Penn State, Rutgers Astronomy Teams Discover Ancestors of Galaxies Like the Milky Way
Astronomers at Penn State and Rutgers universities have discovered galaxies in the distant universe that are ancestors of spiral galaxies like our Milky Way. These ancient objects, some of the first galaxies ever to form, are being observed as they looked when the universe was a mere two-billion years old. Today, scientists peg the age of the universe at 13.7-billion years, so light from these galaxies traveled almost 12 billion years to reach Earth. The discovery by a team led by Caryl Gronwall, senior research associate in Penn State's Department of Astronomy & Astrophysics, and Eric Gawiser, assistant professor in the Department of Physics and Astronomy in the Rutgers School of Arts and Sciences, will be presented at the annual meeting of the American Astronomical Society (AAS) in Austin, Texas, to be held from January 7 to 11, 2008.
The newly discovered galaxies are quite small -- one-tenth the size and one-twentieth the mass of our Milky Way. They also have fewer stars -- only one-fortieth as many as are in the Milky Way. From ground-based telescopes, they look like individual stars in size. Recent images made by the Hubble Space Telescope, however, reveal them as regions of active star formation.
"Finding these objects and discovering that they are a step in the evolution of our galaxy is akin to finding a key fossil in the path of human evolution," Gawiser said. The team led by Gronwall and Gawiser determined that these galaxies were fertile breeding grounds for new stars, which burned hot and bright. These stars ionized the hydrogen atoms around them, stripping them of their electrons and causing them to emit a tell-tale sharp band of ultraviolet light known as Lyman alpha. The researchers also noted that several of these galaxies, sometimes 10 or more, pulled together over the ensuing few-billion years to form a single spiral galaxy.
Compact Lyman alpha emitter
Extended, chain-like Lyman alpha emitter
Credit for both images: NASA, ESA, Caryl Gronwall/Penn State
"The Hubble Space Telescope has delivered striking images of these early galaxies, with 10 times the resolution of ground-based telescopes," Gronwall said. "They come in a variety of shapes -- round, oblong, and even somewhat linear -- and we are starting to make precise measurements of their sizes." Gawiser will preview Gronwall's early Hubble image analyses at the AAS meeting, and the research team will publish the complete results later in a scientific journal.
The astronomers discovered these galaxies as part of a five-year-old census of galaxies in the early universe, a project called MUSYC (Multi-Wavelength Survey by Yale and Chile). Gawiser, while a National Science Foundation (NSF) astronomy and astrophysics postdoctoral fellow at Yale, initiated a search for several types of galaxies that could be precursors of Milky Way-type spirals; Gronwall led an investigation into the luminosity, density, and distribution of the distinctive Lyman-alpha emitters. The team's statistical analyses and computer simulations of how galaxies cluster led them to the conclusion they first reported in the 10 December 2007 issue of the Astrophysical Journal in a paper titled "Lyman alpha emitters are the ancestors of spiral galaxies." "We knew by our understanding of cosmological theory that spiral galaxies had to evolve from low-mass galaxies such as these," Gawiser said. "The challenge was to actually find them. We'd seen other early universe galaxies, but they were bigger and destined to evolve into elliptical galaxies, not spirals."
Clumpy, extended Lyman alpha emitterExtended Lyman alpha emitter which shows evidence of merging
Credit for both images: NASA, ESA, Caryl Gronwall/Penn State
The astronomers undertook four types of observations to find and characterize the objects they were seeking. They performed the first step -- actually finding amid all the visible objects of deep space those galaxies that emit ultraviolet light in the Lyman-alpha range - using the Blanco four-meter telescope at the National Science Foundation's Cerro Tololo Inter-American Observatory in Chile. To measure their distance, they used the Magellan Telescope at Las Campanas Observatory, also in Chile, to measure redshift -- an effect that shows how fast an object is receding from view due to a rapidly expanding universe. (The redshift at which they studied these galaxies is 3.1.) To determine how many stars are in the galaxies, they used the NASA Spitzer Space Telescope's Infrared Array Camera. And to determine how big the galaxies are, they used the NASA Hubble Space Telescope's Advanced Camera for Surveys.
"Astronomy has long used a model where big surveys are followed by detailed studies of the interesting objects they find," said Nigel Sharp, program officer in NSF's Division of Astronomical Sciences. "This work nicely couples the large-area, wide-field view of our ground-based telescope with the sharp focus of the Hubble to probe to the faintest light levels. This team has come the closest yet to finding young galaxies that resemble our own Milky Way in its infancy."
Additional study participants are Robin Ciardullo at Penn State; Harold Francke and Paulina Lira at Universidad de Chile; C. Megan Urry, Pieter van Dokkum and Shanil Virani at Yale; Kamson Lai, Giovanni Fazio and Jia-Sheng Huang at Harvard, Kevin Schawinski at Oxford; Alvaro Orsi, Felipe Barrientos, Leopoldo Infante and Nelson Padilla at Pontificia Universidad Católica de Chile; Ryan Quadri and Edward Taylor at Leiden Observatory; John Feldmeier at Youngstown State; Ezequiel Treister at the European Southern Observatory; and Guillermo Blanc at the University of Texas at Austin. Additional members of MUSYC are listed at http://www.astro.yale.edu/MUSYC.
The research was funded by grants from NSF, including an Astronomy and Astrophysics Postdoctoral Fellowship, and from NASA's Hubble Space Telescope and NASA's Spitzer Space Telescope. The Cerro Tololo Inter-American Observatory is part of the National Optical Astronomical Observatory, funded by NSF and operated by the Association of Universities for Research in Astronomy. The Spitzer Space Telescope is managed by the NASA Jet Propulsion Laboratory and CalTech. The Hubble Space Telescope is managed by the NASA Goddard Space Flight Center and science programs are managed by the Space Telescope Science Institute.
[ Carl Blesch / Barbara Kennedy ]