Astronomers Use Chandra Images to Craft Unprecedented Detail of Nearby Galaxy

CUTLINE 1: An x-ray image shows the inner portion of the Circinus Galaxy, with north at the top of the image and east to the left. In terms of X-ray energies, red represents low energy, green intermediate and blue the highest observed energies. The emission is resolved into a number of distinct components, many of which are associated with a central black hole. A bright, compact emission source is present at the center of the image. That nuclear source is surrounded by a diffuse X-ray halo that extends out several hundred light years. The X-rays directly to the northwest of the nucleus appear red, indicating predominantly soft energies, while the X-rays to the southeast are blue, indicating only hard energies. Because soft X-ray energies are absorbed by gas more easily than hard energies, the sharp contrast suggests that the red emission to the northwest originates from the near side of the disk of the Circinus Galaxy while the blue emission is more highly absorbed and must come from the gas within the disk or on the far side. Such geometry corresponds to the disk of the galaxy as seen in optical and radio images. A bright, soft X-ray plume of emission extends approximately 1,200 light years (380 parsecs) to the northwest and coincides with an optical region containing gas ionized by the nucleus. There is a very strong correlation between the X-ray emission and the high-excitation ionized gas seen in emission-line images obtained by the Hubble Space Telescope and ground-based telescopes. The x-ray image was made with NASA's Chandra X-ray Observatory and the Advanced CCD Imaging Spectrometer (ACIS).

Astronomers using the Chandra X-ray Observatory have made unprecedented observations of the inner regions and general structure of the nearby Circinus Galaxy, enabling them to study both the supermassive black hole in the center of the galaxy and numerous potential smaller black holes or neutron stars sprinkled throughout the spiral disk of the galaxy. They have detected periodic variability in one of the emission sources, representing the first time such an effect has been seen in a black hole outside our own galaxy. They also have gained a better understanding of the complexity, composition, and structure of the matter near the central supermassive black hole.

The astronomers from Penn State and George Mason University—whose results appear in papers in The Astrophysical Journal and The Astronomical Journal, the most recent of which will be published in the July 2001 issue of the latter journal—have determined that numerous X-ray-emitting sources are sprinkled throughout the galaxy, and that those sources are probably the result of systems in which a small black hole closely orbits an evolving normal star. Those systems are known as X-ray binary systems.

"Bright X-ray point sources have been detected in the disks of many nearby galaxies such as Circinus. However, it has never been clear whether these sources were simply intrinsically brighter examples of the X-ray binary systems seen in the Milky Way, or if they represented a new class of X-ray sources," said Franz Bauer, a postdoctoral scholar at Penn State who detailed the group's results in one of the papers. "Periodic variability is the key signature of an X-ray binary system, but it is difficult to detect in distant objects. Usually, we can do that only with the closest sources, but thanks to Chandra we were able to get a glimpse for the first time at such a system outside our own galaxy."

Chandra's detailed X-ray spectrum allowed astronomers to detect emissions and gather information not possible with other X-ray observatories. Results associated with the Chandra images included an analysis of 16 emission points, known as "point sources," and a determination that nearly half of the sources coincided with the star-forming features of the galaxy established by previous optical and radio images.

Earlier studies have established that Circinus, located 13 million light years from Earth, contains a central supermassive black hole and that matter accreting onto that black hole radiates with an intensity of 50 million times the luminosity of the Sun. Such galaxies are often called "active" galaxies, and their centrally located black holes are called "active galactic nuclei" or AGN, because the intensity of the radiation is much greater than that of "passive" galaxies such as our own Milky Way. Because Circinus is the closest active galaxy to Earth, further study of it might provide important information about regions close to the centers of other active galaxies.

Chandra's detailed images and spatial resolution of the galaxy allowed the astronomers to determine that its emission is resolved into a number of distinct components that are associated with a central black hole. A bright, compact emission source is present at the center of the image, and that nuclear source is surrounded by a diffuse X-ray halo that extends out several hundred light years. In addition, there is a strong correlation between the X-ray emission and the highly excited ionized gas seen in emission-line images obtained by the Hubble Space Telescope and ground-based telescopes.

The astronomers also conducted a detailed analysis of the gas distribution and physical conditions near the center of the galaxy and identified at least two different gas components. One, a warm gas that is photoionized by the radiation field from the black hole, contains strong emissions of highly ionized elements such as argon, calcium, iron, magnesium, neon, silicon, and sulfur. The second gas component is cooler and features a strong iron emission line. In addition, the astronomers were able to show that the two gas components have different distributions, with the warmer gas being spread over a much larger region around the black hole than the cooler gas.

"The Chandra observations of Circinus are showing us that the gaseous environment of supermassive black holes can be very complex," said Rita Sambruna, assistant professor of physics and astronomy at George Mason University. "Because it is close and thus easy to study, Circinus provides an important testbed for what might be happening in other, more distant AGNs."

Along with Sambruna and Bauer, the research group included Hagai Netzer of Tel-Aviv University and the following collaborators from Penn State: Niel Brandt, assistant professor of astronomy and astrophysics; George Chartas, senior research associate; Eric Feigelson, professor of astronomy and astrophysics; Gordon Garmire, Evan Pugh Professor of Astronomy and Astrophysics; John Nousek, professor of astronomy and astrophysics; and Shai Kaspi, a postdoctoral researcher. Observations with Chandra, using the Advanced CCD Imaging Spectrometer (ACIS) and the High Energy Transmission Grating Spectrometer (HETGS), were made on 6 June and 7 June 2000. Chandra's ACIS detector was conceived and developed for NASA by Penn State and Massachusetts Institute of Technology under the leadership of Garmire. The ACIS detector is a sophisticated version of the CCD detectors commonly used in digital cameras or video cameras.

CONTACTS:

Franz Bauer, Penn State (814) 863-7111 / feb3@psu.edu

Rita Sambruna, George Mason University (703) 993-4165 / rms@physics.gmu.edu

Steve Sampsell, PIO, Penn State (814) 865-1390 / sws102@psu.edu

IMAGES AVAILABLE:

Chandra images regarding this research are available at http://chandra.harvard.edu

CUTLINE 2: A figure representing the light curve of the Circinus Galaxy X-ray binary, as derived with the Chandra X-ray Observatory and the High Energy Transmission Grating Spectrometer (HETGS), shows variations that appear to be periodic, with a cycle of approximately 7.5 hours. In that time, the flux varies by more than a factor of 20. Those variations imply that the X-ray source has a binary companion.