Full-Synthesis Images of CO(1-0) Emission in ``the Antennae'' (Arp 244)



[CO and HST images of Arp 244]

Figure 1:

CO(1-0) emission is shown in contours overlaid on a false-color image of Arp 244 (``The Antennae'') that was made from archival HST WFPC2 data (made possible by the work of Brad Whitmore). This image is a full-synthesis map which combines observations made with BIMA (The Berkeley - Illinois - Maryland Assosciation) Array and the NRAO 12m radio telescope. Below is a short description of exactly what "full-synthesis" means, but first a few more words about the image :)

The white contours shows CO(1-0) emission. CO is carbon monoxide, it tends to form in dense clouds of gas which are generally referred to as ``molecular clouds.'' Molecular clouds are the sites where stars form in galaxies. This image of the ``the Antennae,'' which are two spiral galaxies that have recently ( < 100,000,000 years) passed a little too close to one another. The result is that the gas in the disks of these galaxies has been stirred up and large dense concentractions have formed in each nucleus and in between the two galaxies. These sites are now actively forming stars as the two galaxies crash together.



Why does this guy keep going on about a full synthesis map?


In radio astronomical jargon, an aperature synthesis image is the output from an observation with an interferometric telescope (like BIMA). In such an observation the signals from many small telescopes are combined (or correlated) to determine very small size features. The spacing between the telescopes determines what size features the interferometer is sensitive to. The further apart the telescopes are, the smaller size the interferometer is sensitive too.

The problem that arises is that an interferometer cannot probe all sizes and therefore the images may ``miss'' emission from a source. In particular there are sizes that cannot be probed easily by an interferometer because you cannot place two radio telescopes closer together than the size of the dishes (well not without really upsetting some people). Thus a full-synthesis image is never possible for a traditional radio interferometric observations.

In order to make a full-synthesis image (and not miss all the large size) features, astronomers use a single-dish telescope to observe the same source and then mathematically ``extract'' the information needed to ``fill-in'' the measurements the interferometer could not make. The single dish observations of this source are shown in the next figure.



[Single-dish CO measurements of Arp 244]

Figure 2:

This is an image made from observations with the 12-meter radio telescope operated by the NRAO (National Radio Astronomy Observatory) on Kitt Peak in Arizona. More that 70 positions have been observed (indicated by red dots). The contrast in the resolution between the two maps should be obvious. The single dish observations have a 55" beam (size of the resolution) while the interferometer map previously shown had a 5"x2" beam. Because ``the Antennae'' are much larger than the telescope's beam, the original measurements - made over a decade ago - detected only a small part of the total emission. A detailed mapping of this emission was necessary to accurately determine the total molecular gas content of ``the Antennae.''

[Radio continuum measurements of Arp 244]

Figure 3:

This third picture shows another radio observation. This time of the 20 cm radio continuum emission as observed by the Very Large Array (VLA), another radio interferometer in New Mexico. The emission contours have been overlaid on a false color, ground-based image of ``the Antennae.'' This is another part of the same project which we have been working on. For a larger overview of the project (The BIMA Luminous Infrared Galaxy Key Project) try this link.

Arp 244 References