Historic Hubble Image Involves Texas A&M Astronomer

Although the latest image released by the Hubble Space Telescope Science Institute as part of the Hubble Heritage Project may have originated in deep space, it has local Texas A&M University ties.

The image, unveiled Tuesday (August 10), is a deep-exposure shot depicting NCG 4911, a majestic face-on spiral galaxy located deep within the Coma Cluster of galaxies, which lies 320 million light years away in the northern constellation Coma Berenices. As home to nearly 1,000 galaxies, the Coma Cluster is one of the closest very rich collections of galaxies in the nearby Universe.

Texas A&M astronomer Dr. Lucas Macri was one of several international astronomers whose data — gathered during 28 hours of exposure time using Hubble’s Advanced Camera for Surveys (ACS) and Wide Field Planetary Camera 2 (WFPC2) instruments in 2006, 2007 and 2009 — was combined to create the natural-color image. Macri and his research teammates were using Hubble to search for Cepheid variable stars in NGC 4911 and NCG 4921, two spiral galaxies among the cluster’s 1,000 galaxies that could be used to measure the distance to the Coma Cluster and, hence, the expansion rate of the Universe, which is also known as the Hubble constant. This parameter yields a direct measurement of the age of the Universe.

“Our intention was to measure the Hubble constant directly, using the ratio of velocity to distance for the Coma Cluster,” said Macri, a member of Texas A&M’s George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy. “Discovering Cepheids in those two galaxies within the Coma Cluster would give us the distance to the cluster.”

According to Macri, because the Coma Cluster is very far from the Milky Way (around 320 million light years) and very massive, its redshift is almost entirely due to the expansion of the Universe and, therefore, essentially unaffected by the gravitational pull of nearby objects. Given that it is at the very limit of what the Hubble Space Telescope can deliver in terms of resolving individual stars in other galaxies, he and his fellow astronomers had to expose Hubble’s camera for a total of 3 hours to get a single image deep enough to reveal Cepheid variables.

To actually discover Cepheid variables, however, would have required about 12 different images during a period of 100 days — an unfortunate fact, given that the ACS suffered a short circuit at the end of January 2007 when only one-third of the team’s data had been obtained.

“We have detected some interesting variables, but we need additional Hubble time before we can complete the project,” Macri said. “We hope we can complete it during a future Hubble Space Telescope observing campaign and eventually study these variables at near-infrared wavelengths with the next-generation James Webb Space Telescope.

“Regardless, the images we obtained go very deep, enabling some spectacular views of the stars and dust lanes in NGC 4911 and 4921. Our project required two colors — green and red — and the Hubble Heritage Project obtained the third one — blue — to make these beautiful images.”

The Hubble Heritage Project was founded in 1998 as a monthly showcase of new and archival Hubble images considered to be some of the most beautiful and spectacular ever taken by the Hubble Space Telescope. The project has been recognized for its contribution to public inspiration in producing some of the most aesthetically-pleasing images ever produced in astronomy.

To learn more about the Hubble Heritage Project, go to http://heritage.stsci.edu/.

For more information about Macri’s research or Texas A&M Astronomy, visit http://astronomy.tamu.edu/.

-aTm-

Contact: Shana K. Hutchins, (979) 862-1237 or shutchins@science.tamu.edu or Dr. Lucas Macri, (979) 862-2763 or lmacri@physics.tamu.edu