Hubble Spies Most Distant Galaxy Yet

Dan Coe with CLASH (PI Marc Postman)

paper accepted for publication in the Astrophysical Journal (pdf) (arxiv 1211.3663)

also see the official press releases at STScI, NASA, and ESA

Hear our discussions on WYPR and WBAL radio, and watch Space Fan Hangout #4.
The Cluster Lensing And Supernova survey with Hubble (CLASH) is a large Hubble program observing 25 galaxy clusters.  It began November 2010 and is scheduled for completion July 2013.  Our 50+ member collaboration is led by Marc Postman. 
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.)

"These go to 11"  -- Spinal Tap

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.