Infrared waves are invisible; they can be sensed as ‘heat’. Even an ice cube emits infrared. The infrared region of the electromagnetic spectrum covers wavelengths between 0.7 to 100 micrometers. This region is subdivided into near infrared (0.7-1.3 nm); middle infrared (1.3-3 um) and far infrared (7-15 um). Far infrared is also known as thermal or emissive infrared.
The infrared region can reveal a wealth of information about the Universe, as the radiation is in infrared from dust, proto-stellar region, planets, cosmic microwave background as well as rotational spectra of molecules and solids. Several complex molecules and dust in the interstellar space can be studied only through infrared radiation.
It is the cooler objects that emit most strongly in the infrared. The diffuse glow of the Milky Way galaxy, dust particles in the solar system and outside, tiny red dwarf stars as well as red giants and organic molecules in space-all emit infrared.
Infrared is essential for the study of the early Universe, in other words, key aspects of its origin. As the Universe is in constant expansion, the visible and ultraviolet radiation in the early Universe is now into the infrared region. Infrared cannot easily penetrate the atmosphere and hence only satellites could detect it. Highly red-shifted objects at cosmological distances are easily detected in infrared, as the optical radiation of objects going away from the observers goes into the infrared region.
Though William Herschel discovered infrared radiation in 1800, real progress was made only after the Space Age . The most impressive record yet on the discovery of infrared sources is held by the Infrared Astronomical Satellite (IRAS), launched in 1983. It mapped more than 96 per cent of the sky during its 10-month life.
The Spitzer Infrared Space Telescope, named in honour of Lyman Spitzer (1914-1997), is a fine example of what can be detected in the infrared region. The telescope can pierce through the dust that fills galaxies like a fine fog. Launched in 2003, two of its three telescopes have to be kept within 10º of absolute zero, with liquid-helium coolant. At any one time, Spitzer can see less than 1/20,000 of the area that a human eye can see!
The First Facility in India
The first major facility in India designed for ground-based infrared observations of celestial objects is the Infrared telescope at Mount Abu, a hill resort in Rajasthan. The observatory, run by the Physical Research Laboratory (PRL), Ahmedabad is at an altitude of 1680 meters adjacent to Guru-shikar, the highest peak in central India. Moreover, the site has very low precipitation (1-2 mm during winter) and about 150 cloud-free nights in a year. The telescope can be used for imaging studies at both optical and infrared wavelengths. A 1.2-m paraboloid primary mirror, with a standard Cassegrain configuration, is used for the optical system. There is provision for observing infrared sources from faint objects in the presence of bright objects.
The Hanle Observatory also has an infrared detector, as that site has low atmospheric water vapour. The sky at Hanle is much darker in infrared than at sea level, as the temperature and aerosols decrease with altitude. The infrared imaging camera has been used for the study of galactic star-forming regions and possible extra-solar planetary systems. There are plans to equip the Hanle telescope (HCT) with an infrared spectrograph in the future. The high demand for time at the optical telescope in Hanle justifies an independent infrared telescope.
Since Hanle’s infrared telescope started seeing the first light, some discoveries in the infrared Universe have been reported. A team of astronomers at the Subaru and Keck telescopes on Mauna Kea, Hawaii, announced their discovery of a giant three-dimensional filament of galaxies, extending across 200 million light years of space. At least two large concentrations of gas have been found, one of which extends to 400,000 light years, more than four times the diameter of our Milky Way galaxy.
Till recently, only galaxies that are 50 million light years across have been found. A study of the gas concentration and the speed of the materials shows that the region is ten times more massive than the Milky Way. More interestingly, they have found that the galaxies were only two billion years old after the Big Bang. Thus they are the earliest structures ever discovered in space. The telescopes have identified very faint objects including 33 large gas blobs along the filament structures. The discovery demolishes the long-held view that the early Universe was relatively smooth.
The latest discovery confirms the recent finding by another telescope, also placed in Hawaii, but designed to probe the Universe in the infrared region of the spectrum; a UK telescope has found massive galaxies at a much earlier stage of development than expected. They are too faint to be seen in optical telescopes. The Institute of Astronomy at the University of Edinburgh has collected through this telescope a mind-boggling volume of data, which would fill the equivalent of 15,000 CDs! It can produce 10,000 images a night.
The Herschel telescope (2009) is the largest space telescope with a 3.5 m diameter mirror to collect long wavelength infrared. It will be the only space observatory to cover the spectral range from far infrared to sub-millimetre wavelength.
The telescope would be positioned at L-2 (Lagrangian point), 1.5 million km away from the Earth—about four times the distance from the Earth to the Moon. Herschel has three instruments: a photo-detector array, a spectral and photo-metric imaging receiver and a heterodyne for extremely high resolution in the far infrared. Herschel will be launched along with Planck, a satellite telescope designed to study the cosmic microwave background radiation.
The James Webb telescope (2013) will have unprecedented accuracy and capacity to see back in time. The visible and near IR camera is designed to catch the light from the ‘first’ stars, galaxies in the very distant past in the process of formation, only about 400 million years after the Bing Bang; discover supernovae in distant galaxies and young stars in the Kupier belt. The middle infrared camera and spectrograph would register hydrogen from ‘unthinkable’ distance while another spectrograph would reveal chemical elements that formed back in time as well as gaseous intergalactic regions.’