What will the IRS tell us?

The Spitzer Space Telescope's infrared technologies will enable us to peer deeper into space than ever before and allow us to investigate many phenomena that have previously been little understood. These studies should provide insights into many fundamental questions. Our primary research interests can be mostly divided into four main areas:

  1. Brown dwarfs and super planets - Stars that have too little mass to ignite the fusion reactions that power stars like our sun, but are larger and warmer than planets. We hope to learn more about their physical characteristics and population.
  2. Debris disks around stars - These disks are thought to be indicative of planet formation and we hope to be able to trace the evolution of dust clouds to planets by observing these.
  3. Ultraluminous Infrared Galaxies and Active Galactic Nuclei - These galaxies emit most of their radiation in infrared wavelengths; Spitzer will provide a unique new view of them and be more sensitive than any previous telescope in finding them. Such galaxies are thought to be powered by colliding galaxies or black holes. IRS studies should provide insight into the nature of their origin, evolution, and physical characteristics.
  4. The Early Universe - The IRS studies should provide better observations of the cosmological redshift leading to a better understanding of the origin of the universe itself.

The IRS team has organized its initial research agenda into the following specific programs. Clicking on one of the links will take you to the relevant text below.

  • Molecular Hydrogen Measurements
  • Starbusts and Active Galactic Nuclei
  • Dusty Stars
  • Ultraluminous Infrared Galaxies
  • Dusty Wolf-Rayet Stars
  • The Galactic Center
  • Dust in the Early Universe
  • Extremely Red Objects
  • NOAO Deep Wide-Field Survey
  • Planets and Moons
  • Interacting Galaxies and Starbursts
  • Massive Stars and the Interstellar Medium
  • Ring Galaxies
  • Bipolar Outflows in Young Stars
  • Towards a better Deuterium ratio
  • High Power Radio Galaxies and Quasars
  • Faint Obscured Galaxies
  • M-Dwarf Stars
  • Formation of the First Generation Stars
  • Reflection Nebulae
  • Starburst Galaxies
  • Brown Dwarfs
  • Planet Formation and Protostellar Disks
  • Nearby Active Galactic Nuclei
  • Low Luminosity Radio Galaxies
  • Tidal Dwarf Galaxies
  • IRS Nearby Star Observations
  • IRS Observations of Chandra Sources
  • Unknown IRAS Sources
  • Planetary Nebulae
  • Gamma Ray Bursts
  • Outflows from Star Forming Regions

Molecular Hydrogen

Much of the matter in the universe is undetectable with optical telescopes because it does not emit light. In particular, cold molecular hydrogen gas may constitute a considerable fraction of the dark matter in galaxies, yet it is not directly measurable with current techniques and its amount must be inferred indirectly.

The IRS team will directly map molecular hydrogen using its infrared emission in two spiral galaxies, one face-on and one edge-on. The amounts of hydrogen in molecular form will be compared with the amount of ionized hydrogen across the two galaxies and compared with existing indirect methods for measuring quantities of hydrogen.

Starbursts and Active Galaxies

The most luminous galaxies in the universe are powered either by intense star formation (starbursts) or by accretion of material toward a massive black hole in a galaxy's center (active galactic nuclei, or AGN, of which quasars are the most extreme examples). Most of this luminosity appears as infrared radiation from dust which obscures the primary source of energy, whether starburst or AGN. A selection of 40 well known, bright AGN, starburst galaxies, and quasars will be observed with the IRS for later comparison with the many expected unclassified ultraluminous infrared objects which we expect to discover over the life of the mission.

Dusty Stars

Most objects which will be observed with Spitzer and the IRS are shrouded and obscured by solid particles (dust). Because infrared light penetrates dust much better than visual light, Spitzer can see many objects hidden to ordinary telescopes. But the dust itself also glows with the warmth of thermal radiation which Spitzer can see.

Much of this dust was originally produced in the atmospheres of giant and supergiant stars. In one IRS observing program some fifty of these stars, which are near the end of their lives, will be studied so that the spectral fingerprints of dust from a variety of sources will be on hand in order that unknown sources can be compared to this library, and a classification scheme of dusty sources produced.

Ultraluminous Infrared Galaxies

Ultraluminous Infrared Galaxies (ULIRGs) are prodigiously bright - but almost all of their radiation is emitted in infrared wavelengths. As a result, they were not detected until Spitzer's predecessor mission IRAS (Infrared Astronomical Satellite) in 1983.

These objects are among the most luminous objects in the universe - as bright as quasars, which they outnumber - but are so shrouded in dust that only infrared radiation can leak out.

Spitzer's cameras can identify new ULIRGs, but only the IRS can record the fingerprints that will reveal how these objects are powered. The abundances of certain types of dust and the temperature and excitation levels of gas in the cores of these objects will be measured for a selection of known ULIRGs - if similar objects are found by Spitzer at extremely large distances, their properties will be compared, and some understanding will be gained of the earliest bursts of star formation in the universe.

Wolf-Rayet stars and dust

Wolf-Rayet (WR) stars are massive objects ending their lives amidst great ejections of mass and dust. Since massive stars live such short lives, they are often found in their natal gas clouds among still-forming stars.

Their luminosity and dust production can dramatically influence the motions, chemistry, and temperature of these star-forming gas clouds, yet many details about WR stars and how they affect their environment are still unknown. The IRS will analyze the dusty environment around selected WR stars and for the first time record fingerprints that may allow the identification of specific properties of WR dust to distinguish it from other dust found in interstellar space.

The Galactic Center

The center of our Galaxy contains a number of unusual structures that manifest the physical extremes found there. The IRS will look at a variety of known objects there, including several that appear to be shocks produced when jets of gas of different speeds and temperatures collide. The sensitivity of the IRS and its ability to see infrared features heretofore hidden will enable the most precise measures yet made of the motions, temperatures, and interactions of gas and dust in our home galaxy - and enable us to better understand similar processes underway in the hearts of other galaxies.

Dust in the Early Universe

Some of the most distant objects known will be observed in this IRS project which seeks to understand the nature of dust in the early universe. What sorts of dust were formed first - silicate or rock-like dust, or hydrocarbon "smog-like" dust? Starting at the beginning will make it easier to trace the evolution of dust content in galaxies throughout the history of the universe.

Extremely Red Objects

Extremely Red Objects (EROs) are believed to be either very dusty galaxies in the process of massive amounts of new star birth, or alternatively, very old and quiescent elliptical galaxies. Since they are seen at probable distances that make them about half the age of the universe, each interpretation has a significant impact on our understanding. If they are starburst galaxies, then they produce a substantial fraction of stars in the universe at that epoch. If they are quiescent ellipticals, then the birth of these galaxies must have occurred earlier than is ordinarily thought.

The IRS, for the first time, can easily untangle the mystery by revealing the presence of dust and gas in these objects, and further map how the universe and the objects within evolved over its lifetime.

The NOAO Deep Wide-Field Survey

The largest area of the sky observed to the limits of ground-based telescopes will be surveyed by Spitzer with its cameras, and then the IRS will be unleashed to identify objects which cannot be otherwise classified by the colors of the objects in the survey images.

The primary science goal of these observations would be to determine distances, particularly of targets so faint that even the largest ground-based telescopes could not study them. This will reveal any population of high redshift galaxies or quasars so obscured that they cannot be identified in optical surveys.

Planets and Moons

Several of the planets and moons in the outer solar system will be studied with both the cameras and the IRS instrument aboard Spitzer. These instruments will map the temperatures of the surfaces and atmospheres (if any), will record the reflectivity from the surface in infrared wavelengths, and also study aspects of surface composition which are revealed from the infrared spectrum.

Together with earth-based observations and data collected by visiting spacecraft, the Spitzer data will contribute toward a better understanding of the origins and evolution of the moons with respect to their parent body.

Interacting Galaxies and Starbursts

Understanding the properties of starbursts is crucial for understanding large scale star formation in the universe. It is known that galaxy interactions can trigger starbursts. In this IRS project, a variety of starburst activity triggered by interaction with neighboring galaxies will be studied, and an attempt made to understand how the variations in composition, age, size, and motions in interacting systems affect the triggering and evolution of starburst activity.

Massive Stars and the Interstellar Medium

Massive stars radiate so brilliantly that they ionize and heat gas surrounding them. They live short but dramatic lives and do not usually move far from the areas of intense star formation that typically give them birth.

This IRS project will sample several areas in our galaxy in which rapid star formation is occurring, and measure the temperatures, excitations, densities, and other general physical properties of the environment surrounding massive stars. The goal is a better understanding of the interplay between these objects and the regions that spawned them.

Ring Galaxies

Ring galaxies are spectacular examples of galaxy transformation through gravitational interactions. They are created by the passage of a companion galaxy through the disk of a spiral along the rotation axis. This interaction reorganizes the spiral's disk, concentrating >90% of gas and dust in the galaxy into an expanding ring.

This ring may persist for up to 400 million years, with a gas density that promotes the growth of very massive star-forming cloud complexes. Some star-forming regions are so large that they may mimic the behavior of the large cloud collapses that give rise to globular clusters in the early history of galaxy formation.

All of the Spitzer instruments will be brought to bear on two dramatic ring galaxies to determine what triggers and regulates star formation in the ring.

Bipolar outflows in Young Stars

Jet-like flows of gas have now been detected around a number of young protostars and seem to be an important characteristic of the star-forming process. The IRS will examine two of the best-resolved examples of this phenomenon to better understand the physical conditions that exist in the jets and how they interact with the surrounding star-forming medium.

Towards a better Deuterium Ratio

In the first minute of the "Big Bang" which describes our early history, the expanding universe cooled rapidly. Nuclear reactions were only possible in the first few seconds as temperatures remained high enough for protons and nuclei to collide with one another. The remaining primordial gas carried with it a composition in which was imprinted the conditions and cooling rate of the universe' first moments.

Small amounts of deuterium, which is a fragile variety of hydrogen, were produced then. Measuring its quantities in undisturbed gas in the modern universe is a critical tool of cosmology and even of the particle physics which describes interactions in those hot, distant times.

Measuring deuterium along the sightlines to certain types of objects is now routine in astronomy. But the key to understanding its signature of earlier times is to know the ratio of deuterium to hydrogen. And getting reliable hydrogen densities along sightlines to a distant object has been problematic.

The IRS instrument is conveniently capable of recording certain diagnostic colors of light emitted by hydrogen which should provide a better measure of its quantity than previous methods. Together with additional infrared hydrogen and deuterium measurements that will be taken by the airplane-based SOFIA telescope which will begin operations this decade, a new degree of precision in deuterium ratio measurements will become possible.

Radio Galaxies and Quasars

These similar objects are thought to both be powered by the accretion of matter into giant black holes in their cores. The differences concern the rate of matter ingestion, the size and thickness of the disks surrounding the black holes, and different viewing angles for which we see the objects.

Astronomers have constructed models to simulate the behavior of the entire range of active galaxies, and these predict certain behaviors of these objects in the mid and far infrared spectrum. To date, it has been impossible to verify these predictions because no telescope has had the sensitivity. Spitzer and the IRS, together with the MIPS camera, will give the first proper test, and verify or correct our understanding of these rare but interesting objects.

Faint Obscured Galaxies

Several surveys that Spitzer will conduct with its cameras are expected to reveal many objects which are unseen in optical wavelengths. Most of these are expected to be distant active or starburst galaxies highly obscured by dust.

The IRS will study a sample of these to categorize the population of faint infrared sources without optical counterparts. In particular, an attempt will be made to establish distances to a number of these objects by gathering spectra of sufficient quality to allow determination of the redshift.

M-Dwarf Stars

M-dwarf stars are the lowest mass stars that support hydrogen burning in their cores. Studies of these objects are important for two reasons: they mark the transition in mass between planets and stars, and the low surface temperatures allows a rich molecular chemistry to occur, which is highly dependent on the temperature and abundances of the particular star.

Blue Compact Dwarf Galaxies

These interesting galaxies are nearby heavy-element poor small objects, which are thought to mimic the earliest episodes of star formation when primordial gas gave birth to the first stars. In these galaxies, the first, or more likely the second episode of star formation is now occurring.

The IRS will examine several of these objects to reveal the physical conditions present in the gas and dust in these objects, and to attempt to relate them to the much more distant objects that Spitzer will study that mark a much earlier epoch of the Universe.

Reflection Nebulae

Reflection nebulae are clouds of gas and dust that glow in both visible and infrared wavelengths. Some of their emission comes from scattering of light from the bright stars that illuminate them, but more comes from the radiation of excited atoms heated by these stars.

The IRS will examine seven reflection nebulae which are heated by stars of different temperatures, and which vary in other characteristics as well. The results will be compared with models that describe the transfer of energy from the high-energy radiation received from the stars to the mix of wavelengths radiated by the nebula.

Starburst Galaxies

A very thorough study of six nearby starburst galaxies will be made by the IRS instrument in this initiative. By combining ground-based optical data with studies by both the MIPS camera and the IRS instrument, a full account of gas, dust, star populations, and how they interconvert in these objects will be possible. These detailed studies of nearby, bright, resolved objects dovetail nicely with the "to the limit" efforts to find the furthest and faintest objects for which only sketchy details can be found in their weak spectra.

Brown Dwarfs

Only within the last decade have these enigmatic objects been discovered. Too heavy to be considered "ordinary planets", not bound to any star, but too light to ignite hydrogen in their cores, brown dwarves seem to be very numerous, only very unobtrusive.

Because of their low temperatures, these objects are most luminous in the near infrared. The IRS will attempt to understand previously unobservable features, such as the presence of dust and hydrocarbons in the atmospheres of these objects, and better refine our understanding of the temperature and chemical structure of their atmospheres. The clues to their origin that this may yield has direct consequences for our understanding of planet formation - and therefore our origins.

Planet Formation and Protostellar Disks

Two of IRS's programs take a detailed look at the disks around young stars. Considered the nursery for planet formation, young stellar disks were first detected only twenty years ago. Many have now been detected by the Hubble Space Telescope in star forming regions, but only by obscuration of visible light by disk material. The IRS will look directly at disk emission in the distance range from Mercury to Uranus. In particular, dips in the spectrum indicating absence of disk material at particular temperatures, and therefore distances, may give a hallmark to planet formation, as planets sweep sections of a disk clear of material.

The IRS will also look at eight specific resolved disks where it can sample regions at specific distances from the parent star without having to rely on indirect spectral methods to infer the distance of specific disk material.

Nearby Active Galactic Nuclei

As in the earlier survey of active galactic nuclei (AGN), the IRS will also look at a sample of the nearest such objects where the brightness and spatial resolution will permit the most detailed study to be made. This program will be used to bootstrap knowledge into the several studies that look at AGN at increasing distances.

Low Luminosity Radio Galaxies

It is known that in clusters of galaxies at increasing distance, the fraction of galaxies possessing low luminosity radio emission increases. It is not known whether these objects are starburst galaxies or some other population.

The IRS can directly resolve this question and will observe a sample of these objects.

Tidal Dwarf Galaxies

Tidal Dwarf Galaxies (TDG's) are formed from material stripped from the disks of spiral galaxies, which are undergoing tidal interactions with a nearby companion. These galaxies provide important clues to our understanding of galaxy formation, evolution and cosmic recycling.

The IRS will be used to gather measures related to star formation activity, and learn about this process in these objects, which have a different gas fraction and heavy element fraction than do Blue Compact Dwarfs, Starbursts, and other active star forming objects.

IRS Nearby Star Observations

Spitzer will conduct a survey of the nearest stars. Most of these objects are observed by the IRS in other programs devoted to M-star observations and nearby stars with known disk emission. The objects to be observed here are those that are not in this program nor are so bright that they would saturate the IRS instrument.

IRS Observations of Chandra Sources

The Chandra X-ray observatory, like Spitzer will do, has opened a unique new window on observations of the cosmos. Many sources have been discovered by Chandra which are optically faint or unidentifiable.

The IRS will be used to survey a variety of these sources in order to identify their nature and, if possible, observe spectral features to enable a distance determination to be made.

Unknown IRAS Sources

Spitzer's predecessor telescope, and the first astronomical infrared satellite, was the IRAS (Infrared Astronomical Satellite) instrument launched in 1983. Many IRAS sources remain unidentifiable or very faint in optical wavelengths, even with the largest ground-based telescopes.

The IRS will follow up on its heritage by conducting a survey to identify previously unidentifiable IRAS sources and when possible measure their distance. Planetary Nebulae The IRS will be used to observe a number of planetary nebulae in our galaxy's closest substantial companions, the Large and Small Magellanic clouds. The amount of heavy elements in these small galaxies is substantially less than than in our own galaxy. Therefore these objects, which consist of gas shells thrown off by dying stars but still illuminated by the brilliant high-temperature core of the parent star, are expected to exhibit phenomena different from those seen in our own galaxy.

Gamma Ray Bursts

Beginning with the nuclear age, monitoring spacecraft whose primary task was to detect unauthorized bomb tests have instead recorded bursts of gamma rays from space. Only within the last decade has some understanding of these objects been gained - and only a small number have actually been localized with new generation X-ray satellites to permit their discovery and follow-up by ground based telescopes.

These events are now known to emanate from distant galaxies, making them among the most energetic explosive events in the universe. But there is still controversy concerning their origin.

The IRS will be able to better identify the characteristics of the parent galaxies in which these bursts have been detected, and perhaps shed better light on their nature.

Outflows from Star Forming Regions

Stars in formation stir up violent motions in the gas from which they arise - particularly their jets and stellar winds plowing outwards into the dense medium surrounding them.

Several IRS projects study star forming regions - this one seeks to map the chemistry and temperature of shocks arising from the bulk motions in the gas.

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-- DonBarry - 28 Sep 2004

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