Credit: NASA/JPL-Caltech /R.E.Ryan, Jr.(STScl)
Spitzer
has revealed that the stars shining in two of the young SURFS UP
galaxies, discovered by Hubble, look surprisingly mature. The finding
suggests these stars formed earlier than expected, and thus began
lifting the cosmic fog sooner than previously thought.
"We have
discovered galaxies that are among the most distant ever spotted, having
formed a mere 500 million years after the Big Bang," said Maru?a
Brada?, a physicist at the University of California, Davis and lead
author of one of two papers describing the findings. "The stars within
these relatively young galaxies already appear mature, pushing back the
time when the first stars and galaxies arose and began illuminating the
Dark Ages."
Spitzer teamed up with Hubble to gather the results
as part of the Spitzer UltRaFaint Survey, or SURFS UP. The joint project
will image 10 massive, foreground galaxy clusters, whose strong gravity
magnifies the light of background objects. This so-called cosmic
lensing causes objects such as the distant, dim, young galaxies that
SURFS UP is investigating, to appear more than 10 times brighter than
they normally would, allowing the team to study the stars within them.
The
Spitzer observations, in infrared, reveal key characteristics, such as
mass and ages, about older populations of stars in the far-off galaxies.
Besides finding the galaxies in the first place, Hubble's observations,
in visible light, speak to the formation rate of young stars. Taken
together, the data paint a richly detailed portrait of galactic
evolution and its effect on the wider cosmos.
SURFS UP is one of
several new observing programs taking advantage of cosmic lensing to
probe deeper into the universe than ever before. The project will
complement Frontier Fields, another such program using Spitzer and
Hubble data, by looking at eight additional clusters not covered by
Frontier Fields. Overall, SURFS UP will look for traces of cosmically
lensed background galaxies representing typical, "normal" galaxies from
just several hundred millions years after the Big Bang.
The
universe was a very different place at the time the first stars and
galaxies began lighting up an otherwise dark cosmos. Neutral hydrogen
gas, which is a very good absorber of ultraviolet light, still filled
the cosmos like a fog, making the universe partially opaque to that part
of the spectrum. Rather like how the warmth of the sun "burns away"
fog, the first stars and galaxies likely ionized, or knocked an electron
loose from, the neutral hydrogen. This ionization prevented the
hydrogen from absorbing ultraviolet light, clearing the "fog." In this
way, the rise of stars and galaxies that brought the Dark Ages to a
close also rendered the universe transparent to some of these objects'
light.
To understand definitively how and when this process
occurred, scientists need an accurate picture of star formation rates
and history. In a paper accepted for publication in the Astrophysical
Journal, Brada? and colleagues used Spitzer and Hubble to measure the
properties of stars in a 13.2 billion-light-year-distant galaxy, made
possible thanks to the cosmic zoom lens provided by the galaxy cluster
MACS J1149.5+2223. The overall reddish hue of starlight visible to
Spitzer in the distant galaxy indicates the presence of an older
population of stars than was expected in this relatively young galaxy.
A
second paper generated by the SURFS UP team involves the magnification
of a galaxy by the massive Bullet Cluster of galaxies. The paper, in The
Astrophysical Journal Letters, describes an extremely distant galaxy,
12.9 billion light years away, which is so faint that Spitzer could only
detect it using the extra boost of a foreground gravitational lens.
Careful analysis of the stellar properties of this galaxy suggests it
also harbors stars of an already reasonably advanced age.
"These
are among the first direct detections of mature stars in young galaxies
seen at the time when the universe was only a few percent of its present
age," said lead author Russell Ryan, a postdoctoral fellow at the Space
Telescope Science Institute. "When SURFS UP completes, we will have a
census of stars in early galaxies that will help us determine what
exactly ended the Dark Ages."
NASA's Jet Propulsion Laboratory,
Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's
Science Mission Directorate, Washington. Science operations are
conducted at the Spitzer Science Center at the California Institute of
Technology in Pasadena. Spacecraft operations are based at Lockheed
Martin Space Systems Company, Littleton, Colo. Data are archived at the
Infrared Science Archive housed at the Infrared Processing and Analysis
Center at Caltech. Caltech manages JPL for NASA.
