Press Release

3-D View of Stephan’s Quintet

By SpaceRef Editor
October 28, 2011
Filed under , , ,

Subaru Telescope has added another dimension of information about one of the most studied of all compact galaxy groups — Stephan’s Quintet. Located within the borders of the constellation Pegasus, Stephan’s Quintet consists of a visual grouping of five galaxies, four of which form a compact galaxy group; one additional galaxy appears in images of the group but is much closer than the others. Refinements in observations of the quintet are revealing more about its members. A comparison of images (the left and right images in Figure 1) compiled by using a suite of specialized filters with Subaru’s Prime Focus Camera (Suprime-Cam) has shown different types of star-formation activity between the closer galaxy NGC 7320 and the more distant galaxies in Stephan’s Quintet. They show the quintet in 3-D.

These new images are the product of Suprime-Cam’s ability to capture images of objects in a wide field of view and to use specialized filters to focus observations according to particular research objectives. To learn about the star-forming regions in Stephan’s Quartet and their structure, observers used an extremely narrow band filter (H-alpha), which lets in a very specific wavelength of light to indicate distinctive hydrogen emissions during active star formation (an H-alpha spectral line). They used two H-alpha filters, each with a different recession velocity, i.e., the speed at which the object is moving away from the observer. They used one H-alpha filter with a recession velocity of 0, which means that the speed at which the object is moving away from the observer is 0 and that it is not far distant. They used another H-alpha filter with a greater recession velocity of 4200 miles (6,700 km) per second, an indicator of distant objects. In addition to the red color of the H-alpha emission, green and blue filters captured light so that the composite tricolor images aligned with human color perception in red, blue, and green.

Processing of the filtered images resulted in the two different views of Stephan’s Quintet shown in Figure 1. The image on the left shows the galaxies when the observers used the H-alpha filter with a recession velocity of 0, while the one on the right shows them when the they used the H-alpha filter with a recession velocity of 4,200 miles per second. The left image shows H-alpha emissions that indicate an active star-forming region in the spiral arms of NGC 7320 in the lower left quadrant but not in the other galaxies. The right image contrasts with the left and shows a region of H-alpha emissions in the upper three galaxies but none from NGC 7320. Two (NGC 7318A and NGC 7318B) of the four galaxies are shedding gas because of a collision while a third (NGC 7319) is crashing in, creating shock waves that trigger vigorous star formation. Figure 2 depicts the relationship of the galaxies. Gas ejected from stars born during such vigorous star formation emits bright light, which the H-alpha filter reveals. The researchers believe that NGC 7319 as well as NGC 7318A/B are driving the star-forming regions in the H-alpha emitting region around NGC 7318A/B.

In addition to star-forming activity, the images indicate the distances of the galaxies. Different recession velocities help observers spot cases where objects located at different distances appear in proximity in the same image. The contrasting images show that NGC 7320 is closer than the other galaxies, which show active star formation at a significantly higher recession velocity (4,200 miles per second) than NGC 7320 (0). NGC 7320 is about 50 million light-years away while the other four galaxies are about 300 million light-years away. This explains the intriguing arrangement of the galaxies in Stephan’s Quintet.

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Figure 1: Composite tricolor images of Stephan’s Quintet using H-alpha filters with a recession velocity of 0 (left image) and a recession velocity of 4,200 miles per second (right image).

Figure 2: A diagram of the member galaxies of Stephan’s Quintet. NGC 7320 is a close galaxy and has a recession velocity of 0. The remaining four are a cluster of galaxies 300 million light-years away. The researchers believe that the merging of NGC 7318A/B and NGC 7319’s crashing into them are responsible for the active star formation regions in the H-alpha emitting region around NGC 7318A/B.

Observation Parameters:
Object Name: HCG 92 (Stephan’s Quintet)
Telescope Used: Subaru Telescope (8.2 m diameter primary mirror), prime focus
Instrument Used: Subaru Prime Focus Camera, Suprime-Cam
Filters: B (0.45 micron), R (0.65 micron), NA656 (0.656 micron), NA671
(0.671 micron)
Composite Color Schemes: blue (B), green (R), red (NA656; Figure 1
left), blue (B), green (R), red (NA671; Figure 1 right)
Observation Dates: 2009-05-25 (R), 2009-05-25 (NA671), 2009-05-26 (B),
2009-08-22 (NA656)
Exposure Times:180s x 4 (R), 900s x 5 (NA671)
300s x 4 (B), 240s x 4 (NA656)
Picture Orientation: Up corresponds to North, Left corresponds to East
(6′ width, 44’height, rectangular field of view)
Coordinates: RA (J2000.0) 22h36m, Dec (J2000.0) +33*58′ (constellation Pegasus)

PIO Contact:
Suzanne Frayser
Press Officer, Subaru Telescope, Hilo, HI, U.S.A.
+1 808-934-5022

Science Contacts
Masafumi Yagi
NAOJ, Japan

Michitoshi Yoshida
Hiroshima University, Japan

Text & Graphics:

SpaceRef staff editor.