Q. Why is understanding how and how fast galaxies evolve an important issue, and what, more specifically can it tell us?
Understanding the star-formation history of the Universe is one of the last great
mysteries. The cosmic microwave background radiation tells us a great deal about
what the universe was like when it was a mere 100,000 years old (and even before
that). We can, and have, looked around us to see what the universe is like today.
Unfortunately, the history of the universe from when it was millions of years old
to, say, a billion years ago or so, is poorly understood. We don't know yet when
most of the stars formed, what types of galaxies they formed in, or even when and how the
galaxies formed. Some indications are coming in, but we have a ways to go.
Q. What is the "usual" rate of star formation in a galaxy, as opposed to a starburst galaxy's higher rate of formation?
A. A better way to think of star formation is to ask: At the observed rate of star formation, how long could such a galaxy sustain that rate, given it's observed supply of gas (hydrogen, helium, a little other stuff)? Most galaxies fall into two broad categories. Elliptical galaxies generally have almost no gas, and almost no star formation. Although they may have been spectacular in the past, they now appear to be relatively quiescent. Many appear as huge (trillions) collections of ancient stars. Spiral galaxies have gas and exhibit star formation. For ellipticals, the question is kind of moot. The answer to the question for typical spirals is usually many billions of years, and sometimes over 10 billion years. For starburst galaxies, the answer is less than 1 billion years. In other words, starburst galaxies are consuming their gas quickly compared to the age of the universe.
Q. What is a confusion limit?
The fainter you look, the more stars and galaxies you see. They pile up
quickly. Eventually, they become so crowded that you cannot discern them from each
other. If the random "bumps" you see on the sky, which are a result of
lots of galaxies within the resolution element of the telescope are as big as 20% of the
faintest galaxies that you are trying to find - then you are at the confusion limit.
You can't look reliably fainter than that.
Q. What is the significance of studying luminous protogalaxies?
A. Protogalaxies are infant galaxies, possibly the result of giant starbursts. If we find luminous protogalaxies at high redshifts, it means that giant elliptical galaxies may have formed in colossal starbursts when the universe was young. Finding them tells us when the galaxies formed. Studying them tells us how they formed. The current most popular theory, which includes the mysterious, undetected, hypothetical Cold Dark Matter (CDM) predicts that galaxies must have assembled themselves from small pieces. The medium-sized pieces formed then continued to assimilate into the full-sized galaxies that we see today. If lots of very luminous protogalaxies are discovered, it would imply that these large galaxies formed all at once, and would bring the CDM theory into further question.
Q. What is the purpose of the Cryostat? What exactly does it do?
A. The cryostat keeps the telescope cold. If the telescope was as warm as the room you are in, then it would be emitting IR radiation, the same thing you are trying to see from galaxies far away. The detectors would be blinded by the telescope. Cool it off, and the peak of the thermal spectrum moves out into radio wavelengths, where it will not interfere with the gathered data .
Q. What is the Quantum efficiency?
A. Quantum Efficiency. is a measure of how efficiently the detector can detect photons. If the QE=50%, that means that half of all photons that hit the detector contribute to the signal, or "photocurrent." The QE of your eyes is ~10%.
What is a Dichroic beam splitter?
A. Chromos means color, so dichroic must mean something that divides light into two colors: 12 and 25mm are the two colors. They are two different wavelengths of light that are both in the infrared region of the electromagnetic spectrum.
What is the advantage of WIRE over ISO, i.e.why was it not possible to
answer the questions WIRE is supposed to answer from ISO data?
A. The WIRE instrument represents improvements over ISO in three important areas. First, WIRE has two very large-format IBC detector arrays that observe simultaneously. This means that more than 32,000 pixel elements will be observing the sky simultaneously. ISO's CAM instrument, by far its most useful imaging tool, utilized about 1,000 pixel elements. Second is the greatly improved sensitivity and uniformity of these detectors over past generations of infrared detectors. Lab tests of these detectors show that they behave extremely well even when exposed to the intense radiation they will see in space. Third, one of WIRE's detector arrays observes at a wavelength of 25um, compared to the long-wave cutoff of the CAM instrument of 15um. Most of the sources are expected to be 2-3 times brighter when observed at 25um than at 15um - especially the most powerful sources that will be seen at the greatest distances. The result is that WIRE will be able detect as many distant sources in a single day as ISO would have detected in many months using CAM.
Of the three original questions posed as WIRE science objectives (prior to the launch of ISO), the first is the easiest to answer. In fact, the IRAS mission (launched in 1983) almost answered it. ISO has confirmed the IRAS results (as have studies with HST and the Keck telescopes) and provided an important first look at the faint mid-infrared sky. So the first of the WIRE questions is largely answered (it will only take a few hours for WIRE to confirm this at 25um).
The second question is much more problematic. It requires a very large sample to accurately determine the rate and nature of the evolution of star-forming galaxies. WIRE is ideally suited to this task because it observes simultaneously at two wavelengths, as mentioned above. These two infrared colors together serve as immediate distance indicators (when used on a very large sample of galaxies, which is our intent) that will help unravel the nature of what is changing in these systems. The largest ISO samples are measured in hundreds of galaxies, and they have been observed at a single wavelength usually. Thus, it will require a lot of follow-up observations with ground-based telescopes to begin to unravel the details of this story. WIRE will detect close to 200,000 galaxies, and their infrared colors will provide a strong constraints on our interpretations of the evolutionary history of star-forming galaxies during the past 5 Gyr or so. Further, these same colors will allow us to select for only the most powerful galaxies out of this very large sample which will help us concentrate our efforts with ground-based telescopes much more effectively. WIRE will have a very large sample, and the infrared colors will allow us to take full advantage of it by selecting for galaxies of a given luminosity.
The third question that WIRE poses is by far the most difficult. To detect galaxies at very high redshifts (2 and beyond), one has to survey a lot of volume of the early universe. The best way to do that is to cover lots of sky. This is WIRE's strongest aspect. By surveying almost 2000 square degrees of sky, WIRE will conduct by far the largest survey for luminous infrared galaxies in the early universe that has ever been undertaken. By contrast, the largest survey conducted by ISO is the ELIAS survey which is expected to cover about 15 square degrees and will be less sensitive than WIRE's wide-angle survey to distant, powerful galaxies. In fact, WIRE will be able to detect dusty, powerful quasars out to redshifts of 10. If there are only 30 of them in the whole sky at that distance, WIRE should find one. Detection of such a system at that distance would be very exciting indeed.
What are the total cost of the WIRE mission?
A. The total cost of the WIRE mission, including the telescope, spacecraft, the rocket, all of the launch costs, operations, data system development, and data analysis for three years after the mission ends is $75M. Counting the recent launch slip from last September to this month (due to rocket unavailability), the cost has risen to $79M.
Q. Why has it been limited to just four month?
A. The lifetime of the mission is limited by the supply of the solid hydrogen cryogen. WIRE is a small mission that we hope will produce some very exciting results. The WIRE telescope and spacecraft barely fit within the small Pegasus rocket fairing. "She is small, but she is mighty".
Have any more questions? Ask them by e-mail. They will be answered as soon as possible.
Last Updated: 2/2/99