Infrared astronomy

Infrared astronomy is an experimental part of the field of astronomy, which uses the radiation emitted by astronomical objects infrared radiation. This radiation is in a part of the electromagnetic spectrum, which can not be perceived by the human eye.

• 3.1 Penetration of interstellar dust
• 3.2 Observation of cold objects

• 4.1 In the solar system
• 4.2 In the Milky Way
• 4.3 Outside our Milky Way

Observation area

The infrared region, also known as thermal radiation, situated between the optical (wavelength <700 nm ) and the sub-millimeter range (> 300 microns ) and is divided into three regions, the

• Near infrared (700 nm -4 microns )
• Mid-infrared (4-40 microns )
• Far infrared ( 40-300 microns ),

The exact limits vary both the infrared region and the subregions slightly depending on the source. In Astronomy, these regions are further divided into wavelength bands, in which the atmosphere is substantially transparent. These bands are designated with capital letters after the name of the optical filter, which can only radiation of the appropriate wavelength to happen: I ( 0.8 microns ), z ( 0.9 microns ), Y ( 1.0 microns ), J (1.25 microns ), H (1.65 micron ), K ( 2.2 microns ), L (3.45 microns ), M ( 4.7 microns ), N (10 microns) and Q ( 20 microns ). Outside these bands air containing water vapor is practically opaque.

Instrumental conditions

The above about 2 micron increasingly disturbing heat radiation of the atmosphere, the telescope and the instruments themselves imprinted in large part for tool development.

Locations for telescopes

Infrared radiation is strongly absorbed by the Earth's atmosphere, particularly by the atmospheric water vapor. Only less than 1 micron, and in some small windows to about 40 microns observation with ground-based telescopes is possible. Ground-based infrared telescopes are therefore preferably built on high, dry sites. Examples are the Mauna Kea Observatory or the observatories of the European Southern Observatory ( ESO). The ice sheets of Antarctica, because of their height, cold and dryness of interest. Often large telescopes are used for both optical and infrared observations, but there are also some specifically optimized for infrared observations telescopes.

As the altitude increases the absorption decreases strongly, since the 1960s, infrared telescopes were used in high-flying balloons and ballistic sounding rocket. Since the 1960s, also high-flying aircraft ( Lear Jet Observatory, Kuiper Airborne Observatory, SOFIA ) are used. In space, not only the atmospheric absorption disappears, it is also possible to cool smaller telescopes on the whole to very low temperatures and thus suppress their excessive heat radiation. Since the 1980s, therefore, space telescopes are increasingly being used for the infrared region, were the first IRAS and ISO, were more important ASTRO -F and Herschel. Currently (Jan 2014) are active Spitzer and WISE, both, however, only at shorter wavelengths, as the coolant are depleted. In the foreseeable future, the James Webb Space Telescope ( JWST ) will be started.

Instruments

The instruments of infrared astronomy are similar in concept to the cameras and spectrographs of visual astronomy. However, they must be over cooled. Most are used with liquid nitrogen or helium -cooled cryostat or mechanical cooling equipment. The optical materials such as lenses for use in the infrared region, however, differ from those commonly used for visible light.

Frequently Change infrared instruments in a chopping process called regularly observing direction between the object under study and an adjacent sky position. By subtracting the measured at both positions, the source signals can be lifted from the background better.

Since the 1990s, is possible for observations in the near infrared, the use of adaptive optics to correct the atmospheric turbulence ( seeing ). In order to achieve large ground-based telescopes their full diffraction-limited resolution and can compete in this respect with the Hubble Space Telescope.

Detectors

About the wide wavelength range of infrared astronomy several types of detectors are used. Up to about 1 micron wavelength normal, common in the visual astronomy CCD detectors are sensitive. For larger wavelengths specific detectors are required.

After the Second World War began with detectors made ​​of lead sulfide ( PbS ), the rise of infrared astronomy. Today functioning detectors are used particularly for the near infrared on the principle of photo-diode of semiconductor materials such as indium antimonide InSb and mercury cadmium telluride (Hg, Cd) Te. According to the principle of the photo- detectors operating resistance of doped silicon (e.g., Si: Ga) and germanium (for example, Ge: Ga ) are used at longer wavelengths. In addition, thermal detectors ( bolometers ), are now particularly at the longest wavelengths used. These demonstrate the thermal energy generated by the radiation in the detector. Until the 1980s, infrared detectors were almost always single detectors that had to be performed for larger images across the sky. Since then, detector arrays have become available at long wavelengths up to 2048 * 2048 elements in the short wavelengths and up to a few thousand elements.

Peculiarities of infrared astronomy

Penetration of interstellar dust

The attenuation (extinction) of electromagnetic radiation by interstellar dust varies greatly with wavelength. At 2 microns in the near infrared it has already fallen to the visible light to approximately 1/10. This verborgenene behind dust areas are observable, such as young stars, the galactic center and the nuclei of infrared galaxies.

Observation of cold objects

According to Planck's radiation law cold celestial bodies such as brown dwarfs or even deeply embedded in molecular clouds, stars radiate mainly in the infrared. Many common in the interstellar medium atoms, ions and molecules have important radiative transitions in the infrared. Particularly suitable is infrared spectroscopy to determine the composition and the physical conditions of the gas temperatures of a few hundred Kelvin. Cold (<100 Kelvin) dust in the interstellar medium emits the absorbed light in the far infrared again, and is often a great contribution to the energy balance of astronomical objects. In the mid-infrared, there is strong emission of organic compounds in the interstellar medium that are related to polycyclic aromatic hydrocarbons.

Due to the cosmological redshift the light emitted by galaxies in the early universe visible or UV light is observed on Earth in the near infrared. This is for example important for the interpretation of the James Webb Space Telescope.

Objects of observation and scientific objectives

In the solar system

Planets, satellites, comets and asteroids in our solar system are being closely monitored in the infrared. From IRAS eg some new asteroids and comets, and three bands of dust have been discovered in the region of the asteroid belt, which are probably caused by collisions within the asteroid belt. A new target are properties of trans-Neptunian objects in the Kuiper belt and the Oort cloud.

In the Milky Way

Many infrared observations in the Milky Way are aimed at understanding the formation of stars. Large-scale search for young stars in all stages of development and for brown dwarfs are combined with high -resolution images and spectroscopy. Circumstellar dust disks showed the first signs of the formation and evolution of planetary systems around other stars. In the Galactic center the vicinity of the next supermassive black hole is investigated in the infrared. Evolved stars and their mass ejection is a further object of infrared astronomy in our Milky Way.

Infrared spectroscopy is used to investigate the state and chemical composition of the interstellar medium. From IRAS also a diffuse infrared radiation and filamentary dust clouds has been discovered that extend up to high galactic latitudes.

Outside our Milky Way

Infrared galaxies shine in contrast to the Milky Way and most other galaxies up to 99 % of their total luminosity in the far infrared from. Interactions and collisions with other galaxies contribute to their formation. The infrared astronomy explores the contribution of high rates of star formation in starbursts and active galactic nuclei to this phenomenon.

The evolution of galaxies in the early universe is studied more and more intense in the infrared. In the near infrared, the red-shifted light from the stars of these galaxies is observed in the far infrared and submillimetre the choked by dust and re- radiated share.

Historical Development and Outlook

After William Herschel discovered infrared radiation in 1800 the sun, Charles Piazzi Smyth was in 1856 for the first time demonstrate an infrared component in the spectrum of moonlight. William Coblentz was from 1915 infrared radiation detected by 110 stars and is considered one of the founders of IR spectroscopy. These early measurements were usually obtained with bolometers or thermocouples.

In the 1950s, the lead sulfide (PbS ) detectors brought a sensitivity leap in the near infrared. As with many later detector developments for the near and mid-infrared astronomy benefited here from the military interest in sensitive detector systems, eg for tracking of aircraft and missiles. Around 1960 developed Harold L. Johnson and his group the first photometric system for the infrared. 1963 were carried out with the first balloon missions infrared observations of Mars and in 1967 was a series of rocket flights, the first mapping of the entire sky carried out in the mid infrared, in this case, for a total observation time of only 30 minutes, more than 2000 infrared sources have been discovered. In the same year the Mauna Kea Observatory was founded, which still houses the largest infrared telescopes today. Early 70s was a military C- 141A Transportjet converted to an infrared telescope, which performed in 14 km altitude from 1974 to the Kuiper Airborne Observatory ( KAO) observations.

The breakthrough of infrared astronomy, however, came in the 1980s with the first satellite missions. 1983 by IRAS studied the sky. COBE was launched in 1989 and discovered anisotropies of the cosmic background radiation. Followed in 1995 with the Infrared Space Observatory (ISO), the first true space observatory for the infrared camera, photometers and spectrometers. 1997 was followed by the upgrade of the Hubble Space Telescope with the infrared NICMOS instrument, 2003, the Spitzer Space Telescope was launched. 2009 started the missions Planck, Herschel and WISE.

The development of infrared astronomy is currently mainly in two directions:

• Observations with high spatial resolution from the ground, using adaptive optics or interferometry as the Very Large Telescope Interferometer (VLTI ). Planned giant telescopes such as the European Extremely Large Telescope are unthinkable without adaptive optics.
• Further increase in the sensitivity of aircraft and satellite telescopes. In the construction phase, the airborne observatory SOFIA and the Space Telescope James Webb Space Telescope. Are discussed for the more distant future TPF (NASA ) and Darwin ( ESA), with which the direct observation exosolarer planet might be possible for the first time.