Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above 1011 L☉. LIRGs 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 ultraluminous infrared galaxies (ULIRGs). Many of the LIRGs and ULIRGs are showing interactions and disruptions. Many of these types 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 hyper-luminous infrared galaxies, HLIRG.
The most luminous class are the extremely luminous infrared galaxies, ELIRG.
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 comes 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, visible, ultra-violet, X-ray and gamma ray wavebands. These types of galaxies were discovered in 1983 with IRAS. In some cases a LIRG's luminosity may come from starburst, the creation of stars, and also an AGN.
These galaxies emit more energy in the infrared portion of the spectrum, not visible to the naked eye. The energy given off by LIRGs is comparable to that of a quasar, which formerly was known as the most energetic object in the universe.
The reason that one is not able to see the visible light from these galaxies is that 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 LIRGs are known to have an active black hole. LIRGs are known to live in denser parts of the universe than non-LIRGs.
LIRGs are also capable of becoming Ultra Luminous Infrared Galaxys (ULIRGs) but there is no perfect timetable because not all LIRGs turn into ULIRGs, Newtonian mechanics is used in the calculations and because the constraints are not quite approximate. Studies have shown that ULIRGs are more likely to contain an AGN than LIRGs
According to one study a ULIRG is just part of an evolutionary galaxy merger scenario. In essence, two 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 a ULIRG. It then becomes a quasar and in the final stage of the evolution it becomes an elliptical galaxy. This can be evidenced by the fact that stars are much older in elliptical galaxies than those found in the earlier stages of the evolution.
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 rate. 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 not have been recorded in previous surveys. It is now clear that the reason the number of detections has risen is that the majority of LIRGs 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 LIRG Survey (GOALS) is a multi-wavelength study of luminous infrared galaxies, incorporating observations with NASA's Great Observatories and other ground and space-based telescopes. 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||ULIRG||Ophiuchus||a well studied nearby infrared galaxy|
|Arp 220||ULIRG||the closest ULIRG, it is in the process of merging two galaxies.|