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Accretion: An object with a large mass has a large gravitational pull on matter in its vicinity. Using this ability, a supermassive black hole is able to swallow a lot of nearby matter. The latter process is refered to as accretion.
Bridges: A band or rather, stream, of stars connecting the nuclei of the two interacting galaxies. A good example of a bridge can be seen in the image of the pair of interacting galaxies known as `The Mice'.
Diluted infrared light: The brightness of a bulb, star or galaxy falls of with the square of the distance, because the light gets spread out (is diluted) over a larger and larger area around the source. Hence the brightness decreases by a factor 4 by doubling and a factor 9 by tripling the distance.
Dust: What do astronomers know about dust? Do they ever vacuum? In astronomy `dust' is a term used for tiny particles composed of silicates (sand) and carbonaceous substances (soot or coal) that are a less than 1% component of gas clouds. Despite their small numbers, dust is very effective at blocking light from objects behind it. A good example is the dark cloud `Barnard 68'.
Event horizon: A black hole is black because light trying to escape does not have a high enough velocity to actually escape the gravtitational pull of the black hole. The event horizon is the distance from the center of the black hole at which the gravitational pull of the black hole has dropped to a level that light that is produced there can escape. The event horizon is clearly visible in this artist impression of a black hole and accretion disk.
Feature: In this context, another word for a distinct wobble in a spectrum that we indentify as a finger print of a chemical substance. PAH molecules have strong PAH features in the infrared spectrum, which serve as their finger prints. Also silicates (sand) can be recognized by their strong features in infrared spectra.
Glow from dust grains: Warm dust grains give of a glow, just like a camp fire or an infrared heater do. The perceived glow actually is infrared light which we would see if our eyes would be sensitive to infrared light.
Gravitational energy: You know you've got gravitational energy in you when you jump off a diving board and fall into the pool without any effort! During the free fall, stored gravitational energy is released and turned into 'kinetic' energy (speed energy). Upon impact on the water the kinetic energy is used to create a splash, waves and short-lived currents. Were Earth's gravitational pull twice as low, you would have only half the gravitational energy to play with. If you replace yourself by a star and Earth by a supermassive black hole, you have got a lot of gravitational energy to loose by spiraling down into the black hole!
Gravitational interaction: Any object with mass exerts an attracting force on another object with mass. Without this force galaxies would fly by each other rather than interact (change course) and merge. Gravitational interaction between Earth and you keeps your feet firmly on the ground and the Earth in orbit around the Sun.
Infrared light: A color of light redder than the human eye can see. Infrared light has wavelengths that are at least twice as long as green light. Besides visible light and infrared light there are many more types of light. These differ from each other by their wavelength. Click here for a nice graphic overview of existing colors and note what a tiny fraction of colors we call visible light!
Lightyear: The distance traveled by light over the period of a year. Given the speed of light (299 792.458 km/s or 186 282.397 miles/s) this amounts to 9 460 730 472 580.8 km or 5 878 625 373 183.61 miles.
LIRG: Luminous InfraRed Galaxy. These galaxies produce at least a hundred billion times the power of our Sun in infrared light. Most of them are colliding galaxies.
Luminosity: Light power emitted by an object. The luminosity of a 75 Watt light bulb is 75 Watt, that of the Sun is 385 000000 000000 000000 000000 Watt! Luminosity should not be confused with brightness. The latter decreases with distance, the former is a property of the bulb, star or galaxy which does not change. A 75 Watt bulb is still a 75 Watt bulb even if it is a mile away.
Merger phase: Other word for stage of collision. This assumes that as a result of a galaxy collisions the two galaxies merge to become one bigger galaxy. Alternatively, under the right conditions, the galaxies may part ways again.
Solar mass: A solar mass is the amount of mass contained in the Sun. Astronomers rather express the mass of an astronomical object in solar masses than in kilograms or tons. One solar mass amounts to 2 000 000 000 000 000 000 000 000 000 000 kg. That's why.
Soot: particles found in smoke and flames, based on carbon. The yellow/orange color of flames in a wood fire or candle flame is caused by particles of soot set aglow by the burning fire.
Tidal tails: Long filaments of stars and gas clouds ejected from colliding galaxies in the collision process. The most famous tidal tails give `the Antennae' galaxies their name.
ULIRG: Ultra-Luminous InfraRed Galaxy. These galaxies produce at least a thousand billion times the power of our Sun in infrared light. Nearly all of them are colliding galaxies.
