UKIRT Oversight Committee in the mid-1970s, submillimetre (or far infrared astronomy as it was often called then) was in it infancy (Robson 1979) and we were very determined to maximise the effectiveness of the telescope for our work. Additionally, I could bring first-hand experience of observing on Mauna Kea to the debates.
My first contribution was to remove the ‘folded-prime’ configuration that had been selected as the location for the submillimetre photometer in order to retain a low f-number. While this would allow the detector to be more easily optically fed, it meant that the instrument would sit above the primary mirror and access (and helium fills) would be through the Cassegrain hole. This was less than a practical way forward and I could see that it would not help in getting telescope time, so we agreed that scrapping the folded prime and working at Cassegrain would be an acceptable option. This was a very popular decision and as a result I suspect someone in the project was the recipient of a 1 m diameter perfectly polished flat piece of glass – a brilliant coffee table no doubt. This then enabled the submillimetre to be accepted as part of the instrument suite: Cass-mounted, readily accessible and ‘part of the general system’. My second contribution was to help persuade SERC to have the UKIRT headquarters in Hilo, rather than Waimea, in spite of the fact that land had already been purchased there. My third contribution was to agree on the colour of the telescope and mount with Gordon Carpenter, the Project Manager.
Submillimetre Photometry
One aspect that had been clear from our extensive experience on many telescopes was that in order to control background radiation (in the absence of chopping secondaries, which were not yet developed) we needed a purpose-built photometer, with careful consideration of tertiary chopping and background suppression. Our design (Robson et~al. 1978) was tested on the Flux Collector at Izaña and this produced a large improvement, albeit the site was not conducive to submillimetre observing.
With the opening of UKIRT we were one of the first groups to gain access to the telescope and mounted our submillimetre photometer at the f/9 Cassegrain focus and began opening up the discovery space. The photometer was directly fed at the Cassegrain focus and came equipped with a large field-of-view offset guider (eyeball driven), which got around the problem of poor telescope pointing and the ability to guide in regions of the sky such as dark clouds, where there were few guide stars. However, the QMC photometer (as it was called) required someone (usually a research student) to guide the telescope from the Cass focus, often balanced precariously on the top of steps in the pitch dark (and freezing conditions) of the dome (health and safety was different in those days). The ‘real’ astronomers sat in the control room in the relative warmth, trying to manage the observing and data taking, get the data out of the PDP11/40 computer and the signals on the strip chart (no on-line data analysis) and try and make sense of whether it was worthwhile integrating further or go to another target (Fig. 4.1). Communicating with the ‘guider’ in the dome was via radio mikes and headsets. On the other hand, guiding through a large eyepiece on a 4 m telescope produced some spectacular views that we will all remember (Fig. 4.2).
In those early days the continuum detectors were still quite primitive, being liquid helium cooled doped germanium bolometers with rudimentary broad-band filtering. The end-product was that progress at the time seemed disappointingly slow and was limited by telescope, weather and instrument problems, with the result that only the brightest submillimetre objects were detected, and usually only at 800 μm. Nevertheless, looking back, this was a time when valuable knowledge and experience of understanding the ‘observing system’ (filters, atmospheric attenuation, noise sources) were gained, all of which would lead to improvements and ultimate breakthroughs. Throughout this time, calibration was always difficult. Mars was the primary calibration source and there was very little else. From this early era one paper stands out: ‘The submillimetre spectra of planets: narrow-band photometry’ (Cunningham et~al. 1981).
Nevertheless, in spite of this UKIRT was well up with the state-of-the-art in submillimetre astronomy at the time, with other groups (notably Caltech, Chicago, Arizona) making similar observations from various telescopes and the KAO. The challenges facing us on UKIRT and all the ground-based telescopes were overcoming the high and variable sky-background power with the resulting baseline instability and of course the sky-noise itself. Our very fast 110 Hz chopping removed the high-frequency component well enough but still left major baseline drifts. Also, as wavelength filtering was in its infancy, understanding the effects of an unknown spectral index of the source and potential high- and low-frequency filter leaks were other factors that had to be taken into account. Improvements were targeted at: better control of filtering; improved detector sensitivity; and crucially, having a chopping secondary on the telescope.
The Game Changers
The first two avenues were completed during 1980–1981 with new and narrower-band filters possessing excellent leak rejections. This was the speciality of Peter Ade, who went on to become the filter supplier to the Universe of astronomers. The detector sensitivity was vastly improved by using helium-three cooled germanium bolometers; being cooled to 350 mK rather than 1.2 K with pumped helium-four. This came about through collaboration with Ira Nolt and Jim Radostitz of the University of Oregon, who developed the helium-three systems. The really big step came with the f/35 chopping secondary being introduced on UKIRT in late 1980 and becoming the only top-end available a year later. From this point until the introduction of UKT14, improvements would be incremental. The chopping secondary came with a new Cassegrain mounting unit (the famous ISU 1, or ‘gold dustbin’) incorporating a ‘straight-through’ optical TV guider, which was sensitive enough that all future observing would be done from the control room with the source or offset guiding being done with the intensified TV camera. At a stroke the QMC photometer was made redundant – the submillimetre radiation was now fed directly into the detector cryostat from the tertiary dichroic and this arrangement became known as the ‘QMC-Oregon’ photometer (Ade et~al. 1984). It was essentially a compact cryostat with all the filtering done internally via a filter wheel at 4.2 K (Fig. 4.3).
The chopping secondary produced huge improvements in baseline stability. The chopping frequency was usually around 12.5 Hz and the two beams, each of about 64 arcsec diameter at 1.1 mm, were separated by 120 arcsec on the sky. The new wavelength filtering also better isolated the atmospheric windows and for the first time separated out the 450 and 350 μm windows. Although this resulted in a small reduction of flux from the source, the benefit of removing a large and varying atmospheric emission power was dramatic and far outweighed the loss of flux in terms of detectivity. This was a crucial feature for UKIRT because most other groups continued to use very wide-band filters for many years.
The results of all these improvements led, by 1982, to an improvement in the noise-equivalent flux density from around 200 Jy Hz−0.5 to around 5–10 Jy Hz−0.5 at 1.1 mm. This was the breakthrough we had all been waiting for and the discoveries followed thick and fast. A further benefit was that on-line data analysis was now available for the first time to the submillimetre continuum community – but this was another story, which started off with a bang (see Radio Quiet Quasars below).
The trend to design a photometer system that basically fitted onto the telescope like any other facility instrument was also used by other submillimetre groups headed up by the University of Chicago with Roger Hildebrand and involving astronomers from various institutes in California. UKIRT observations of a range of evolved stars (such as IRC + 10216, CRL2688, NGC7027 and VYCMa – which would all go on to become well-observed objects from UKIRT and subsequently the JCMT) in 1981 (Sopka et~al. 1985) using the Chicago submillimetre photometer (Whitcomb et~al. 1980) demonstrated the success of this technique. Again, these groups began with He-4 and like ourselves, moved into He-3 for greater detectivity.
Atmospheric Attenuation
The move to narrower-band filters not only helped the photometry and noise reduction but also enabled atmospheric attenuation to be better understood. Extensive work was undertaken on this by Bill Duncan and myself, including experiments with two- and three-position chopping/nodding techniques but regrettably this was never fully written up. As well as the experiments in sky-background/noise cancellation it was also vitally important to determine the attenuation as accurately as possible because certainly in the submillimetre the attenuation coefficient was high and so any change of airmass between calibrator and source could mean a very significant difference in flux estimation. The problem of the changing atmospheric attenuation would plague submillimetre astronomy, limiting the accuracy of observation, for many years until SCUBA on the JCMT along with sky emission monitors essentially solved the problem.
In the UKIRT days the best that could be done was to make frequent measurements of calibration sources and produce secant plots of flux versus airmass. Of course this was a time-consuming task as it had to be done in each filter, which added overhead to the astronomical programme. For stable nights this gave good results, allowing an attenuation coefficient to be derived at each filter and so some degree of confidence in astronomical fluxes. For many nights, however, secant plots were anything but stable, leading to uncertainty in the attenuation and final data. It also seemed that there were never enough stable nights to be able to determine accurately the attenuation coefficient as a function of precipitable water vapour, or even know what this latter value was (apart from through models). Nevertheless, most of us accepted these observing overhead in the quest for best photometric accuracy of the data.
Steady-State Submillimetre Progress
Some of the first submillimetre measurements from UKIRT in late 1980 dealt with emission from the planets, the thrust being to use some of these as calibration sources, but also to understand the atmospheric emission models and for the first time the contribution of Saturn’s rings to its total emission (Cunningham et~al. 1981) and hence dust particle size distributions. Mars was the primary calibrator for all submillimetre observations due to it having essentially no atmosphere, a solid surface, and had been extensively studied by the Mariner 6 and 7 probes. This allowed models of the far-infrared emission to be developed (Wright 1976), which became the basis for all subsequent calibration. The work on the planets continued throughout the 1980s, with observations making more inroads on the physics of planetary atmospheres as the detectivity improved along with the narrow-band filters. Highly detailed measurements of Jupiter were made by Griffin et~al. (1986) who found that from 350 to 450 μm observations the Jovian atmosphere is best described by the addition of ammonia ice particles of size between 30 and 100 μm. However, the authors also pointed out that if the absolute calibration scale (due to Mars) was increased by only 5 %, then there would be no need to incorporate additional absorbers in the Jovian atmosphere.
A novel collaboration took place between groups working on the IRTF and UKIRT. It was clear that the IRTF was better at the thermal infrared, especially 10 and 20 μm, and so it was agreed to undertake joint programmes of observations utilising the best features of both facilities. UKIRT provided the submillimetre and both telescopes the near-IR as suited. The programmes covered a range of objects but one of the first observations was of the asteroid, (10) Hygiea (Lebofsky et~al. 1985). This was the first detection of an asteroid in the submillimetre and along with the thermal- and near-IR data provided the output spectrum over a crucial range of wavelengths and enabled a discussion of the temperature and surface properties to be undertaken. Subsequently (1) Ceres was later also detected at all submillimetre wavelengths.
The Arrival of UKT14
The popularity and productivity of continuum submillimetre astronomy had resulted in the procurement of a common-user photometer from the ROE. This was called UKT14 (number 14 in the series of ROE photometers) and although originally designed primarily for UKIRT, was capable of operating on the JCMT with a minor modification. Bill Duncan was the Project Scientist and I was the external Project Scientist. The photometer was designed to incorporate all the lessons and advances that had been learned from the years of work on UKIRT with the QMC systems (Duncan et~al. 1990). The detector in UKT14 was a composite germanium bolometer, cooled by helium-3 to 350 mK. The radiation was channelled to the detector through a variable aperture stop, stray-light minimising optics, a 10-position filter wheel and a low-pass filter. This represented a step-change, not so much in raw detectivity, but in noise suppression and bandpass filter control, the latter being upgraded to the latest state-of-the-art filters in mid-1988.
UKT14 was the pinnacle of single-pixel devices and it would be a hugely successful instrument, doing excellent work on UKIRT and then the JCMT. The impact of UKT14 is often forgotten following the advent of SCUBA, but it laid the foundations for a community of astronomers and much of the work that SCUBA would revolutionise (Fig. 4.4).
The Link with IRAS
One of the key events in far infrared astronomy was the IRAS mission. This promised to provide a wealth of new discoveries and we were particularly keen to capitalise on this by undertaking follow-up observations as early as possible. Funding was awarded for a 6-month programme of submillimetre IRAS Follow-Up observations on UKIRT using the early morning twilight hours – also necessary due to the IRAS orbit and observing pattern. Unfortunately, this was not successful due to a number of reasons-technical, logistic and the information coming out of the IRAS data centre at RAL-and was eventually curtailed after about six weeks.
However, IRAS follow-up observations continued through regular observing sessions and a number of discoveries were made. I will pick just one example, the measurement of the submillimetre spectrum of Arp220 in May 1984 (Emerson et~al. 1984). The 350 and 760 μm observations were augmented by 20 μm measurements, the latter showing that all the emission came from within the 4-arcsec beam. The data, along with the IRAS fluxes gave a dust temperature of 61 K and a luminosity of 1012 L⊙. This is around 100 times that of the nearby starburst galaxy M82 and pioneered the class of super-starburst galaxy referred to at the time as ‘IRAS high luminosity galaxies’ (to become LIRGS and ULIRGS in due course).
Galactic Observations
IRAS also played a part in galactic observations from UKIRT. These continuum studies had lagged some way behind their extragalactic counterparts for one very good reason, they were mostly extended and mapping with a single pixel detector was a non-trivial process, being both time-consuming (requiring stable weather conditions over many hours) and extensive data reduction by hand as there were no software packages available at the time. Although a number of sources were observed, very few were ever written up apart from in Ph.D. theses. Galactic star forming regions were undoubtedly the realm of the heterodyne instruments, which made very good progress on UKIRT.
Nevertheless, one of the first measurements was the submillimetre emission from the planetary nebula NGC7027 in 1982 and 1983 by Gee et~al. (1984). This was at 350 μm and showed that the emission was above an extrapolation of the free-free emission, being almost certainly due to thermal emission from dust with a temperature of order 20 K. This was the first detection of cold dust in the halo of a planetary nebula and the mass of dust was consistent with the gas mass deduced from CO observations. Another example of Galactic (almost point-source observations) was GL490, a deeply embedded bipolar far infrared outflow source. This was part of the IRAS follow-up work and the UKIRT 350 μm mapping observations (Gear et~al. 1986b) showed that the dust was most likely in the form of a disc, but that it was incapable of channelling the outflow, which must be collimated on a much smaller size-scale.
A more extensive programme of submillimetre observations of a sample of compact HII regions was undertaken by Gear et~al. (1988), who used the newly arrived common-user photometer, UKT14, early in 1986 to determine the emissivity index, β, of the dust emission using matched beamsizes at 350, 800 and 1,100 μm. The result was a mean value of β = 1.75 ± 0.20. Indeed, the arrival of UKT14 in 1986 allowed some major submillimetre mapping programmes to be undertaken on selected regions. It also turned out that some of the last submillimetre continuum observations to be done on UKIRT were galactic, dark-cloud mapping programmes. Ward-Thompson and Robson (1990) undertook IRAS follow-up observations by mapping the W49 complex at 350, 800 and 1,100 μm in 1988 revealing extensive cool dust of Td ∼ 50 K and an emissivity index, β of 1.8. The far infrared luminosity of 2.7 × 107 L⊙ made this complex one of the most luminous star-forming regions in the Galaxy and the data showed that the dust cloud must be fragmented on small-scales, possibly indicating continuous star formation. This was followed up by mapping of the ρ Ophiuchi dark cloud complex, again in 1988, by Ward-Thompson et~al. (1989). The submillimetre mapping was combined with IRAS maps to reveal the extent of the cool emission and the highlight was the discovery of a new source, labelled SM1, which was very cold (Td ∼ 15 K) and of sufficient mass that it was a candidate for a new protostellar object. However, the very nearby (less than 1 arcmin) Class-0 discovery source, VLA1623 would have to await observations using UKT14 on the JCMT, a few years later.
The Radio Quiet Quasar Story
Turning to the extragalactic submillimetre programmes on UKIRT, a group from Europe had produced a paper claiming a high ratio of detections at 1 mm wavelength of a sample of radio-quiet (actually radio-weak) quasars using the prime focus of the 3.6 m ESO optical telescope at La Silla, Chile (Sherwood et~al. 1982). This was exciting because if the observations were correct, then this meant a possible new source of emission as the 1 mm point lay way above the radio extrapolation and the radio-1 mm spectral slope was too steep for thermal emission from dust. We set about checking this claim in 1982 and with the first QSO we obtained an instant detection – a 5-sigma result in just a few tens of minutes. This repeated with the next two sources, but then caution set in, this seemed far too good to be true. Sure enough, looking at the raw data quickly revealed that there were as many negative signals as positives, which prompted immediate suspicion and a quick check on a HP45 calculator showed zero detection.
So, what had gone wrong? The puzzle was quickly solved; a software bug in the brand-new, on-line facility data analysis package that was being used for the first time had allowed the modulus of the signals to be used rather than their absolute values. Hence we were doomed to detect anything we looked at, even blank sky. Of course for bright sources such as the planets, which we had been observing for calibration and set-up (pointing/focus) purposes, this had no effect.
Euphoria rapidly turned to disappointment when over the next two nights every one of the quasars in our sample showed zero detection, well below the claimed detected values. We extended the sample and wavelength range and eventually published the result (Robson et~al. 1985) removing the possible new physics and the earlier claim was quietly forgotten, showing the perils of the early days of the field. One of the lessons we had learned was that we had to be extremely careful to be sure that we had taken into account all possible sources of noise and systematics in determining the putative detection of these hard-to-observe sources that were on the threshold of detectivity, or below. In fact, detecting radio-quiet quasars in the submillimetre was to remain a challenge for almost the next decade.
Extragalactic Observations
As noted above, point-like objects were always easier targets than very extended objects and so the extragalactic field opened up rapidly, albeit always fighting sensitivity challenges. One of the first successes was, looking back on it, an amazing detection of the nearby active galaxy NGC5128 (Centaurus-A). This was clearly done on a very dry night as the elevation of Cen-A from Hawaii is only 27° at transit! Also, this was only 1981 and so very early in the lifetime of submillimetre astronomy. The 370 μm flux was 59.5 Jy (a 4σ detection, which gives an idea of the sensitivity at the time). Detections were also made at 770 and 1,070 μm which allowed the thermal emission from dust to be well characterised (Cunningham et~al. 1984).
This observation was followed by a multi-wavelength study from UKIRT ranging from the near IR through to the millimetre. In this case the target was another AGN, NGC1275 (Perseus A). Unlike Cen-A, the observations did not unambiguously show thermal emission from dust, but rather a smooth spectrum extending throughout the wavelength regime. The observational programme was undertaken on three occasions from 1980 to 1982 and so variability was always a concern. Nevertheless, this was one of the first sets of observations of its type from UKIRT (Longmore et~al. 1984). Later observations of NGC1275 (Gear et~al. 1985b) would go on to show that while the millimetre through near-IR emission was dominated by synchrotron radiation from a very compact component, thermal emission from heated dust, very similar to NGC1068, was also present. Furthermore, the material responsible for the star-formation causing the far-infrared emission was undoubtedly derived from the in-falling, cooling-flow from the Perseus cluster itself.
Radio-loud, flat-spectrum AGNs had been a target of the group for some time, using Kitt Peak prior to UKIRT (Rowan-Robinson et~al. 1978). These programmes continued and an early success was with the radio-loud quasar 3C273, which had been part of the multi-wavelength (millimetre through infrared – and IRTF thermal IR observations as noted above). We had managed to obtain sufficient observations that the ‘baseline’, or ‘quiescent’ emission spectrum of 3C273 was known (Gear et~al. 1984) and so when the object had a major flare in early 1983 we were able to monitor the emission and subtract this baseline to obtain the spectrum of the emission component (Robson et~al. 1983). The data showed that the flare propagated to longer wavelengths while decaying at shorter wavelengths and the timescale was suggestive of an event in the central 0.1 pc of the source. The flare was modelled by Marcher and Gear (1985) as a shock-in-jet, and this was to form the basis for the physics of the high frequency variability in Blazar-jet models for the future.
Continued monitoring of 3C273 eventually revealed a new infrared component in the emission, which was believed to be from free-free emission from the broad-line clouds (Robson et~al. 1986). The work on 3C273 opened up what would become a long-standing collaboration with Thierry Courvoisier and resulted in the first of a truly multi-wavelength (radio-through X-ray) study as reported in Courvoisier et~al. (1988). This work focused on the origin of the various components in the central engine of these AGNs, especially the X-ray emission (synchrotron-self-Compton etc.), and hinged on the temporal variations of the longer or short-wavelength components. The later data in these papers were all from the JCMT.
Blazar monitoring from the near-mm through IR became one of the well supported programmes on UKIRT for many years, producing a new generation of submillimetre astronomers in the process along with a wealth of papers. The ‘Multifrequency observations of Blazars’ series, from paper I through IV (Gear et~al. 1985a, 1986a; Brown et~al. 1989a, b) made breakthroughs in terms of the jet physics from blazars and further demonstrated the viability of the Marscher and Gear model. This series of observations essentially spanned the most productive epoch of submillimetre observing on UKIRT, mostly with UKT14.
Although the radio-loud sources were the ‘bread and butter’ targets, strenuous attempts were made to detect submillimetre emission from the ‘starburst’ galaxies and in 1985 the southern object, NGC253, was ‘mapped’ with the QMC photometer (Gear et~al. 1986c) showing that the thermal emission extended along the major axis of the galaxy. This provided an estimate of the interstellar mass in the central kpc of around 3 × 108 M⊙, which, along with CO data suggested that the material is in the form of massive clouds, similar to what is seen in our galaxy but unlike what was thought to be the case for M82 (large ensemble of small clumps).
The Remote Eavesdropping/Observing Experience
Before closing it is worth just mentioning for the record the work on remote eavesdropping and observing that we participated in extensively as part of the monitoring programme. The blazer monitoring was an ideal test-programme in that it was routine, had experienced observers and used standard techniques. It was very clear that although the technology was rudimentary, not only was the dedicated telemetry line a requirement but voice communication between Edinburgh and the summit of Mauna Kea was also a requirement. The experiments were way ahead of their time and showed that careful planning was crucial but that in principle, with well understood instruments at the telescope, the algorithms for observing could be written in advance. However, the key was that solid data reduction pipelines would need to be operating both at the summit (in case of line dropout) and for the remote observers in order that the programmes could be carried out efficiently in real-time. This also meant having a good handle on the atmospheric extinction, which for the submillimetre, posed the greatest drawback to remote observing and confidence building. Nevertheless, these experiments were critically valuable for the data analysis work that would eventually lead to the pipelines and queue-scheduled, priority-based, weather-banded scheduling and observing, that became the feature for SCUBA on the JCMT, UKIRT itself with UKIDSS and now in common use on the major telescopes of today.
So in conclusion, not only did UKIRT provide fantastic discovery science and set ground-based submm astronomy on a firm footing. It also produced a whole generation of submm astronomers who would go on to have highly distinguished careers and leaders in their fields. An amazing achievement for a cheap, flux-collector and a great legacy.
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Robson, I. (2013). Continuum Submillimetre Astronomy from UKIRT. In: Adamson, A., Davies, J., Robson, I. (eds) Thirty Years of Astronomical Discovery with UKIRT. Astrophysics and Space Science Proceedings, vol 37. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-7432-2_4
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