Name: NGC 2207 and IC 2163
Description: Merging Galaxies
Position: R.A. 06h 16m 24.9s Dec. -21° 22' 26"
Constellation: Canis Major
Image Credit: NASA and The Hubble Heritage Team
Distance: 35 Mpc (114 million light-years)
Diameters: NGC 2207 has a diameter of 143,000 light-years
                 IC 2163 has a diameter of 101,000 light-years
Dimensions: The image is 2.5 arcminutes on the vertical side.
Instrument: WFPC2
Exposure Dates: May 25, 1996; November 11, 1998
Exposure Time: 4.5 hours
Filters: F336W(U), F439W(B), F555W(V), F814W(I)
Release Date: November 4, 1999
Orientation: North is toward the bottom of the image.

In the direction of the constellation Canis Major, two spiral galaxies pass by each other like majestic ships in the night. The near-
collision has been caught in images taken by NASA's Hubble Space Telescope and its Wide Field Planetary Camera 2.

The larger and more massive galaxy is cataloged as NGC 2207 (on the left in the Hubble Heritage image), and the smaller one
on the right is IC 2163. Strong tidal forces from NGC 2207 have distorted the shape of IC 2163, flinging out stars and gas into
long streamers stretching out a hundred thousand light-years toward the right-hand edge of the image.

Computer simulations, carried out by a team led by Bruce and Debra Elmegreen, demonstrate the leisurely timescale over which
galactic collisions occur. In addition to the Hubble images, measurements made with the National Science Foundation's Very
Large Array Radio Telescope in New Mexico reveal the motions of the galaxies and aid the reconstruction of the collision.

The calculations indicate that IC 2163 is swinging past NGC 2207 in a counterclockwise direction, having made its closest
approach 40 million years ago. However, IC 2163 does not have sufficient energy to escape from the gravitational pull of
NGC 2207, and is destined to be pulled back and swing past the larger galaxy again in the future.

The high resolution of the Hubble telescope image reveals dust lanes in the spiral arms of NGC 2207, clearly silhouetted against
IC 2163, which is in the background. Hubble also reveals a series of parallel dust filaments extending like fine brush strokes along
the tidally stretched material on the right-hand side. The large concentrations of gas and dust in both galaxies may well erupt into
regions of active star formation in the near future.

Trapped in their mutual orbit around each other, these two galaxies will continue to distort and disrupt each other. Eventually,
billions of years from now, they will merge into a single, more massive galaxy. It is believed that many present-day galaxies,
including the Milky Way, were assembled from a similar process of coalescence of smaller galaxies occurring over billions of years.

Understanding the Discovery

1. How do galaxies meet?
Two galaxies don't have to bump into each other to meet. They merely have to pass close enough to get caught up in each other's
gravitational stranglehold. And "close" could mean 100,000 light-years, the distance between the galaxies in this Hubble picture.
Galaxies possess gravitational forces that can slowly pull close objects toward them. More massive galaxies have stronger
gravitational forces. In this picture, the strong gravitational forces of the heftier galaxy, NGC 2207, lock both objects in an orbital
embrace. These galaxies are the closest they've been in 40 million years. They are destined to continue swirling around each
other, slowly falling closer together until the two become one beefy galaxy.

2. What happens after they merge?
Spiral galaxies contain large concentrations of gas and dust. A merger between two spirals condenses the gas clouds, igniting
star birth. Astronomers believe that many of today's galaxies, including the Milky Way, were assembled in the same way. The
galaxies in this image reside 114 million light-years from Earth in the direction of the constellation Canis Major.