User:LIRRRRG/sandbox/Luminous Infrared Galaxies

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Luminous Infrared Galaxies or (LIRG's) are galaxies with luminosities, the measurement of brightness, above 1011 L. LIRG's are more abundant than Starburst galaxies, Seyfert galaxies and Quasi-stellar objects at comparable luminosity. Infrared galaxies emit more energy in the infrared than at all other wavelengths combined. An LIRG's luminosity is 100 billion times that of our sun.

Galaxies with luminosities above 1012 L, are Ultra Luminous Infrared Galaxies (ULIRG's). Many of the LIRG's and ULIRG's are showing interactions and disruptions. Many of these type of galaxies spawn about 100 new stars a year as compared to ours which spawn one a year, this creates the high level of luminosity.

Origin and Evolution of LIRG's[edit]

Infrared galaxies appear to be single, gas rich spirals whose infrared luminosity is created largely by the formation of stars within them. [1] However some galaxies' luminosity come from an Active Galactic Nuclei or AGN. These AGN reside in a compact region at the centre of a galaxy and have a higher than normal luminosity. The emission of the AGN has been observed in the radio, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. These types of galaxies were discovered in 1983 with IRAS. In some cases an LIRG's luminosity may come from starburst, the creation of stars, and also an active galactic nuclei.[2]

These galaxies contain more energy in the infrared portion of the spectrum, not visible to the naked eye. The energy given off by LIRG's is comparable to that of a quasar, which before was known as the most energetic object in the universe.[3]

The reason that we are not able to see the light from these galaxies is because they are gas rich, meaning that gases within the galaxy absorb most of the light and re-emit it in the infrared. A large fraction of galaxies in the universe with extreme luminosities such as LIRG's are known to have an active black hole. LIRG's are known to live in denser parts of the universe than non-LIRG's.

South America Galaxy taken by the Hubble Space Telescope[4]

Three known LIRG's are:

II Zq 96-an object where a pair of galaxies are merging together

NGC 6240-a well studied nearby infrared galaxy in the constellation Ophiuchus.

Arp 220-the closest LIRG, it is in the process of merging two galaxies.


LIRG's are also capable of becoming ULIRG's but there is no perfect timetable because not all LIRG's turn into ULIRG's, Newtonian mechanics is used in the calculations and because the constraints are not quite approximate. Studies have shown that ULIRG's are more likely to contain an active galactic nuclei than LIRG's[5]

According to one study an Ultra-Luminous Infrared Galaxy is just part of an evolutionary galaxy merger scenario. In essence, 2 or more spiral galaxies, galaxies that consist of a flat, rotating disk containing stars, gas and dust and a central concentration of stars known as the bulge, merge together to form an early stage merger. An early stage merger in this case can also be identified as a LIRG. After that, it becomes a late stage merger, which is an ULIR. It then becomes a quasar and in the final stage of the evolution it becomes an elliptical galaxies. [6] This can be proven by the fact that stars are much older in elliptical galaxies than those found in the earlier stages of the evolution.


The Infrared Astronomical Satellite(IRAS) was the first all-sky survey which used far-infrared wavelengths in 1983. In that survey, tens of thousands of galaxies were detected, many of which would have not been recorded in previous surveys. It is now clear that the reason the number of detections have risen is because the majority of LIRG's in the Universe emitted the bulk of their energy in the far-infrared. Using the IRAS, scientists were able to determine the luminosity of the galactic objects discovered. The telescope was a joint project of the United States (NASA), Netherlands (NIVR), and the United Kingdom (SERC). Over 250,000 infrared sources were observed during this 10 month mission.


The Great Observatories All-Sky(GOALS) is a multi-wavelength study of luminous infrared galaxies. [7] Using information from Nasa's Spitzer, Hubble, Chandra and Galex observations in a study over 200 of the most luminous infrared selected galaxies in the local universe. [8] Approximately 180 LIRGs were identified along with over 20 ULIRGs. The LIRGs and ULIRGs targeted in GOALS span the full range of nuclear spectral types (type-1 and type 2 Active Galactic Nuclei, LINERS's, and starbursts) and interaction stages (major mergers, minor mergers, and isolated galaxies).


  1. ^ "Luminous Infrared Galaxies". Retrieved 23October2013.  Check date values in: |access-date= (help)
  2. ^ "The AGN-Starburst Connection in Warm Infrared Galaxies". Retrieved 12november2013.  Check date values in: |access-date= (help)
  3. ^ "The Curious History of Luminous Infrared Galaxies". Retrieved 24October2013.  Check date values in: |access-date= (help)
  4. ^ "A very bright contortionist". ESA/Hubble Picture of the Week. Retrieved 14 June 2013. 
  5. ^ "Star-formation, AGN and Ultra-luminous infrared galaxies". Retrieved 12novemeber2013.  Check date values in: |access-date= (help)
  6. ^ "The Curious History of Luminous Infrared Galaxies". Retrieved 24October2013.  Check date values in: |access-date= (help)
  7. ^ "Mid-Infrared Properties of Nearby Luminous Infrared Galaxies I: Spitzer IRS Spectra for the GOALS sample". Retrieved 23October2013.  Check date values in: |access-date= (help)
  8. ^ "GOALS". Retrieved 24October2013.  Check date values in: |access-date= (help)

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