Luminous infrared galaxy
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. A 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.
More luminous than ULIRGs are the HLIRG, Hyper-Luminous Infrared Galaxies.
The most luminous class are the ELIRG, Extremely Luminous Infrared Galaxies.
Origin and Evolution of LIRG's
Infrared galaxies appear to be single, gas rich spirals whose infrared luminosity is created largely by the formation of stars within them. However some galaxies' luminosity come from an Active Galactic Nucleus or AGN. These AGNs reside in compact regions at the centres of galaxies and have 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 nucleus.
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.
The reason that we are not able to see the optical 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.
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 nucleus than LIRG's
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 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. 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.
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The Extremely Luminous Infrared Galaxy WISE J224607.57-052635.0, with a luminosity of 300 trillion suns was discovered by NASA's Wide-field Infrared Survey Explorer (WISE), and as of May 2015 is the most luminous galaxy found. The galaxy belongs to a new class of objects discovered by WISE, extremely luminous infrared galaxies, or ELIRGs.
Light from the WISE J224607.57-052635.0 galaxy has traveled 12.5 billion years. The black hole at its center was billions of times the mass of our sun when the universe was a tenth (1.3 billion years) of its present age of 13.8 billion years.
There are three reasons the black holes in the ELIRGs could be massive. First, the embryonic black holes, might be bigger than thought possible. Second the Eddington limit was exceeded. When a black hole feeds, gas falls in and heats, emitting light. The pressure of the emitted light forces the gas outward, creating a limit to how fast the black hole can continuously absorb matter. If a black hole broke this limit, it could theoretically increase in size at a fast speed. Black holes have previously been observed breaking this limit; the black hole in the study would have had to repeatedly break the limit to grow this large. Third, the black holes might just be bending this limit, absorbing gas faster than thought possible, if the black hole is not spinning fast. If a black hole spins slowly, it will not repel its gas absorption as much. A slow-spinning black hole can absorb more matter than a fast spinning black hole. The massive black holes in ELIRGs could be absorbing matter for a longer time.
Twenty new ELIRGs, including the most luminous galaxy found to date, have been discovered. These galaxies were not found earlier because of their distance, and because dust converts their visible light into infrared light.
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. 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. 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).
Some examples of extremely notable LIRGs, ULIRGs, HLIRGs, ELIRGs
|WISE J224607.57-052635.0||ELIRG||22h 46m 07.57s||−05° 26′ 35.0″||Discovered in 2015, the most luminous galaxy known, as of 2015|||
|II Zw 96||LIRG||an object where a pair of galaxies are merging|
|NGC 6240||LIRG||Ophiuchus||a well studied nearby infrared galaxy|
|Arp 220||LIRG||the closest LIRG, it is in the process of merging two galaxies.|
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