Early Results from the Infrared Astronomical Satellite

Plasmonic heterodyne spectrometry for resolving the spectral signatures of ammonia over a 1-4.5 THz frequency range

We present a heterodyne terahertz spectrometry platform based on plasmonic photomixing, which enables the resolution of narrow spectral signatures of gases over a broad terahertz frequency range. This plasmonic heterodyne spectrometer replaces the terahertz mixer and local oscillator of conventional heterodyne spectrometers with a plasmonic photomixer and a heterodyning optical pump beam, respectively. The heterodyning optical pump beam is formed by two continuous-wave, wavelength-tunable lasers with a broadly tunable terahertz beat frequency. This broadly tunable terahertz beat frequency enables spectrometry over a broad bandwidth, which is not restricted by the bandwidth limitations of conventional terahertz mixers and local oscillators. We use this plasmonic heterodyne spectrometry platform to resolve the spectral signatures of ammonia over a 1-4.5 THz frequency range.

Implicit Changes of Model Uses in Astrophysics, Illustrated on the Paris-Durham Shock Model

This paper explores the epistemic status of models and simulations between theory, on the one hand, and observations, on the other. In particular, I will argue that the interpretation of an essentially invariant astrophysical model structure can change substantially over time. I will illustrate this claim using as an example the first 20 years (1985-2004) of development of the Paris-Durham shock code-a numerical model of slow interstellar shock waves (i.e. a disturbance of the medium that travel faster than the local speed of sound). I will show that the model's interpretation and, in particular, its underlying representational ideal-the modeler's (often implicit) goal governing the development and the use of the model-changed notably during this period. Whereas the code was originally used in a purely exploratory fashion, it was later taken to represent and encompass the target phenomenon as completely as possible. It is noteworthy that during this transition the model's change of epistemic status was never explicitly acknowledged or in any way articulated. However, the impetus for the change can, I claim, be found in the role that observational data came to play in the later publications.

Astronomical imaging with infrared array detectors

History shows that progress in astronomy often stems directly from technological innovation and that each portion of the electromagnetic spectrum offers unique insights into the nature of the universe. Most recently, the widespread availability of infrared-sensitive two-dimensional array detectors has led to dramatic improvements in the capabilities of conventional ground-based observatories. The impact of this new technology on our understanding of a wide variety of phenomena is illustrated here by infrared pictures of star-forming regions, of nebulae produced by the late stages of stellar evolution, of the nucleus of our own galaxy(the Milky Way), and of activity in other galaxies.

Infrared Astronomy After IRAS

The 250,000 sources in the recently issued Infrared Astronomy Satellite (IRAS) all-sky infrared catalog are a challenge to astronomy. Many of these sources will be studied with existing and planned ground-based and airborne telescopes, but many others can no longer even be detected now that IRAS has ceased to operate. As anticipated by advisory panels of the National Academy of Sciences for a decade, study of the IRAS sources will require the Space Infrared Telescope Facility (SIRTF), a cooled, pointed telescope in space. This instrument may be the key to our understanding of cosmic birth-the formation of planets, stars, galaxies, active galactic nuclei, and quasars. Compared with IRAS and existing telescopes, SIRTF's power derives from a thousandfold gain in sensitivity over five octaves of the spectrum.

Colliding and Merging Galaxies

Aided by advances in computer technology and observations from space, astronomers have begun to unravel the mysteries of galaxy formation and evolution. Galaxies evolve by interacting with their environment and especially with each other. During brief but often fierce galactic encounters, gravitational forces generate strong tides that survive as telltale signatures for billions of years. Because these so-called collisions dissipate orbital energy, galaxies on bound orbits may eventually merge. Collisions and mergers are responsible for a great variety of phenomena, including the triggering of widespread star formation in galaxies and the fueling of nuclear activity in quasars. Evidence is accumulating that not all galaxies formed shortly after the Big Bang. A sizable fraction of them may have formed later, and many are still experiencing significant dynamical evolution.