This galactic pair is a golden oldie, but it is nice to see it - them
As a color commentator, I must say something about the colors of this particular version of the well-known Hubble image. The Antennae galaxies are two colliding galaxies, similar in size, but in other respects they were quite different before they collided. One of them, NGC 4038, was rich in gas, possibly
similar to a galaxy like NGC 3184
, with a yellow center and a gas-rich disk. The other one member of the Antennae, NGC 4039, was decidedly gas-poor before it collided. It may well have been a lenticular galaxy
, possibly similar to NGC 524
, but smaller.
As the gas-poor and the gas-rich galaxies collided, fascinating things happened to the gas. Parts of it formed a brilliant starforming "arc", turning NGC 4038 into "half a Cartwheel galaxy
". Much of the rest of the gas ended up "at the impact site", forming a tremendous, dark dust cloud bursting with new stars. In today's APOD, this truly gigantic star forming region is the enormous central brown dust area full of yellow, dark orange, red and magenta light.
Just possibly the only area in NGC 4039 that had a gas supply of its own is the central area. Today's APOD shows a small blue arc "hanging down" from the center of NGC 4039. Fascinatingly, the blue arc seems to be connected to the nuclear area by a thin bright "stalk".
NGC 4038, by contrast, is full of bright new stars. It has many extremely bright emission nebulae (likely enhanced in visibility in this picture), and young blue stars are streaming out from these star forming areas in a tangential direction. Note one such "stellar fountain" at about 9 o'clock, another one at about 11 o'clock and a third one at about 12 o'clock.
One of the links in the caption of today's APOD is this one
, which shows the full disk of NGC 4039. Note how large, "undisturbed" and yellow it is. Before its collision, this must indeed have been a galaxy with a yellow, non-starforming disk. Note, nevertheless, the dark dust features seen in silhouette against the yellow disk. They are somewhat similar to the brown dust features in the otherwise all yellow galaxy NGC 1316
, except that the dust features in NGC 1316 are much larger and darker. NGC 1316 is a post-collisional galaxy, one where all the starforming material has been spent and only the ashes, the dark dust, remain.