What happens when galaxies collide?

What happens when galaxies collide?
When two nearby galaxies attracted to each other by their gravitational force (a relatively common event in galaxy evolution), their gases collide violently. As gas and dust smash together at speeds of millions of miles an hour, they combine the raw materials for new born stars in an amazing hot and bright display, and eventually -in some cases- causes a super massive black hole from this collision.

Different Phases of Galactic Collisions
Over millions of years, new stars and even new galaxies can form this way. Highly energetic collisions could also be the cause of rare cosmic objects called quasars, which, despite being much smaller, can be hundreds of times as bright as giant galaxies, allowing them to be observed from great distances.

Whereas some galaxies formed on their own, many others, including our galaxy, the Milky Way, probably developed from the collisions and mergers of smaller ones. And in turn, collisions between two spiral galaxies, like ours, are thought to form still larger elliptical galaxies. Andromeda and the Milky Way are among the largest galaxies in our galactic cluster, known as the Local Group.

Andromeda, which is located about two million light-years away from our galaxy, is on track to collide with it in three to four billion years. When this occurs, many new stars will form and others will go supernova. Nearby exploding stars will probably render Earth uninhabitable. That is, if a larger, hotter sun—something our star will become—hasn’t already done so.

Notable interacting galaxies:

Whirlpool Galaxy (M51), located at a distance of approximately 23 million light years.
Antennae Galaxies (NGC 4038/9).
NGC 2207 and IC 2163.
NGC 520, a pair of colliding spiral galaxies, going through the third phase in galactic collision, is about 90 million light-years away, in the constellation Pisces.
NGC 1097, a two satellite galaxies. NGC 1097A is the larger of the two. It is a peculiar elliptical galaxy that orbits 42,000 light-years from the center of NGC 1097. NGC 1097B is the outermost one and not much is known about that. also has a supermassive black hole at its center.
Mice Galaxies (IC 819/20), About 290 million light-years away, currently they are going through the second phase in galactic collision.

To celebrate the 18th anniversary of the launch of Hubble,
the Space Telescope Science Institute released these images
of galaxies colliding.

Galaxy collisions are now frequently simulated on computers, with all the realistic physics, including gravity forces, gas dissipation, star formation and feedback. Dynamical friction slows down galaxy pairs, which may or may not merge at some point, according to the initial relative energy of the orbits. A few video shows the collision process using various Astor-physics simulators, like the videos below.

4 billion years from now, after M31 pass through our Milky Way, a view of Earth's nighttime sky shows that Andromeda is tidally stretched out and the Milky Way is also warped around (Image: NASA / STScI).

A timeline image of the present day line-up for the collision between our Milky Way galaxy and the Andromeda galaxy. The galaxies are moving toward each other, and being accelerated by their mutual gravity to their final dance (Image: NASA / STScI).

Mosaic image of Earth's night sky as the collision of M31 and the Milky Way occurs: 1. (Upper left): Present - the bright belt of our Milky Way stretches across the sky, while the Andromeda galaxy looks like a small faint foggy object, several times the diameter of the full Moon. 2. (Upper right): 2 Billion Years from now - M31's approaching disk is noticeably larger. 3. (2nd row left): 3.75 Billion Years from now- Andromeda fills the field of view. The Milky Way begins to show distortion due to tidal pull from Andromeda. 4. (2nd row right)-5(3rd row left): 3.85-3.9 Billion Years - during the first close approach, the sky is ablaze with new star formation, which is evident in a plethora of emission nebulae and open young star clusters. 6. (3rd row right): 4 Billion Years - After its first close pass, Andromeda is tidally stretched out. The Milky Way, too, becomes warped. 7. (Final row left): 5.1 Billion Years - During the second close passage, the two galactic cores maintain their separate identity. The level of star formation is much smaller because interstellar gas and dust has been reduced during earlier stages of the collision. 8. (Final row right): 7 Billion Years - The merged galaxies form a huge elliptical galaxy, its bright core dominating the nighttime sky.

Sources: Science Illustrated Magazine Jul/Aug 2010, Wikipedia, NASA and various, http://astromic.blogspot.gr

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