John A. Graham

Star Formation in Radio Galaxy Centaurus A

I am currently engaged in studying star formation occurring under unusual circumstances in the nearby radio galaxy Centaurus A (Cen A). I am working on this project with Caleb Fassett, who was a summer intern at DTM in 1999 and is now at Williams College. The radio emission from giant galaxies such as Cen A fills an enormous volume, making them among the largest objects in the universe. Located about 10 million light-years from us, the whole Cen A structure stretches over several degrees in the sky. Maintaining the huge energy output over astronomical timescales has always been a problem, and it is currently believed that they are powered by particle jets, which stream at relativistic velocities from the central core of the galaxy.

Cen A, although close, is a comparatively mild example of the class, and the radio jet is directly observed only close to the active nucleus of the galaxy. Farther out, it makes its presence known by impacting a stray cloud of dust and gas. Some of the gas is entrained in the jet and shocked into visibility by the impact. It is seen as a long stream of faint filaments (Fig. 3). The cloud is probably quite clumpy, and some material is compressed by the impact to the extent that gravitational cloud collapse is triggered and loose chains of young, luminous blue stars are formed. I have discussed characteristics of the gas in a recent paper, and the ongoing work with Fassett consists of obtaining brightnesses and colors of the blue stars as a means for estimating their ages and predicting their ultimate destinies.

The field we are investigating is outlined by the square in Figure 3. We use images taken with a 2,048 x 2,048 CCD detector on the du Pont 2.5-m telescope at Las Campanas Observatory. Images were obtained with U, B, and V filters to obtain color information. A blue image, shown as Figure 4, indicates the locations of the main concentrations of blue stars. Reduction of the observational material has been carried out with the software developed for the Hubble Space Telescope Key Project on the Distance Scale of the Universe (see Freedman entry in the Observatories' section). The brightest blue stars in the loose groups are close to magnitude 20 and are evidently quite normal and similar to the brightest stars in the Large Magellanic Cloud. Ages can be computed by interpolation of theoretical stellar models. A significant age range is indicated, extending from less than a million to more than 15 million years.

To maintain the observed energy in the radio lobes, the radio jet must be long-lived, with a lifetime of many millions of years at least. We thus have the unusual situation in which shocked gas and the stars resulting from triggered cloud-core collapse can be seen at the same time. In the more local situation in our galaxy, star formation is believed to be triggered by shocks generated in supernova outbursts. In such a case, however, the supernova, its remnant, and the attendant shocks have long disappeared by the time the new stars manifest themselves about a million years later. In Cen A, in contrast, because of the long life of the radio jet, we can see both the triggering mechanism and the consequent star formation at the same time.

Fig. 3. The radio galaxy Centaurus A is shown with contours of the inner radio structure at 1407 MHz superposed. The outer radio lobes extend far beyond the confines of this illustration. The direction of the radio jet is indicated by the orientation of these inner radio lobes. Shown as a single broad white contour is the outer boundary of a neighboring cloud of dust and gas. Note the extended filamentary emission from gas entrained and excited by the radio jet. The square box shows the location of a region where shock-triggered star formation is observed.

Fig. 4. This image shows the southeast quadrant of the square field in Figure 3 showing loose groups of young blue supergiant stars that have formed at the edge of the cloud (bottom left and above center). Most of the bright stars in the field are foreground stars belonging to our own Milky Way galaxy.