FEATURE ARTICLE
Spitzer’s Cold Look at Space
To get a clear view of infrared emissions from celestial objects, the Spitzer Space Telescope has been cryogenically cooled—and what sights it has seen
Michael Werner
A Broader and Deeper View
Because of the expansion of the universe, the radiation from more and more distant objects is increasingly shifted towards the infrared end of the spectrum as it travels toward us. This effect, called the cosmic redshift, makes infrared wavelengths of particular importance for cosmological studies. The radiation from objects at a redshift, or z, of 1, for example, is doubled in wavelength.
The redshift of a distant galaxy is a measure of how far back in space and time the observed light left the object, as well as a measure of the factor by which the universe has expanded since that epoch. Because of the finite speed of light, as we look deeper in space we also look back in time, because it takes longer for the light from a more distant object to reach us. As an illustration, recent measurements indicate the current age of the universe, the time since the Big Bang, is about 13.8 billion years. Galaxies with a z of 1.5 are seen when the universe was about 40 percent of its current size and about 6 billion years old. So those galaxies are literally more than halfway across the universe.
When Spitzer looks back to a z of 6, for example, it is not only looking to an epoch when the universe was 15 percent of its current size; it is also looking back to within about 900 million years of the Big Bang. By observing galaxies both at such great redshifts and also as close as our nearest extragalactic neighbors—the Magellanic Clouds and the Andromeda Nebula—Spitzer has uncovered many of the features of galaxy evolution.
Clusters of galaxies, which can have masses of about a quadrillion (or a million billion) times that of the Sun, are the largest structures in the known universe. The same survey that yielded the brown-dwarf census is producing an equally exciting census of clusters of galaxies in the early universe. Looking at small areas of sky that previously have been observed in other wavelengths, Spitzer’s observations have found a prominent cluster, or gravitationally bound group, of galaxies at a redshift of z=1.4. Spitzer sees these distant clusters quite readily because their optical starlight is shifted into the infrared. The observations have led to the identification of more than 100 clusters of galaxies at redshifts greater than 1 in this one area of sky, many more than had been identified over the entire sky by all previous observations.
The number and properties of high-redshift clusters of galaxies hold clues to the way those structures formed and grew in the early universe. We know from observations of the microwave background radiation still present in the cosmos that the early universe was remarkably smooth and uniform, with minute density and temperature fluctuations superimposed on a uniform background. With time, these fluctuations grew, driven largely by gravitational forces, into the rich variety of structures we see in the universe around us today.
Spitzer’s identification of large numbers of clusters at high redshifts, looking back into space and time, can help us to understand how these and smaller structures developed and evolved. It also could elucidate the roles and perhaps the nature of the mysterious dark matter and dark energy that apparently make up most of the material in the universe.
Studying galaxies at high redshift can also provide clues to their chemical evolution. Spitzer, following up on previous results from IRAS and ISO, has shown that the mid-infrared emission from the interstellar medium in our galaxy and, indeed, from entire galaxies, is dominated by molecules called polycyclic aromatic hydrocarbons (PAHs). These planar hydrocarbon molecules consist of hexagonal carbon rings and their associated hydrogen atoms. PAHs are very familiar on Earth as combustion products; they have been extensively studied by chemists and have also been detected in meteorites.
Despite extensive study over two decades, astronomers still don’t understand the pathways for the formation and evolution of PAHs, or their possible role in bringing organic materials into forming planetary systems. Spitzer has added a new element to this study by measuring galaxies as distant as a redshift of z=2.7 and showing that their PAH emission spectra are virtually identical to those of nearby galaxies. The result suggests that this constituent of the interstellar medium was in place just a few billion years after the Big Bang.
Spitzer can detect galaxies as distant as a redshift of z=7, which means that the light we observe now left the galaxy when the universe was only 12 percent of its present size, and about five percent of its present age. Spitzer’s results indicate that a significant amount of the star formation in the oldest galaxies in the universe took place during an earlier epoch than had been predicted by most models. Controversial but tantalizing Spitzer results suggest extremely massive galaxies may be present at redshifts higher than 6. This result, if confirmed, would represent a major challenge to the theoretical framework of galaxy formation that has been built over the past decade.
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