13.3 billion years ago, when the universe
was just 420 million years old, a galaxy's light began its journey
toward Earth. Over that time period, space itself expanded
in size by a factor of 12, stretching all light along with it to
longer, redder wavelengths. Ultraviolet light from the hot,
young stars in that distant galaxy now appears infrared to us, too
red for our eyes to see and almost too red for Hubble! (
See how
redshift works.)
Astronomers measure distances in units of this "redshift".
This galaxy, named MACS0647-JD, is at redshift 11, the highest yet
observed. Redshift is simply the stretch factor minus one
(redshift zero is today).
We measure redshift in part based on the observed "Lyman
break." Any light emitted bluer than 0.1216 microns is
energetic enough to excite electrons in hydrogen clouds. In
the process, they are absorbed and never reach us.
Hubble observed this galaxy through filters of 17 different
colors, from the ultraviolet to infrared (0.2 - 1.7 microns), but
it only appeared in the two reddest filters (F140W and
F160W). The Lyman break was redshifted to 1.46
microns. If MACS0647-JD were 100 million light years more
distant (redshift 13), its light would be invisible to Hubble's
Wide Field Camera 3 (WFC3), redshifted beyond its limit (1.7
microns).
"JD" is short for "J-band Dropout". The galaxy is not
detected in the so called J-band (F125W), nor in 14 bluer Hubble
filters.
Three Cherries
8 billion years into its journey, the light from this galaxy took
multiple detours around MACS0647+7014, a massive cluster of
galaxies named after the survey which discovered it and its
coordinates on the sky. The mass of this cluster bends space
and time according to Einstein's Theory of Relativity, and light
follows these bends in space. This "
gravitational
lensing" produces three multiple images of MACS0647-JD
magnified by factors of 8, 7, and 2. (A very similar lensing
effect can be seen
through
the base of a wine glass.)
We identified similarly lensed multiple images of eight other
background galaxies. This enabled us to map the mass within
the cluster. It's like a big puzzle. We had to arrange
the mass in such a way that it lenses galaxies to the positions we
observe them. Once we did this, we determined that the three
images of MACS0647-JD are as expected in terms of their positions
and relative brightnesses.
One of the images is much fainter and took us longer to identify,
though we knew it had to be there. Finding it was like
coming up with three red cherries on a slot machine. We'd
hit the jackpot.
Redshifted or just red?
Some galaxies are red without being redshifted. Galaxies
that run out of gas to form new stars see their stellar
populations age. Intense blue stars die off, and cooler red
stars remain. Galaxies may also appear red if they are
enshrouded in dust.
So how did we distinguish between a galaxy at redshift 11 and a
closer galaxy at, say, redshift 2, which is some combination of
red and redshifted?
Out of some 20,000 objects in
CLASH images
of 17 clusters to date, the colors of MACS0647-JD really stand
out. None are redder. Red galaxies at redshift 2 have
been observed before, and they are never as red as these.
And if a galaxy were so red (rather than redshifted), it should be
bright at even redder wavelengths. However, MACS0647-JD does
not show up brightly (if at all) in Spitzer images at 3.6 and 4.5
microns.
We also ruled out individual red stars, brown dwarfs, asteroids,
and more.
Missing Galaxies Found?
Before CLASH, astronomers were puzzled that they had only
discovered one galaxy within 500 million years of the Big
Bang. We had expected to find six such galaxies in deep
imaging of "blank" sky. Instead, only one was discovered in
the Ultra Deep Field.
This had suggested that, perhaps, galaxies were just beginning to
form then? Perhaps we were observing a dramatic rapid
buildup in numbers of the first galaxies.
This dearth of galaxies also presented a mystery as to what
reionized the early universe,
clearing away the
neutral hydrogen fog. If there weren't enough
galaxies, a more exotic energy source may have been required such
as dark matter self-annihilation.
Now thanks to CLASH and these cosmic telescopes, we are beginning
to find such distant galaxies in the numbers we expect. This
includes our previous
report
of a galaxy at redshift 9.6.
However, CLASH may have just been a bit lucky (and the "blank"
field searches unlucky). To be sure, we will need to search
more with Hubble and its successor, the James Webb Space Telescope
(JWST).
Fossil Record
We observe MACS0647-JD as it was 13.3 billion years ago: a
fledgling dwarf galaxy, less than one percent the size and mass of
our Milky Way. Since then it has probably merged with many
other galaxies, growing in size, and perhaps evolving to look much
like our own Milky Way, which we believe to be some 13 billion
years old. Might it be possible that on at least one of the
billions of stars in that galaxy, life evolved? If so,
perhaps they too built large telescopes in space. And
perhaps they are looking back at us through the same gravitational
lens, seeing a magnified image of our Milky Way as it was 13.3
billion years ago, a faint red dot in their images.
Of course we don't yet know whether there is other life out there
or if we are alone in the cosmos.
JWST will discover more distant galaxies like MACS0647-JD and
study them in greater detail. This will improve our
understanding of the earliest galaxies and their evolution which
ultimately produced at least one civilization capable of looking
back in time and discovering its origins.
