Chapter 8: The Antennae Galaxies
The galaxy pair NGC 4038 and NGC 4039 is commonly referred to as the Antennae Galaxies because their distorted streamer-like spiral arms look like insect antennae. Initially two separate spiral galaxies, gravity drew NGC 4038 and 4039 together in a titanic collision.
Figure 26: The Antennae - combined visible and radio image. This is a combination of a radio image by the VLA and an optical image from Cerra Tololo Observatory in Chile. You can easily identify two elongated arcs that intersect to form a heart-shaped center rotated clockwise 90 degrees. Start at 11 o'clock and trace down the first arc until you intersect with the two solid curved areas that are the centers of the two galaxies. This is where the second arc begins and continues down in a separate curve. As the two original spiral galaxies collided and passed through each other, the gas, dust and stars in the spiral arms were drawn out into long streamers of material. This material will either be lost to intergalactic space or fall back into the center of the merged galaxy. Although the central regions of the galaxies are very bright in visible light, the streamers are composed of mostly neutral hydrogen gas, which only emits radio waves.
Even though the galaxies are moving through each other at hundreds of kilometers or miles per second, the merging process takes a hundred million years because of the sheer size of galaxies. For the next three images we zoom in to the central region of the antennae, to see the ongoing collision up close.
Figure 27: The Antennae - visible image. The hollow circles identify the centers of each of the colliding galaxies. The most active star-forming regions are shown with solid irregular shapes. Star formation is taking place on a colossal scale! From the visible image alone, astronomers have identified more than 1000 bright, young star clusters bursting into existence. Such wide-spread star formation is a common result when spiral galaxies collide, unleashing shock waves that trigger the compression of hydrogen gas and dust into giant molecular clouds where new stars are born.
Figure 28: The Antennae - infrared image. Infrared light reveals stars that are hidden in the visible image. This Spitzer image highlights the huge warm dust clouds that are the cocoons of new star birth. It is only when the intense radiation from the young stars breaks through their dusty shroud that we detect their presence in optical light.
Figure 29: The Antennae - X-ray image. When galaxies collide, the stars within the galaxies simply pass by each other, such is their immense separation. Not so the giant clouds of gas and dust in the galaxy, which smash into each other triggering shock waves that initiate the huge burst of star formation. Note different border textures denoting different areas of X-ray intensity from gases heated by the collision and by multiple supernovae. As in the Whirlpool, you'll feel X-ray binary pinpoints that mark the locations of black holes and neutron stars as they pull gas off their companion stars. The brightest point sources are shown as small solid spots.