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John A. Graham

John A. Graham


John Graham has been active in a variety of astronomical societies over the years, among them the American Astronomical Society, where he was vice president between 1984 and 1986, and the Astronomical Society of the Pacific, where he chaired the editorial board from 1988 to 1991. Concurrent with his work at Carnegie, he served as a program director for the Division of Astronomical Sciences at the National Science Foundation from 2000 to 2001. He is currently the secretary of the American Astronomical Society.

 

Radio Galaxy Centaurus A

The radio galaxy Centaurus A is shown with contours of the inner radio structure at 1407 MHz. The outer radio lobes extend beyond the image frame. The direction of the radio jet is indicated by the orientation of the inner radio lobes. The outer boundary of a neighboring cloud of dust and gas is shown as a single broad white contour. The square box shows the location of a region where shock- riggered star formation is observed.

 

The nearby galaxy NGC 5128 is one of the most peculiar in the sky. It is among the brightest emitters of radio waves and, for this reason, is also known as Centaurus A (Cen A). John Graham, Staff Member Emeritus, has discovered groups of young, blue stars within the extended areas of the galaxy’s radio emission. By studying these stars, which orig-inated in an unusual environment, he can gain a deeper insight into the whole process of star formation.

The entire Cen A structure stretches over several degrees in the sky. It is believed that the radio emission is powered by particle jets, which stream at relativistic velocities from the galaxy's core. If the radio jet happens to hit a stray cloud of dust and gas, material may be compressed to the extent that gravitational collapse of the cloud is initiated and loose chains of young, luminous blue stars are produced. The brightness and colors of the blue stars can be used to estimate their ages and predict their destinies. Part of the cloud may be swept up by the jet and, energized by the impact, can be seen as a long stream of faint filaments.

In our Milky Way galaxy, star formation is believed to be triggered by shocks generated during supernova outbursts-the spectacular explosions that end a star's life. However, in these cases 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, researchers can see both the triggering mechanism and the consequent star formation at the same time because of the long life of the radio jet.

Graham and summer intern Caleb Fassett used images taken with a CCD detector on the du Pont 2.5-meter telescope at Carnegie's Las Campanas Observatory in this research. Different glass filters permit color information to be obtained. A blue image, for example, highlights the main concentrations of blue stars in Cen A. The brightest blue stars in the loose groups are close to magnitude 20 and appear to be quite normal and similar to the brightest stars in our neighboring galaxy, the Large Magellanic Cloud. Interpolation of theoretical stellar models leads to an estimation of ages for these stars. A significant age range emerges, extending from less than a million years to more than 15 million years, showing that, in this instance, star formation is a continuing process.

SELECTED PUBLICATIONS
  • Graham, J. A., and C. I. Fassett. 2002. Star formation associated with the X-ray jet in Centaurus A, Astrophys. J. 575, 712-720.

  • Fassett, C. I., and J. A. Graham. 2000. Age, evolution, and dispersion of the loose groups of blue stars in the northeast radio lobe of Centaurus A, Astrophys. J. 538, 594-607.

  • Graham, J. A. 1998. Shocked gas and star formation in the Centaurus A radio galaxy, Astrophys. J. 502, 245-252.

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