Exploring at Other Wavelengths
For a long time, astronomy meant "optical" astronomy: the science of looking at a star and classifying it by its color and brightness. The development of sophisticated detection techniques that enabled astronomers to examine the whole range of the electromagnetic spectrum completely transformed that science.
These days, astronomers are increasingly involved in multiwavelength studies – making observations at energies ranging from the lowest possible (radio) to the highest ever detected (gamma rays).
Spiral galaxy NGC 891
Image Copyright: WIYN Consortium Inc
It is not always possible to study an object across the whole range of energies. The emissions of some objects are too faint to be detected, while some objects are obscured, and some just don't produce any radiation at a given energy. One of the key points of multiwavelength studies is that just detecting radiation at a given energy provides crucial information on the physical conditions existing in the object, because different physical phenomena produce different signature radiation.
Sometimes, a multiwavelength analysis of an object suffices to explain its mystery or at least clarify it. For example, in 2003, scientists were analyzing X-ray images of a supernova remnant called Vela when they discovered an unknown object hidden behind it. Both the image and the scant X-ray spectrum of this new object seemed to indicate that it was another young supernova remnant, but they could not be sure. The follow-up detection of the high energy radioactive decay of Titanium-44 (an element usually formed during supernova explosions) established with certainty that it was indeed a young supernova remnant. Both the X-ray spectra and the very high energy line helped solve this mystery. One piece of information would not have been enough.
X-ray Image of Vela
Image taken by the ROSAT satellite
Optical image of Vela
Image copyright: David Malin, AAO.
Sometimes these astronomical mysteries are not solved completely (not yet, at least), though they are made less puzzling by multiwavelength studies. This was the case with a set of sources discovered by the EGRET instrument on the Compton Gamma-ray Observatory in the 1990s. EGRET discovered a number of sources in the gamma-ray sky for which there were no-known counterparts at any wavelength. Astronomers have studied these sources, searching near their gamma-ray positions for signatures in other wavelengths.
This plot shows an all-sky view of the
sources observed by EGRET. The unidentified sources
those without a counterpart in other wavelengths and whose
nature was unknown at the time of discovery are shown with
The Fermi Gamma Ray Space Telescope's Large Area Telescope (LAT), in addition to other instruments on other observatories, has enabled astronomers to classify the objects. According to Neil Gehrels, chief of the Astroparticle Physics Laboratory at NASA/GSFC, as of June 2011, nearly all of the original EGRET sources have now been identified, but Fermi's LAT has found approximately 500 new unidentified sources.
Gehrels says that some of the EGRET unidentified sources were not seen with the more sensitive Fermi instruments, and astronomers think that these may not have been objects, but artifacts caused by not knowing exactly how much of the diffuse emission to subtract in that region when searching for the objects. He says that it is also possible that a few that disappeared were real sources in the EGRET era that have disappeared, but that it is impossible to know for sure.
Above are optical and X-ray images of a
cluster of galaxies superimposed over each other. The purple
false-color cloud is hot gas.
It can also happen that the mystery only deepens as the result of new information – a scientist's favorite scenario. This is what happened in the study of clusters of galaxies. Optical studies of rotation curves from galaxies revealed that the bright gas that one sees in a telescope could not account for all the matter present. Scientists began the search for this missing substance (or substances) referred to as "dark matter." X-ray observations of clusters of galaxies revealed a hot gas (>10 million degrees) permeating between galaxies that are bound together by the gravitational potential of the group. This gas is invisible at any other energy, but unfortunately, it cannot account for all the missing matter. The combined X-ray and optical data still imply the presence of a hidden mass, sometimes 100 to 300 larger than the one observed. It is called "dark matter" not only because it is invisible at all energies that we have had a capability to see, but also because there are many kinds of this "invisible" matter that could compose dark matter. However, new techniques have been devised for finding these dark matter candidates, which involve not remote observation, but direct detection via the Alpha Magnetic Spectrometer-2, installed in May 2011 on the International Space Station.
There are many more examples of the potential of multiwavelength observations. It will take more time until we are able to study the sky with the same precision at all the energies of the electromagnetic spectrum. When this happens, for the first time we will be able to contemplate the universe as it really is.
Updated: June 2011