Graduating Senior Studies the Shadows of Galaxies
Most of the universe is empty space -- galaxies lie few and far between. Moreover, distant galaxies are difficult, if not impossible, to observe by their meager light. So, what if astronomers studied not the light of the galaxies themselves, but also their shadows?
Undergraduate researcher Nigel Mathes and his advisor Jane Charlton, a professor in Penn State's Department of Astronomy and Astrophysics, do exactly that in their quest to understand how galaxies form and evolve. To study the shadows of galaxies, Mathes and Charlton need a background light source -- a "light bulb" bright enough to shine across billions of light years. Fortunately for them, the universe is full of light bulbs known as quasars -- beacons of light powered by black holes.
"When the light of a quasar shines through the gas of a galaxy, we see it as absorption in the quasar's light spectrum," Mathes said. "Some gas may be isolated, just floating in space, but usually the gas we're seeing is related to galaxies."
"Even a small amount of gas, no bigger than the mass of the Sun, can be seen in absorption," Charlton added. "The technique is that sensitive."
Mathes analyzed the gas absorption using archival spectra from the Keck telescope in Hawaii and the Very Large Telescope in northern Chile. "As I went through the data," Mathes said, "these questions were always in the back of my head: Where is the gas? Is it moving? What is actually going on in this galaxy?"
Mathes's collaborator, Sandhya Rao, a research professor at the University of Pittsburgh, imaged the intervening galaxies themselves using the powerful Hubble Space Telescope. By combining their data, Mathes, Charlton, and Rao were able to connect the properties of the gas to the properties of the stars in the galaxies. "We're looking specifically at absorption from an element produced in supernovae -- the explosions that end stars' lives," Mathes explained. "That means we're able to trace where star formation has occurred."
Mathes and his collaborators find that the brightest galaxies have the strongest shadows, caused by high-speed flows of gas. "With brighter galaxies comes more star formation, and with more star formation comes more supernovae," Mathes said. "What we think we're seeing are large bubble structures due to supernovae that are blowing gas out of the galaxy."
"These results tie in with current theories of star formation, which suggest that stars formed more prominently in the past than in the present," Charlton said.
Mathes has presented his results at two conferences of the American Astronomical Society and is preparing to submit a paper on his work. Mathes said he is looking forward to starting graduate school at New Mexico State University next fall, where he'll use his undergraduate research as a springboard for future work.