Spitzer Space Telescope

Stars and galaxies emit much of their energy in the infrared region of the spectrum. The Spitzer Space Telescope is the latest of NASA’s Great Observatories, designed to observe this infrared radiation and use it to map the sites of star formation in both our Galaxy and external galaxies.

Spitzer was launched on 25 August 2003 from Kennedy Space Center in Florida, USA. The Delta rocket put the observatory into orbit around the Sun and it is currently about 15 million kilometres away from the Earth. This orbit allows the telescope to be kept very cold, at a temperature of around -268 celcius. Warm objects, such as the Earth and the Sun, produce large amounts of infrared radiation which would swamp the faint signals from distant galaxies, so the telescope is kept far away from the planet and is protected from the Sun by a heat shield.

The telescope is relatively small compared to ground-based telescopes, with a primary mirror of only 85cm in diameter. There are three science instruments onboard the observatory: two cameras (IRAC and MIPS) which take photographs in a wide range of infrared ‘colours’, and a spectrograph (IRS) which separates the infrared light into a spectrum for more detailed analysis. These instruments are designed to study the radiation emitted by stars and interstellar dust, in particular around sites of newly-forming stars.

Galaxies

Over the last few years many of the fundamental properties of the universe have been accurately measured. We know better than ever when the universe began, what it is made of, and how it has evolved since the beginning. Moments after the Big Bang, 13.5 billion years ago, the universe was very hot, very dense and very simple – a mixture of energetic particles and radiation, or ‘plasma’. The universe expanded, becoming cooler and less dense, forming atoms, molecules, stars, planets, galaxies and eventually life. The history of our own Sun and planet, and indeed ourselves, has been determined by the history of the galaxy in which we live.

The transformation of the universe from a simple hot plasma into a complex hierarchy of stars and galaxies was a process spanning billions of years. Galaxies are massive systems containing mostly dark matter (mysterious particles that do not emit radiation), hydrogen gas and stars, bound together by the force of gravity. They are the basic building blocks of the universe.

Within individual galaxies, complex physical processes form stars out of hydrogen gas. Within the stars, nuclear fusion reactions turn the hydrogen into heavier elements (carbon, nitrogen and oxygen, for example). As the stars evolve with time these elements are released back into space, chemical and physical processes turning them into larger particles of complex molecules known as ‘dust’. Some of this dust is then incorporated into newly-forming stars in a continuous feedback cycle which is forever changing the physical and chemical composition of galaxies.

When new stars are formed, the surrounding gas and dust in a galaxy is heated, causing the dust to radiate in the infrared. Conversely, the dust absorbs most of the ultraviolet and visible light from new stars making them invisible at these shorter wavelengths. Thus the star formation that drives galaxy evolution can be most effectively studied at long wavelengths, principally in the near-infrared (wavelengths of 1-20 microns) and far-infrared (20-1000 microns) passbands. In fact, something like one-third of all the radiation in the universe is in the far-infrared passband, making it the second most important region of the electromagnetic spectrum in which to study the energy output of the universe (the first is the microwave region, a leftover of the Big Bang itself).

Our work is to study the process of star formation in galaxies, in particular looking at how this process depends on the environment of a galaxy – whether there are lots of other galaxies nearby, or if it’s relatively isolated in space. Due to the constant speed of light, when we observe distant galaxies we actually see them as they were in the past, when the radiation (for example, visible light or infrared radiation) was emitted. Thus we can directly measure how galaxies have evolved over time by comparing distant objects (seen as they were in the past) with those nearby (seen as they are now).

The Cosmic Web

Galaxies are not distributed uniformly across space but are found grouped together in ‘clusters’ of galaxies; these clusters are themselves grouped together into larger structures. The distribution of galaxies and clusters is sometimes refered to as the ‘cosmic web’ because its many strands and connections resemble a spider’s web.

A galaxy cluster is the largest single object in the universe. The force of gravity holds the individual galaxies in orbit around one another so that they stay grouped together. Also contained within the cluster is a large mass of hot (ionized) hydrogen gas, which is a strong emitter of X-rays. However, the total amount of mass in galaxies and gas does not provide enough force to hold the cluster together. There must be another source of gravity within the cluster and this hidden mass is known as ‘dark matter’ (because it doesn’t emit any light or radiation). What is it? We don’t know – the physical nature of this dark matter is one of the greatest outstanding mysteries in all of physics.

As part of the Spitzer Wide-area Infrared Extragalactic legacy survey (SWIRE), we have used the Spitzer Space Telescope to map an area of sky 250-times larger than the full moon. This map is being used to measure the positions of several million galaxies and determine how the cosmic web of galaxies and clusters has changed over time. From these measurements we hope to deduce whether the galaxies (which we can see) are found at the same locations as the dark matter (which we can’t see, but theory tells us must be there). This relationship between galaxies and dark matter, known as ‘bias’, is a key factor in understanding the structure of the universe itself.