NGC 4038, NGC 4039 - Antennae Galaxies

NGC 4038 and NGC 4039 are known as the Antennae, or sometimes called the "rat-tail" galaxies. They are located near western edge of Corvus.

Discovery and Observation

This pair of interacting galaxies was discovered by William Herschel on February 7, 1785, and first classified as a planetary nebula. His son, John Herschel, cataloged them under separate numbers, h 1052 and h 1053, in his 1833 catalog. He carried them over into his 1864 General Catalog as GC 2670 and GC 2671; from there, they found their way into Dreyer's NGC.

In small telescopes, the pair have a high surface brightness, and appear slightly blue. NGC 4038 and 4039 have a visual magnitude of 10.5 and 10.3, respectively, and their cores appear both 5.4 arc minutes across. The nuclei of the two galaxies are joining to become one giant galaxy; two long streamers of ejected stars, gas and dust extend outward from their centers, making two long tails which resemble the antennae of an insect.

Properties and Evolution

Detailed images captured by the Hubble Space Telescope show over a thousand bright young star clusters in the cores of Antennae, the result of a burst of star formation triggered by their collision. These bright knots are infant globular clusters, newly born out of collisions between giant hydrogen clouds in the two galaxies. At the other end of the stellar evolutionary spectrum, images taken by the Chandra X-ray Observatory have revealed dozens of bright point X-ray sources that are probably neutron stars or black holes tearing gas off nearby stars. These collapsed objects are the remains of large stars that formed earlier in the burst of star formation triggered by the collision, and have already died. Two supernovae have been discovered in these galaxies: SN 2004GT and SN 2007sr.

The Antennae lie about 63 million light-years away, in the NGC 4038 Group of galaxies in the constellations Corvus and Crater. The group's best known galaxies are NGC 4038 and 4039; but NGC 3956, 3957, 3981, 4024, 4027, 4033, and 4050 have also been consistently identified as group members. The group may contain between 13 and 27 galaxies. A recent study finds that these interacting galaxies may closer than previously thought, at 45 million light-years.

In any case, the Antennae are the nearest and youngest example of a pair of colliding galaxies. About 1.2 billion years ago, the Antennae were two separate galaxies; NGC 4038 was a spiral, and NGC 4039 was a barred spiral. Before the galaxies collided, NGC 4039 was probably larger than NGC 4038. 900 million years ago, the Antennae began to approach one another, appearing similar to NGC 2207 and IC 2163. 600 million years ago, the Antennae passed through each other, like the Mice Galaxies (NGC 4676 in Coma Berenices). 300 million years ago, the collision began to release the Antennae's stars, as each galaxy's gravitation has drawn out a curved tail of stars from the other. Today the two streamers of ejected stars extend far beyond the original galaxies, spanning a total of some 360,000 light-years.

Within 400 million years, the Antennae's nuclei will collide and become a single core with stars, gas, and dust around it. Observations and simulations of colliding galaxies suggest that the Antennae will eventually form a single elliptical galaxy. This is likely the future of our Milky Way when it collides with the Andromeda Galaxy; most galaxies probably undergo at least one significant collision in their lifetimes.