Lecture 11

Galaxy Encounters and Mergers

Galaxies have random velocities in space relative to the general Hubble Flow. About 20 years ago, it started to become quite clear that galaxies, far from being isolated systems, often encounter each other. These encounters can have dramatic consequences for the shape of the galaxies involved, leading to the formation of global structure, such as seen in grand design spirals, or to the complete reshaping of the galaxies' basic forms.

M31, or the Andromeda galaxy, is moving toward the Milky Way with a velocity of aboyut 70 km/s. This means that it will collide with the Galaxy, more or less directly, in about 1010 years, since it is about 700 kpc from us.

The consequences could be quite spectacular ---
 


Simulation of the Milky Way and Andromeda (M31) merging... (source http://www.cita.utoronto.ca/~dubinski/merger/bigmerger.html)

Collision of the Galaxy with Andromeda. The Galaxy is the smaller galaxy to the upper right in the first frame. The view is down the North Galactic Pole. The
sequence commences shortly after the collision 3 Gyrs into the future. The time between frames is 170 million years.  MOVIE


 

Artist's Impression of the event: (Source http://oposite.stsci.edu/pubinfo/pr/1997/34/af1.html)
 
The Andromeda galaxy appears simply as a spindle-shaped smudge of  light in the Northern autumn sky. Because it is 2.2 million light-years away -- or roughly 20 times the diameter of our Milky Way galaxy - it only appears four times the width of the full moon. As the two galaxies approach each other, Andromeda will grow ever larger in the sky, resembling an eerie glowing sword of light.
When the Andromeda galaxy and our Milky Way galaxy are close enough, clumps of cold, giant molecular clouds, each measuring tens to hundreds of light-years across, will be compressed. These dark knots will light up as millions of stars burst into life. Most of these stars will be in brilliant blue clusters, many of them 100 times brighter than the original globular star clusters already present in the two galaxies.
The disk of dust and stars that for billions of years marked the lanes of our galaxy and the Andromeda galaxy, will also begin to come apart under the gravitational pull of the two galaxies. As Andromeda swings past our galaxy, the sky will grow increasingly jumbled with tattered lanes of dust, gas, and brilliant young stars and star clusters. 
So many new stars will be born that the fraction of massive stars that are present will increase dramatically. These stars will begin popping off like a string of firecrackers as they self-destruct as supernovae.
As the stars gravitationally settle into their new home, through a dynamic process called "violent relaxation", any hint of the Milky Way and Andromeda as majestic spiral galaxies will be gone. The band known as the Milky Way will be gone, but far in the future some astronomers might gaze out onto a starry sky and look all the way into the core of the new elliptical galaxy. They would have no clue that there were once two majestic spiral galaxies, called the Milky Way and Andromeda by a long forgotten civilization.

 

This kind of process can studied using N-body simulations. In such simulations, a large number of stars are represented by point masses in the computer, and the orbit of each "star" within the total potential field of all the other stars is computed. Such simulations are gravitationally self-consistent.

Such simulations began with computers in the 1960's and 1970's, and already with a small number (a few hundred) of particles quite clear results could be obtained. Modern simulations can include up to a million particles. Note that computers are not nearly fast enough to simulate the actual number of particles (stars) in galaxies, which may be as high as 1012.  Computers are of course doubling their processing speeds about every 18 months, and will do for several doubling times to come.

Exampes: (see http://galileo.ifa.hawaii.edu/faculty/barnes/transform.html)

Collision between two disk galaxies

Overview

Gas only

Disk response

Dwarf formation

Retrograde collsion with smaller disk galaxy (here)

Two spirals which simulate the "Mice Galaxies"

Spiral galaxies really do collide : here are the famouse pair called the antennae galaxies


 
 

Here is a simulation of the antennae system forming  http://www.cita.utoronto.ca/~dubinski/antennae/antennae.html


Most galaxies contain gas, often quite a lot of it: treating this in computer codes is quite a challange, because the physics of gas clouds colliding is very poorly understood. However, the gas, while it might contain only 10 or 20% of the mass of a galaxy, can have a big effect on the outcome of a collision.

The following simulations show stars and gas treated together: (source http://zeus.ncsa.uiuc.edu:8080/BlueBook96/Galaxies.html)

Merger of Two Disk Galaxies

This sequence shows star formation in a merger of equal mass disk galaxies. Time is shown in the upper left corner of each frame, with unit time corresponding to 13 million years. Shortly after the initial collision (at T=24), the galaxies become quite distorted, forming long tidal tails and driving large amounts of gas to the inner portions of the galaxies. As the galaxies merge (at T ~ 65), a tremendous burst of star formation is triggered in the center of the merging galaxies, accounting for the extreme physical properties observed in the anomolous "ultraluminous infrared galaxies."

From "Ultraluminous Starbursts in Major Mergers" by J. Christopher Mihos and Lars Hernquist, Astrophysical Journal, 431, L9 (1994).

      Time sequence of a major merger

Merger with Satellite Galaxy

This sequence shows star formation in a disk galaxy accreting a small dwarf companion galaxy (shown as a white circle). Time is shown in the upper left corner of each frame, with unit time corresponding to 13 million years. As the companion falls into the disk galaxy, it triggers a stong inflow of gas and ensuing burst of star formation in the central kiloparsec of the disk. These types of mergers are thought to be common events in the life of many disk galaxies, and may explain the formation of many starburst galaxies and active galactic nuclei.

From "Triggering of Starbursts in Galaxies by Minor Mergers" by J. Christopher Mihos and Lars Hernquist, Astrophysical Journal, 425, L13 (1994).

      Time sequence of a satellite merger
 

One of the main difficulties in N-body simulations is to get stable models of isolated disks --- the entire three dimensional stellar system must be set up in a way which satisfies the Poisson-Boltzmann equations and remains time independent.

An example of such a disk galaxy is shown below...

This disk galaxy exhibits minor thickening with time and also you can easily see that transient spiral arms develop and disappear --- quite a common feature of simulations.


 

What will happen to merging galaxies? After a relatively short time, several billion years, many of these systems continue to evolve into what looks much like an elliptical galaxy. Hence, there is a possibility that elliptical galaxies are formed in this way.


 

One interesting discovery of this work is that small self-gravitating clumps can be formed which later become satellites of the final system! Such objects might correspond to some dwarf elliptical galaxies.

Ring Galaxies

One unusual type of galaxy is dominated by a ring. The classic example is the cartwheel galaxy, shown below.

2

A likely scenario for the formation of this galaxy is that a regular spiral has been penetrated by a smaller galaxy (probably the blue one of the two on the right), and in passing through the nuclear region it has set off a density wave of star formation which is still propagating outwards.

Theys and Spiegel, ApJ, 212, 616,1977


 

Capture of small galaxies


Galactic canabalism is the name given to the process by which larger galaxies accrete smaller units, with greater or lesser effect on the parent. At one extreme, the giant elliptical galaxies at the centers of galaxy clusters are though to have grown to the massive size by canabalising large numbers of regular galaxies which have strayed too close. Perhaps at the other extreme is the process by which regular disks like the Milky Way may have accreted a small number of small galaxies, without them fundamentally altering its shape. This is an interesting constraint on cosmological predictions of the number of small galaxies near a bigger one, since too many small galaxies can upset a disk structure rather easily.

Hernquist and Quinn, Apj, 403, 74


 

Shells


Impacts by small galaxies onto large ellipticals result in shells which have actually been observed. These shells are caused by the decaying orbit of the accreted system.
 


Quite a large number of shell galaxies were discovered in the 1980's when photographic techniques were developed which could reveal very faint structure in optical images.  (Source http://www.aao.gov.au/local/www/dfm/ngc1344.html)