Texas Astronomer uses Hubble Space Telescope to Solve Mystery of Favorite Galaxy
Austin, TX -- For more than a quarter-century, Fritz Benedict's thoughts have been 40 million light-years away, focused on a distant cosmic pinwheel in the constellation Coma Berenices. Now, his findings about spiral galaxy NGC 4314 constitute the cover story of the March 31 issue of The Astronomical Journal. Benedict is a Senior Research Scientist at the University of Texas at Austin's McDonald Observatory.
It began in the bicentennial year of 1976, when Benedict was at McDonald Observatory in West Texas, using the 2.7-meter telescope to track Earth-orbiting satellites for the U.S. Air Force. He found a window of opportunity to point the telescope at other things, and began leafing through the Hubble Atlas of Galaxies for an interesting object.
"I came upon NGC 4314 -- a remarkably smooth galaxy, with not much star formation. It had a bar running through it with a spiral arm attached at each end, and what looked like another little spiral galaxy right in its center," Benedict said. "This piqued my curiosity." He measured the light coming from the central part of NCG 4314 with the 2.7-meter.
"I discovered that the spiral at its center was really a ring of recently formed stars. This raised more questions for me than it answered," he said, "because this galaxy is anemic, relative to star formation -- a condition probably brought on by a collision with another galaxy." Collisions strip the combatants of almost all their gas and dust, leaving them mostly unable to form new stars.
However, simulations predict a small amount of the gas falls back into the galaxies, enough to form a nuclear ring like the one seen in NGC 4314. "Its bar has collected most of that material and delivered it to the nuclear ring and is processing this gas into stars," Benedict said.
Back in 1976, as now, Benedict spent his spare time and weekends puzzling over his 'favorite galaxy,' while officially working on other projects. "We were ramping up for HST," he explains. Since 1977, Benedict has been a member of the Hubble Space Telescope Astrometry Science Team. (He became its Deputy Principle Investigator in 1993.) But as luck would have it, an added bonus of working on Hubble was that Benedict was awarded time to use the newly launched instrument in 1991 -- time he would dedicate to NGC 4314.
To prepare for the HST observations, he began to study NGC 4314 from the ground. This work resulted in the first of a series scientific papers, this most recent being the fourth. "I've used NGC 4314 like a spider web," said Benedict, "to ensnare the help of other astronomers whose understanding of galaxies would increase the scientific value of these observations."
The subject of the next paper was a couple of short exposures obtained with HST in 1991. "Hubble's original WFPC [Wide Field and Planetary Camera] resolved the ring around the galaxy's nucleus into individual star clusters," Benedict said. "I saw a dust pattern there and a hint of another bar, inside the nucleus, in addition to the main bar already seen."
Several years later, a UT-Austin postdoctoral researcher named Beverly Smith (now of East Tennessee State University) suggested looking at Benedict's galaxy in a different light -- literally. Smith and another new collaborator, Jeff Kenney of Yale University, made radio-wavelength observations with the six-antenna Owens Valley Radio Observatory (OVRO) array.
Smith and Kenney helped Benedict to map the distribution of carbon monoxide (CO) gas in the galaxy. CO is a marker of star-formation sites. "We saw that the CO distribution matched the dust distribution, and saw what we interpreted as inflow of gas into the center of the galaxy from a bar," Benedict explained. "We identified the delivery method for the raw material, the gas and dust, required to fuel the star formation in the nuclear ring of NGC 4314."
HST received its first major upgrade in 1993, and in late 1995, Benedict spied on 'his galaxy' once again. He and collaborators (now including D. Andrew Howell of Lawrence Berkely Labs and Inger Jorgensen of Gemini Observatory) studied 76 star clusters in the ring around NGC 4314's nucleus. "Their blue color and hydrogen emission tell us that they're between one million and 15 million years old – very young compared to the age of the galaxy, which is around 10 billion years," Benedict said. His team also concluded that most of the galaxy's star formation occurs in such clusters, and that the formation of new stars has been more or less continuous over the last 20 million years.
Benedict's HST studies of the two spiral arms just outside the nuclear ring reveal that they seem to be in an outer ring. The arms' colors indicate that the stars there are 50 to 200 million years old, he said.
"The age difference between the inner ring of young stars and the larger oval-like feature containing the blue arms makes us think that the inner region around the nucleus contains a reservoir of gas that's becoming more compact over time," Benedict said. He speculates that as the gas concentrates more and more, it has entered two special locations near the nuclear ring, triggering two distinct epochs of star formation. These two special locations are called resonances. "Just as a child on a swing oscillates with a unique cadence, stars and gas orbiting the center of a galaxy sway in and out. At a resonance we find the gravitational attraction of the swaying stars combining to trap gas and concentrate it," Benedict said.
Benedict plans a future paper based on observations with the Canada-France-Hawaii Telescope on Mauna Kea, Hawaii to uncover new details about the star formation process in NGC 4314.
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