Astronomers are peeking into the universe's early eras using the light from galaxies that existed several hundred million years after the Big Bang. Spurred on by their success, they're embarking on a 3-year project with NASA's Hubble Space Telescope.
Have you ever noticed how kids are fascinated by stories of themselves? Particularly tales from before they can remember, such as of their birth? Astronomers are the same way with the universe. As humans we can only "remember" back thousands of years, but we want to know what came earlier — before our planet formed, before our galaxy existed. What did the early universe look like?
Galaxy research presented at the winter American Astronomical Society meeting fleshes out a picture of the cosmic history we can't remember.
In this picture, galaxies started forming in the first couple hundred million years after the Big Bang. These galaxies weren't the pretty spirals or fat yellow ellipticals that we see today; they were small, blobby irregulars, like the Magellanic Cloud dwarf galaxies. From about 400 million years to 3 billion years (or from a redshift, z, of 12 to 2, in astrobabble), star formation ramped up in the universe. Then it tapered off. Today it's maybe one-tenth what it was in the universe's heyday.
Astronomers base this picture on observations of galaxies at various cosmic epochs. Viewing objects at great distances as a way to go backward in time, they have identified about 1,000 galaxies that emerged between 1 and 1.5 billion years after the Big Bang, about 700 at 950 million years, and another 700 between 650 and 800 million years. But they only have about two dozen before that.
Among those rare gems are four bright galaxies Garth Illingworth (University of California, Santa Cruz) and colleagues studied using the Hubble and Spitzer space telescopes. The light from this foursome was emitted 500 million years after the Big Bang. The galaxies are oddballs in that they're 10 to 20 times brighter than previous galaxies detected in this era. While theory predicts such objects should exist, the researchers were so surprised by the detections that they wondered if they were doing something wrong, Illingworth said during a press conference on January 7th.
Using the infrared capabilities of Spitzer, the astronomers could study the galaxies' light output more carefully and estimate their masses. They determined that these objects have on the order of a billion solar masses' worth of stars, on par with a recently discovered star-forming galaxy that existed about 200 million years later.
But most galaxies at early times were fainter than these. In the past few years, astronomers have begun to realize that these fainter galaxies were the primary source of the ultraviolet radiation in the universe during the first several hundred million years — radiation that ionized the ubiquitous neutral hydrogen in the so-called era of reionization.
These galaxies are, naturally, challenging to spot. To do it, astronomers use a sneaky trick known as gravitational lensing, in which the gravitational force of a closer, massive object bends and concentrates the light from a more distant object around the nearer object, just as a glass lens bends light to a focus.
As Anahita Alavi (University of California, Riverside) reported at the AAS meeting, she and her colleagues were able to use the lensing power of galaxy cluster Abell 1689 to reveal 58 faint galaxies that shone between 3.7 and 2.8 billion years after the Big Bang. These galaxies are about 100 times fainter than those previously studied in this era, which is roughly when star-formation rates peaked. Even when magnified, the galaxies appear to be at most tens of thousands of light-years across, a tenth the Milky Way's size. Yet Alavi says it's these galaxies that account for more than 70% of the universe's star formation at the time.
Because astronomers have a fairly large census of galaxies both during this era (around redshifts of 2 and 3) and back to about 700 million years after the Big Bang, they infer that the universe's makeup didn't change dramatically during the intervening time, Illingworth said. So this same type of faint, small galaxy probably predominated all the way back to the cosmos's earliest days.
Buoyed by these results, Hubble scientists are setting out to study lensed galaxies in earnest. As Jennifer Lotz (Space Telescope Science Institute) reported at the meeting, the Frontier Fields project is a 3-year collaboration to sift out lensed galaxies surrounding "nearby" galaxy clusters in long-exposure images.
After scrutinizing their first object, the galaxy cluster Abell 2744 (redshift of 0.3, corresponding to 10 billion years after the Big Bang), Lotz and her team found tiny magnified images of almost 3,000 distant galaxies. These would have been invisible without the lensing power of Abell 2744's gravity, which magnified them 10 to 20 times, and they existed 1 to 2 billion years after the Big Bang.
How far back Hubble can see with lensing — in theory, to 400 million years after the Big Bang — will depend on a bit of luck. Light from even earlier times has been redshifted so much by the universe's expansion that the starlight that once began as optical and ultraviolet has been stretched to infrared wavelengths. Hubble's infrared capabilities are somewhat limited, which is why astronomers want the infrared-optimized James Webb Space Telescope. If observers are already detecting the building blocks of today's galaxies with Hubble, they're optimistic that JWST will reveal the early universe in enough detail to unmask the mysteries of our cosmic infancy.
P. A. Oesch et al. "The Most Luminous z~9-10 Galaxy Candidates yet Found: The Luminosity Function, Cosmic Star-Formation Rate, and the First Mass Density Estimate at 500 Myr." Posted to arXiv.org on January 6, 2014 (version 2).
Anahita Alavi et al. "Ultra-faint Ultraviolet Galaxies at z~2 behind the Lensing Cluster A1689: The Luminosity Function, Dust Extinction, and Star Formation Rate Density." Astrophysical Journal, January 10, 2014.