Astronomers Set a New Galaxy Distance Record
The universe is incredibly big. But how do astronomers know that? Billion-mile-long tape measures can't be found at the hardware store. Instead, astronomers use the expansion of the universe itself to establish milepost markers. The light from remote objects is attenuated and weakened as space stretches like a rubber band. The consequences are that starlight will look redder relative to a nearby star of the same temperature. When starlight is spread into its component color via spectroscopy, features in the light will be shifted to the red end of the spectrum. This "redshift" can be used to reliably calibrate distances. The challenge is the farthest objects in the universe are typically too faint for spectroscopy to work. So instead, astronomers deduce a galaxy's distance by precisely measuring its colors in visible and infrared light. This technique has found candidates for the farthest object in the universe.
Now, in a synergy between the Hubble and Spitzer space telescopes, and the giant W. M. Keck Observatory, astronomers have set a new distance record to the farthest redshift-confirmed galaxy. It is so far away the light we receive left the galaxy over 13 billion years ago, and it is just arriving now. Hubble found the galaxy in deep-sky surveys, and Keck's 10-meter-diameter segmented mirror is powerful enough to collect a spectrum from the unusually bright galaxy. The new observations underline the very exciting discoveries that NASA's James Webb Space Telescope will enable when it is launched in 2018.
An international team of astronomers, led by Yale University and University of California scientists, has pushed back the cosmic frontier of galaxy exploration to a time when the universe was only 5 percent of its present age of 13.8 billion years. The team discovered an exceptionally luminous galaxy more than 13 billion years in the past and determined its exact distance from Earth using the combined data from NASA's Hubble and Spitzer space telescopes, and the Keck I 10-meter telescope at the W. M. Keck Observatory in Hawaii. These observations confirmed it to be the most distant galaxy currently measured, setting a new record. The galaxy existed so long ago, it appears to be only 100 million years old.
The galaxy, EGS-zs8-1, was originally identified based on its particular colors in images from Hubble and Spitzer and is one of the brightest and most massive objects in the early universe. "It has already grown more than 15 percent of the mass of our own Milky Way today," said Pascal Oesch, lead author of the study from Yale University, New Haven, Connecticut. "But it had only 670 million years to do so. The universe was still very young then." The new distance measurement also enabled the astronomers to determine that EGS-zs8-1 was still forming stars very rapidly, about 80 times faster than our Milky Way galaxy today (which has a star-formation rate of one star per year.)
Only a handful of galaxies currently have accurate distances measured in this very early universe. "Every confirmation adds another piece to the puzzle of how the first generations of galaxies formed in the early universe," said Pieter van Dokkum of Yale, second author of the study. "Only the most sensitive telescopes are powerful enough to reach to these large distances." The discovery was only possible thanks to the relatively new Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) instrument on the Keck I telescope, which allows astronomers to efficiently study several galaxies at the same time.
Measuring galaxies at these extreme distances and characterizing their properties is a main goal of astronomers over the next decade. The observations see EGS-zs8-1 at a time when the universe was undergoing very important changes: the hydrogen between galaxies was transitioning from an opaque to a transparent state. "It appears that the young stars in the early galaxies like EGS-zs8-1 were the main drivers for this transition, called reionization," said study co-author, Rychard Bouwens of the Leiden Observatory, Leiden, Netherlands.
These new Hubble, Spitzer, and Keck observations together give a new glimpse into the nature of the infant universe. They confirm that massive galaxies already existed early in the history of the universe, but that their physical properties were very different from galaxies seen around us today. Astronomers now have very strong evidence that the peculiar colors of early galaxies seen in the Spitzer images originate from a very rapid formation of massive, young stars, which interacted with the primordial gas in these galaxies.
The new observations underline the very exciting discoveries that NASA's James Webb Space Telescope will enable when it is launched in 2018. In addition to pushing the cosmic frontier to even earlier cosmic times, the Webb telescope will be able to dissect the infrared galaxy light of EGS-zs8-1 seen with the Spitzer Space Telescope and will provide astronomers with much more detailed insights into its gas properties. "Our current observations indicate that it will be very easy to measure accurate distances to these distant galaxies in the future with the James Webb Space Telescope," said Garth Illingworth of the University of California, Santa Cruz. "The result of Webb's upcoming measurements will provide a much more complete picture of the formation of galaxies at the cosmic dawn." The team's results will appear May 5 in the online edition of The Astrophysical Journal Letters.