Luminous infrared galaxy

Some space clouds are very bright.

They shine with a special light. This light is hard for eyes to see. These clouds make many new stars. Making stars helps them glow.

Often, two big space clouds crash together. This crash makes them even brighter. Dust and gas hide their regular light. The dust turns that light into special heat light. These bright spots are very far away. They are some of the brightest things in space.

Some galaxies are very bright. We call these LIRGs. LIRGs stand for luminous infrared galaxies. They give off most of their power as infrared light. This is a type of light we cannot see.

LIRGs are much brighter than our own Milky Way. They can make about 100 new stars every year. Our galaxy only makes one star a year. Many LIRGs happen when two galaxies crash into each other. This is called a merger. When galaxies merge, they create lots of gas and dust. This dust hides visible light. The dust then lets out heat as infrared light.

Some LIRGs are even brighter. We call the brightest ones ULIRGs. There are also HyLIRGs and ELIRGs. One ELIRG is 300 trillion times brighter than the Sun. It was found by a tool called WISE. These big galaxies often have a bright center. This center is an active galactic nucleus, or AGN. An AGN is a very active part of a galaxy. It can be a huge black hole.

Some galaxies are incredibly bright and special. We call them Luminous Infrared Galaxies, or LIRGs for short. These galaxies give off more energy in infrared light than in any other kind of light combined. Infrared light is a type of energy that our eyes cannot see. A single LIRG can be 100 billion times brighter than our Sun.

How do these galaxies get so much energy? Often, two or more spiral galaxies crash into each other. This event is called a merger. When galaxies merge, they create huge amounts of gas and dust. This dust acts like a thick blanket. It absorbs the visible light from stars. Then, the dust releases that energy as heat in the infrared spectrum. Many LIRGs also have an active galactic nucleus, or AGN, at their center. An AGN is a very active part of a galaxy that can release massive amounts of energy.

Scientists first discovered these galaxies in 1983. They used a tool called the Infrared Astronomical Satellite, or IRAS. This was a joint project between the United States, the Netherlands, and the United Kingdom. The IRAS mission looked at the whole sky for ten months. It found over 250,000 infrared sources. This helped scientists see galaxies that were hidden by dust. Later, NASA's WISE mission discovered even brighter objects.

There are different levels of brightness for these galaxies. ULIRGs are ultraluminous infrared galaxies. They are even brighter than LIRGs. HyLIRGs, or hyper-luminous infrared galaxies, are even more powerful. One HyLIRG named IRAS F10214+4724 is very bright because of gravitational lensing. ELIRGs, or extremely luminous infrared galaxies, are the brightest of all. The galaxy WISE J224607.57-052635.0 is an ELIRG. It is 300 trillion times brighter than the Sun.

These galaxies help us understand how the universe changes. A merger might start as a LIRG. It can then become a ULIRG and then a quasar. Finally, it might become an elliptical galaxy. We know this because stars in elliptical galaxies are much older. These massive galaxies also have huge black holes. Some black holes might grow very fast by breaking a rule called the Eddington limit. This limit usually controls how fast a black hole can eat gas.

Luminous infrared galaxies, or LIRGs, are massive systems that emit enormous amounts of energy. These galaxies are defined by having a luminosity above 10^11 solar luminosities. This means they are at least 100 billion times brighter than our Sun. LIRGs are unique because they emit more energy in the infrared spectrum than at all other wavelengths combined. They are more common in the universe than starburst galaxies, Seyfert galaxies, or quasi-stellar objects of similar brightness. Because they shine so brightly in infrared, they are often called submillimeter galaxies (SMGs) by researchers.

How do these galaxies produce such intense infrared light? The process usually involves massive amounts of gas and dust. In many LIRGs, the visible light from stars is absorbed by this thick dust. The dust then re-emits that energy as thermal energy in the infrared spectrum. This makes them appear much brighter in infrared than in the optical spectrum. Additionally, many LIRGs contain an active galactic nucleus, or AGN. An AGN is a highly energetic center of a galaxy. This center can contribute significantly to the total luminosity of the system.

Astronomers categorize these galaxies into different classes based on their brightness. LIRGs are the baseline group. Galaxies with luminosities above 10^12 solar luminosities are called ultraluminous infrared galaxies, or ULIRGs. Even brighter systems are known as hyper-luminous infrared galaxies, or HyLIRGs. These have luminosities exceeding 10^13 solar luminosities. The most extreme class is extremely luminous infrared galaxies, or ELIRGs. These represent the highest tier of infrared brightness discovered in the universe.

Scientists first discovered these infrared-heavy galaxies in 1983 using the Infrared Astronomical Satellite, known as IRAS. This was a joint mission between the United States, the Netherlands, and the United Kingdom. The IRAS survey lasted ten months and observed over 250,000 infrared sources. Before this mission, many of these galaxies were hidden from view. This is because their visible light is trapped by dust. The IRAS mission allowed scientists to determine the true luminosity of these objects. Later, NASA's Wide-field Infrared Survey Explorer, or WISE, discovered even more extreme examples like ELIRGs.

Many of these galaxies are the result of cosmic collisions. A common scenario involves the merger of two or more gas-rich spiral galaxies. A spiral galaxy is a flat, rotating disk with a central bulge of stars. When they collide, they create an early-stage merger, which can be identified as a LIRG. As the merger progresses into a late stage, it becomes a ULIRG. This evolution may continue until the system becomes a quasar. Eventually, the process ends when the system becomes an elliptical galaxy. We know this because stars in elliptical galaxies are much older than those in earlier stages.

Some specific examples show the incredible scale of these objects. The ULIRG Arp 220 is the closest known example and is currently merging two galaxies. Markarian 231 is another ULIRG that contains a quasar. One HyLIRG, IRAS F10214+4724, has an intrinsic luminosity of about 2 x 10^13 solar luminosities. Its light is actually amplified by a factor of 30 due to gravitational lensing. The most luminous galaxy found as of 2015 is the ELIRG WISE J224607.57-052635.0. This galaxy is 300 trillion times brighter than the Sun. Its light has traveled for 12.5 billion years to reach us.

These extreme galaxies also provide clues about the growth of black holes. The black holes in ELIRGs are massive, sometimes billions of times the mass of the Sun. Scientists suggest they might grow by exceeding the Eddington limit. This limit is a balance where the pressure of emitted light pushes gas outward. This pressure normally limits how fast a black hole can absorb matter. However, a black hole might break this limit to grow very quickly. Another possibility is that slow-spinning black holes absorb gas more easily. This allows them to accumulate mass over a very long time.