One class of galaxies has proved especially amenable to study using infrared wavelengths. This is the so-called ULIRGs, for Ultraluminous Infrared Galaxies. These galaxies are marked by high production rates of stars. The radiation from the stars heats up the dense clouds of dust and gas, causing them to glow brightly in the IR. Here are images of ULIRGs made by the SST:
Multispectral images made by different space telescopes covering separated regions of the EM spectrum can lead to some striking images, as seen in this view of Galaxy M82. The Chandra image is rendered blue; the Hubble image of excited Hydrogen is shown in green; and the Spitzer ST image is assigned red:
The SST has a spectrometer that can recognize chemical components of a star and its surrounding cloud. For HH 46/47, shown again in the inset, the spectral curve contains strong absorption bands that indicate the presence of water ice, methyl alcohol, silicate particles, and carbon dioxide.
On February 22, 2006 NASDA, the Japanese Space Program, launched ASTRO-F, an Infrared Observatory. Nicknamed "AKARI" ("Night"), the spacecraft looked like this:
Its main sensor has 8 infrared channels. Each produces its characteristic expression of thermal energy distribution, as exemplified:
Infrared imagery has proved especially sensitive to detecting Deep Field galaxies. These have been redshifted by the Universe's expansion so as to radiate in the infrared. This image shows some of the stars at great distances from Earth that glow brightly in the IR:
The Herschel Observatory, shown below, was launched on May 14, 2009. It operates mainly in the far infrared. Herschel has been placed at the Lagrangian point L2 (where the gravity of the Sun just balances that of Earth) about 150,000 km from Earth. Read about it on its ESA website.
Photodetector Array Camera and Spectrometer (PACS), a camera and a low- to medium-resolution spectrometer for wavelengths up to about 205 micrometres. It uses two bolometer detector arrays in the camera and two photo-conductor detector arrays in the spectrometer.
Spectral and Photometric Imaging Receiver (SPIRE), a camera and a low- to medium-resolution spectrometer for wavelengths longer than 200 micrometres. It uses five detector arrays: three to take images of infrared sources in three different infrared 'colours' and two to fully analyse the longer infrared light being released from the source.
Heterodyne Instrument for the Far Infrared (HIFI), a highly accurate spectrometer that can be used to obtain information about the chemical composition, kinematics, and physical environment of infrared sources.
The first image from Herschel released to the public is shown below. It is the Whirlpool galaxy, near the Milky Way. Contrary to the usual convention, red denotes relatively cooler temperatures while blue is warmer.
An indication of the improvement in resolution afforded by Herschel is gained from this comparison of galaxy M51 as seen by Spitzer (left) and by Herschel's PACS (right):
Similar improvements are evident when Herschel's SPIRE instrument is compared with its Spitzer equivalent, using galaxy M74 as the target.
Herschel is spending much of its observing time looking at the Milky Way as it appears in the Infrared. The gases between stars are generally quite cold but the sensitive PACS and SPIRE instruments can detect low temperature outputs, giving a better understanding of how the gas is distributed. Here is a small area within the M.W. as seen individually by these instruments, and then as an image made by combining them:
Herschel's HIFI is also working well, as evidenced by this data set that shows detection of carbon in a galactic medium:
Herschel in mid-summer of 2009 was still in the instrument readiness phase but routine data gathering is expected by November.
Another IR satellite, WISE, the Widefield Infrared Survey Explorer was launched on December 14, 2009. Read about it at this University of California-Berkeley website. It will produce two complete sky maps by October, 2010. These will have more detailed images in the infrared than ever before. Here is the first image:
Typical of the kind of images produced by WISE are these three views, at different wavelengths, of the nearby Andromeda galaxy:
Now, lets move still farther out to longer wavelengths in the EM spectrum. Astronomical objects, in particular galaxies and supernovae, emit the gamut of radiation across the spectrum. Galaxies are usually strong emitters of microwave radiation, in particular in the radio region. Radio waves are generated by excitation of neutral Hydrogen. A good general review of radio astronomy has been prepared by the Haystack group at MIT.
The specialized field of radio astronomy utilizes large "dish" antennas to capture the long wavelength radiation. One of the first radio wave monitors is the famed Arecibo site in Puerto Rico, in which the parabolic receiver is embedded in a limestone sink in the jungle. The dish, 305 meters (just over 1000 ft) wide, is fixed in orientation and must use the rotation of the Earth to examine parts of the astronomical heavens.
The largest movable telescope in the world is the 100 meter radio antenna facility at Effelsberg in Germany. It can both rotate and swing up and down.
Resolution of celestial targets from which radio waves emanate can be improved by developing a synthesized aperture by means of electronically hooking together individual radio telescopes. A major facility in the National Radio Astronomy Observatory group is the Y shaped array of 27 radio telescopes, each 25 m (81 ft) in diameter, located in the flats 70 miles west of Socorro, New Mexico. This creates an effective resolution of 36 km (22 miles). This Very Large Array (VLA) mode uses principles of Interferometry to process the signals from each telescope as a unit.
In essence, the same signals are received almost simultaneously at different receivers. When added together these may be out of phase and may cancel out or reenforce at specific wavelengths; computer processing allows a new interference signal to be produced.
Radio telescopes separated by hundreds and even thousands of kilometers can be tied together by electronic wiring or radio signals to each other to produce an array called VLBI (Very Long Baseline Interferometry). The effect of integrating the telescoope signals is to increase the resolution significantly, so that smaller features in radio objects can be discriminated.
One of the major tasks of radio astronomy was to survey the sky at 21 cm to pick up the distribution of neutral Hydrogen in the Milky Way and the halo around our galaxy. Here is the result:
More details about the central region of the Milky Way appear in this radio telescope image made at 90 cm.
Within the Milky Way, regions of more intense radio wave sources can be identified and mapped. These regions have been called "radio blobs", referring to their somewhat diffuse nature. The blobs consist mostly of ionized Hydrogen gas (plasma). The green areas in nearby galaxy NGC3603 exemplify radio blob distribution.
Most galaxies are so far away that their internal localized sources of radio waves cannot be resolved. Whole galaxies are imaged at the 21 cm H wavelength. Here is M81:
In the early days of radio astronomy, many radio sources in deep space were discovered but when the same region was examined by optical telescopy often no obvious galaxy or other stellar body was found at first. Later observations at non-radio wavelengths have now detected the astronomical feature, usually a galaxy (many galaxies are very strong radio wave emitters). One of the best examples of powerful energy emitters in which visible images do not detect any obvious sources is Cygnus A, from a galactic center about 700 million light years away. Cygnus A is the strongest radio wave emitter in our part of the Universe. Consider these images:
In the above image, the upper left shows a visible light image (star groupings in bright blue) but with no obvious galactic shape. Superimposed, as colorized in red, are two distant lobes representing radio wave signals associated with Cygnus A. The lower left image is another radio wave rendition of signals received at 6 cm. The lower right image, made by HST, reveals some strong radiation coming from the central region of Cygnus A.
Here is a galaxy seen in the Infrared, on which is superimposed the intensity contours associated with two radio sources in the limbs that once seemed isolated from this distinct galaxy.
This next image shows an L-Band image of the Starburst Galaxy (M82); this was made at the Jodrell Bank Radio Telescope Observatory near Manchester, England, one of the premier facilities in the field. The signals were obtained from the MERLIN (Multi-Element Radio Linked Interferometer Network) array.