What Does SWIRE Stand For?SWIRE stands for Spitzer-space-telescope, Wide-area, InfraRed Extragalactic survey. It describes the kind of astronomical survey of the celestial sky aimed at by this project.
Goal of SWIRE SurveyNASA's Origins program seeks to understand how life arose on Earth, and to determine if life exists anywhere else in the Universe. The Sun powers almost all life on Earth and, in fact, is responsible for the planet's very existence. Yet the Sun is an ordinary star, one of approximately 100 billion in the immense conglomeration of stars that is our galaxy. In order to understand the origin of life, it is necessary to understand the origin of stars, the galaxies that contain them, and the structure that galaxies lie within. The goal of SWIRE is to understand how the universe we see today arose. To do so it will chart the history of star formation, quasar formation, and the formation of large scale structure in the Universe, Where SWIRE Will Look
Why Infrared Observations Will Be MadeCurrent observations suggest that stars and their planetary systems form when massive interstellar clouds of gas and dust collapse. Thus, most star forming regions are shrouded by clouds that block the visible and ultraviolet light emitted by young stars. This makes the star formation process inaccessible to ordinary visible light telescopes. However, infrared light can pass through these clouds. Even more importantly, the ultraviolet and optical light given off by stars (and more exotic phenomena such as active galactic nuclei) heats the dust. The energy that the dust absorbs is then reradiated away in the form of infrared light. The SIRTF telescope makes its observations using infrared light. Light of this type is not visible to the human eye, and is perceived as heat. The SWIRE project exploits all of the wavelengths of light visible to SIRTF. These range from what astronomers call the mid-infrared to the far-infrared. Mid-infrared light has a wavelength that is about 10 times longer than that of the light we see with our eyes. It corresponds to temperatures near room temperature. The far-infrared has wavelengths that are about 300 times longer than optical light, and they correspond to temperatures of around 300 degrees below zero, Fahrenheit. Star-Formation StudyAlthough the star formation process is brief by astronomical time scales, it is extremely long compared to human experience, lasting perhaps hundreds of thousands to millions of years. No human observation could ever view a single star through all its birth phases. Thus, SWIRE will make observations of many stars in different phases, with the intention of providing enough data to assemble a history typical of all stars. This is similar to how botanists study tree growth. It would be impractical to observe a single tree throughout its entire lifetime, so botanists survey an entire forest, hoping to find different examples of a certain species at different stages in its life cycle. SWIRE will survey a "forest" by observing entire galaxies rather than individual stars. It will take the star formation inventory of several million galaxies within a region of the sky equal to 300 times the area of the full moon. This large area may allow SWIRE to detect rare objects or new phenomena. Astronomical Objects To Be SurveyedSWIRE will concentrate its observations on several areas. SWIRE will see many galaxies at its shortest wavelengths. These galaxies are like "ordinary" galaxies found locally. SWIRE will also see starburst galaxies, extremely bright galaxies that are producing new stars at ten times the rate of typical galaxies. It will also search for ultraluminous galaxies, very bright, possibly extremely distant galaxies with intense star formation. SWIRE will examine the remote Universe to determine the large scale distribution of galaxies, which will be used to test competing theories of galaxy formation. Observing distant objects is important when studying the history of the Universe because distance equals time. This is because light travels at a finite speed. For example, if an object is 100 light years away from us, the light we are seeing from it now was actually emitted 100 years ago, because it took 100 years for its light to travel to Earth. This means we are actually seeing the object as it appeared a century ago. Its image acts as an astronomical "fossil", preserving a record of the object's past. Some of the ultraluminous galaxies may be billions of light years away, possibly representing the first galaxies to form. SWIRE will search for a kind of "failed" star, called a brown dwarf. Brown dwarfs are relatively small objects that shine faintly, mostly in infrared light. They do not shine for the same reasons that a normal star shines. A normal star shines because of energy supplied by nuclear fusion reactions in its central core. Brown dwarfs, at about 80 times as massive as Jupiter, still have too little mass to sustain nuclear fusion reactions in their cores. Instead, they glow dimly with the heat left over from their contraction. SWIRE should be able to detect several tens of these objects. When a star forms from a collapsing cloud, it often is surrounded by a disk of material from that cloud. It is believed that planets form out of the material in these disks. The circumstellar disks absorb light from the star and radiate it again as infrared light. While looking out of the Milky Way at the distant universe, SWIRE will also see many "foreground" stars located within our own galaxy. We have a pretty good idea of what a "naked" star should look like - stars found by SWIRE to have an excess of infrared light are likely to have circumstellar disks. Thus, SWIRE will possibly find clues to planet formation. SWIRE will also see objects within our own solar system as well. One of SWIRE's seven survey fields lies close to the plane occupied by the disk of the solar system (called the "ecliptic"). In this field we expect to see several thousand new, small asteroids. We will be able to detect them because they will move relative to the fixed background of stars. The SWIRE survey is designed to look at each piece of sky twice, allowing us to see these moving objects. Measurement ResolutionAlthough SWIRE will be surveying a very large area, it's spatial resolution will not be very high. This is because for a given sized telescope, the blurriness of an image is directly related to the wavelength of light being looked at. Because SWIRE is using such long infrared wavelengths, the blurriness of our images will be similar to or worse than an optical telescope on Earth. For individual objects, we cannot produce the detailed images that the Hubble Space Telescope does. SWIRE instead relies on the brightness of objects (which tells us how much power they produce) and the color of objects (which tells us about their temperature and what they are). Measurement SensitivityBecause SWIRE is so sensitive, it will detect the cold dust that fills our galaxy, which is called "cirrus" because it resembles the wispy cirrus clouds in Earth's atmosphere. Even though SWIRE is looking out of the galaxy and away from the galactic plane (where the densest, brightest cirrus is found), it will still see the cirrus "overhead". SWIRE will see finer structure in this faint cirrus than has been seen before. |
