The work contained in this thesis is concerned with the design and construction of a near-infrared spectrometer for astronomical applications, Cooled Grating Spectrometer 4 (CGS4). The results of two observational projects carried out with the instrument on the United Kingdom Infrared Telescope are presented.
CGS4 is one of the first infrared spectrometers to incorporate a 2D detector array, and the design of the spectrometer is driven by the desire to maximise the improvements in sensitivity which can be obtained with such an array. The need for high throughput and good image quality is discussed, and the way in which they have been achieved outlined. Other factors which affect the sensitivity of the instrument are the sky-background emission, thermal background from the instrument and telescope and detector noise. The method by which each of these is minimised is outlined. CGS4 was designed as an instrument which could be applied to the disparate projects which benefit from observations in the NIR waveband range. These include observations of molecules in star-forming regions, studies of emission lines from active galactic nuclei, and studies of gas dynamics.
Two aspects of optimising observations in the NIR are discussed in detail. For observations in the 1- 2.3/im region, emission from hydroxyl in the Earth’s atmosphere dominates the sky background. The intensity of the line emission from OH varies by ~10% on timescales of ~10mins. As a result, on-source exposure times should be restricted to 60s, setting a fundamental limit to the sensitivity achievable for observations of extended sources. Flat-fielding a 2D array spectrometer, to calibrate the relative gains of the pixels, is another area which is explored in this work. The reasons for excluding the methods used for optical astronomy and NIR imaging are discussed. The solution for CGS4 was to provide with a tailor-made “calibration unit” . The design of this unit and the resulting success in flat-fielding observations is detailed.
The astronomical results reported concern observations of emission from the hydrogen molecule. There are two predominant methods of exciting H2 : radiative excitation by UV photons with energies less than 13.6eV or collisional excitation in shocks. Excited H2 decays by transitions in the rotational-vibrational bands of the ground electronic state, emitting NIR photons. The emission spectrum, specifically the 1-0 S (l)/2-l S(l) line ratio, is frequently used to diagnose the excitation mechanism. Recent models of the emission from H2 have shown that for gas densities above a critical density, the emission from radiatively excited gas can emulate that from a collisionally excited gas, and that radiative excitation of H2 may be more widespread than previously thought.
The planetary nebula, Hubble 12, was identified by Dinerstein et al. (1987) as asource of fluorescent emission from H2 . A spectrum taken with CGS4 confirms this result, in the light of the recent models, through observations of the emission from higher rotational-vibrational levels not previously detected in this source. The emission is shown to arise from a gas of density 104-105cm-3 illuminated by a source of UV fieldstrength 104 times that of the interstellar medium.
H2 emission from a ring of molecular material which surrounds the Galactic centre
was identified as being shock excited when first measured by Gatley et al. (1984). Conditions at the Galactic centre, and the evidence for recent star—formation, have prompted a re-measurement of the H2 spectrum from this source. It is shown that, indeed, radiative excitation of a dense (~ 10(6)cm(- 3)) gas by a UV field of 10(5)G(o) is sufficient to explain the H2 observations. The dynamics of the gas has also been investigated. The observations confirm that the ring has a radius of 1.54pc, at which point the velocity of the gas is 100km s-1 , and that the velocity has a keplerian dependence on radius. This is consistent with the gas orbiting in the gravitational field of a compact, central source of 4xl0(6) M0.