A team of scientists led by astronomers at the University of California, Riverside has used NASA’s Hubble Space Telescope and the W. M. Keck Observatory to uncover the long-suspected underlying population of galaxies that produced the bulk of new stars during the universe’s early years.
The galaxies are the smallest, faintest, and most numerous galaxies ever seen in the remote universe, and were captured by Hubble deep exposures taken in ultraviolet light, and confirmed using the mighty Keck I telescope on the summit of Mauna Kea on the island of Hawaii.
Study results appear in the Jan. 10 issue of The Astrophysical Journal, and will be presented today (Jan. 7) at the 223rd meeting of the American Astronomical Society in Washington DC.
The 58 young, diminutive galaxies spied by Hubble were photographed as they appeared more than 10 billion years ago, during the heyday of star birth.
The team then confirmed the redshift, and the age of the galaxies, on 12 of the sample 58 galaxies with Keck Observatory’s 10-meter telescope fitted with the LRIS instrument. “This was important in determining that our selection of these galaxies was correct,” said study leader Brian Siana, an assistant professor of physics and astronomy. “The LRIS instrument on Keck is ideally suited for obtaining redshifts for these galaxies because it is the most sensitive spectrograph in the world at blue wavelengths, which is where many (if not most) of these galaxies emit most of their light.”
Just finding the galaxies in the first place took a bit of clever trickery. While, the newly discovered galaxies are 100 times more numerous than their more massive cousins, they are 100 times fainter than galaxies detected in previous deep-field surveys of the early universe, and normally too faint for Hubble to see. But the astronomers were able to detect them by using a natural zoom lens in space, produced by the gravity of Abell 1689, a giant foreground galaxy cluster. The cluster is so massive that it magnifies the light from faraway galaxies behind it due to a phenomenon called gravitational lensing, where the curvature of space acts like a giant funhouse mirror to stretch and brighten distant objects.
“There’s always been a concern that we’ve only found the brightest of the distant galaxies,” said Siana. “The bright galaxies, however, represent the tip of the iceberg. We believe most of the stars forming in the early universe are occurring in galaxies we normally can’t see at all. Now we have found those ‘unseen’ galaxies, and we’re really confident that we’re seeing the rest of the iceberg.”
Siana’s team believes it has completed the census of galaxies at an epoch when the universe was roughly 3.4 billion years old. If this sample of galaxies is representative of the entire population at this early time, then the majority of new stars formed in these small galaxies.
“Though these galaxies are very faint, their increased numbers means that they account for the majority of star formation during this epoch,” said team member Anahita Alavi, a Ph.D. graduate student in Siana’s lab, and the first author of the research paper.
Siana explained that uncovering these galaxies also helps bolster claims that hot stars in small galaxies pumped out enough radiation to ionize hydrogen by stripping off electrons. This process, called “reionization,” occurred about 13 billion years ago, within the first billion years after the Big Bang. Reionization made the universe transparent to light, allowing astronomers to look far back into time.
“Although the galaxies in our sample existed a few billion years after reionization, it’s presumed that galaxies like these, or possibly some of these galaxies, did play a big role in reionization,” Siana said.
The early galaxies do not look like the majestic spiral and elliptical galaxies seen in our galactic neighborhood.
“The gravitational lensing stretches out the apparent shape of the distant galaxies, resolving them,” Alavi said. “Without the lensing, some of the galaxies would be just point sources to Hubble. We now have an idea about their sizes that previously were impossible to measure because the galaxies were unresolved.”
The Hubble analysis shows the galaxies are small, irregularly shaped objects measuring just a few thousand light-years across.
“Even when fully mature, these galaxies will be about one-tenth to one-hundredth the mass of our Milky Way,” Siana said. “Because they are undergoing a firestorm of star birth, their light is dominated by the ultraviolet glow of fledgling stars.”
The Low Resolution Imaging Spectrometer (LRIS) is a very versatile visible-wavelength imaging and spectroscopy instrument commissioned in 1993 and operating at the Cassegrain focus of Keck I. It records the spectra of up to 50 objects simultaneously, especially useful for studies of galaxies in the most distant reaches, and earliest times, of the universe. Beamsplitters separate the light into two arms. The red and blue cameras may be operated simultaneously, and together they may acquire spectra covering a wavelength range from 3200 to 10,000 Angstroms. The enhanced lower range of sensitivity, compared to most instruments, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum) to the blue light from star formation, to the red light of very distant objects.
The research team used Hubble’s Wide Field Camera 3 to search for faint, star-forming galaxies in ultraviolet light, a reliable tracer of star birth. The galaxies existed when the universe was undergoing a “baby boom” of star formation, estimated to have peaked between 9 billion and 12 billion years ago.
Siana and Alavi were joined in the study by Alberto Dominguez and William R. Freeman at UC Riverside; Johan Richard at Université Lyon, France; Daniel P. Stark and Brant Robertson at the University of Arizona, Tucson; Claudia Scarlata at the University of Minnesota, Minn.; Harry I. Teplitz and Marc Rafelski at Caltech, Pasadena; and Lisa Kewley at the Australian National University.