The galaxy pair NGC 4038 and NGC 4039 are commonly referred to as the Antennae because wide-angle, visible-light photographs from ground-based telescopes are suggestive of antennae on an insect. They are located in the southern constellation of Corvus. These galaxies are in the process of undergoing a titanic collision, which actually started perhaps 100 million years ago, and are producing widespread bursts of star formation as a result.
Visible: DSS and Visible: AAO (right)
The visible-light images shown above are zoomed into the heart of the intergalactic collision, with the field of view being only 15 percent of the diameter of a full Moon. The black-and-white DSS image (above left) over-exposes the galaxies. Nonetheless, you can easily see some obscuring dust throughout the photograph, especially near the image center. A more detailed color image (above right) begins to reveal the spectacular nature of this galaxy pair. Both members of the interacting pair are thought to have originally been spiral galaxies. The yellowish light is from older stars, while the bluish light identifies regions where massive and hot young stars have recently formed from the maelstrom. The pink regions denote emission nebula within the turbulent interstellar medium of these colliding galaxies. Dark dust filaments are also seen throughout the photo. The points of light scattered around the periphery are foreground stars within our own Milky Way Galaxy.
Visible: HST, Mid-Infrared ISO, and Mid-Infrared Spitzer
The visible-light image (above left) is one of the best obtained to date with the Hubble Space Telescope (HST). The chevron-shape of the image is a byproduct of the unusual field of view afforded by the Wide Field/Planetary Camera-2 aboard HST. The surrounding black border has been added so that the image size matches the others in our gallery. The bright yellow-orange light identifies the center of each of the colliding galaxies. Twisting streams of blue-white light denote the regions where new stars are being born. As this image attests, star formation is taking place on a colossal scale. From this image alone, astronomers have identified more than 1000 bright and young star clusters bursting to life. Such widespread star formation is a common result when spiral galaxies collide. The collision rips through the interstellar medium (ISM) , the fabric of each galaxy, as the galaxies undergo their violent gravitational embrace. The resulting shock waves ripple through the ISM of each galaxy, triggering the compression of hydrogen gas and dust into giant molecular clouds, the nurseries from which new stars are born.
Now turn your attention to the mid-infrared view of ISO obtained with a camera aboard Europe's Infrared Space Observatory (ISO). The first observation to make is that the composite infrared image, obtained at wavelengths of 7 and 15 microns, is of relatively poor resolution. This image serves as an important example of the role technology plays in astronomy. The ISO detectors used to make this image were of lower resolution than those used in HST. In general, infrared detector array technology is about a decade behind the performance of comparable arrays used at optical wavelengths. However, recent and ongoing developments in IR technology hold great promise for a new generation of IR observatories, such as Spitzer. Moreover, the ISO telescope mirror was smaller than that aboard HST, and larger mirrors normally provide better resolution.
The higher resolution Spitzer image (above-right), shows more detail than the ISO image and reveals stars which are hidden in the HST image. This picture was taken by the infrared array camera and is a combination of infrared light ranging from 3.6 microns (shown in blue) to 8.0 microns (shown in red). The dust emission (red) is by far the strongest feature in this image.
Despite having lower spatial resolution, the ISO and Spitzer images offers intriguing comparisons with the HST photo. Take a moment to examine the relative orientation of emission peaks within each image, and then do the same between the images. It turns out that many of the brightest spots in the infrared image correspond to the darkest portions of the visible-light picture.
The yellow circles are superimposed on the nucleus of NGC 4038. This region is bright in the HST visible-light image (left), the ISO infrared photograph (center) and the Spitzer infrared image (right), although it is partly obscured by dust in visible light. Now shift your attention to the green square and the red diamond. In the HST picture, the area within the red diamond, corresponding to the nucleus of NGC 4039, is much brighter than the heavily obscured region inside the green square. It is this darkened area, however, that shines brightest in the mid-infrared! The area within the green square is the scene of massive star formation, mostly hidden by dust from our visible eyes. But the visible-light and ultraviolet photons from the hot and bright new stars are absorbed by the dust grains and are re-radiated at longer infrared wavelengths. You can think of dust as the processing agent that converts visible and UV light into IR emission. This comparison wonderfully illustrates why astronomers need to observe objects at a variety of wavelengths if they hope to successfully interpret the underlying phenomena.
Far-Infrared: IRAS and Radio: VLA
These images are long-wavelength depictions of the Antenna galaxies. In the low-resolution far-infrared image (above left), red denotes the brightest emission. The far-infrared photo, obtained at a wavelength of 60 microns, lacks adequate spatial resolution to reveal any detail. The oblong shape of the emission is a consequence of the unusual rectangular detectors used by IRAS in the early 1980s. The radio image (above right) shows CO (carbon monoxide) contours superposed on a 20-cm radio continuum image. CO tends to form in dense molecular clouds which are the sites where stars form in galaxies.
Finally, we present a high-resolution x-ray view of the Antenna galaxies, courtesy of the Chandra X-Ray Observatory (CXO), one of NASA's Great Observatories. The bright fuzzy emission represents superbubbles produced by the accumulated power of thousands of supernovae. These cosmic explosions are the fate of massive stars that live short lives. The same stellar population, albeit at an earlier evolutionary stage, was evident as blue-white light in the HST visible-light image studied earlier. The fainter fuzzy x-ray glow is produced by clouds of hot gas within the galaxies and by the accumulation of x-rays from countless fainter sources. On the other hand, the dozens of bright point-like sources are neutron stars or black holes in the process of stripping hot gas from nearby stars.