The CANDELS Survey
CANDELS: A Cosmic Odyssey
CANDELS is a powerful imaging survey of the distant Universe being carried out with two cameras on board the Hubble Space Telescope.
- CANDELS is the largest project in the history of Hubble, with 902 assigned orbits of observing time. This is the equivalent of four months of Hubble time if executed consecutively, but in practice CANDELS will take three years to complete (2010-2013).
- The core of CANDELS is the revolutionary near-infrared WFC3 camera, installed on Hubble in May 2009. WFC3 is sensitive to longer, redder wavelengths, which permits it to follow the stretching of lightwaves caused by the expanding Universe. This enables CANDELS to detect and measure objects much farther out in space and nearer to the Big Bang than before. CANDELS also uses the visible-light ACS camera, and together the two cameras give unprecedented panchromatic coverage of galaxies from optical wavelengths to the near-IR.
- CANDELS will exploit this new lookback power to construct a "cosmic movie" of galaxy evolution that follows the life histories of galaxies from infancy to the present time. This work will cap Hubble's revolutionary series of discoveries on cosmic evolution and bequeath a legacy of precious data to future generations of astronomers.
- CANDELS will also test the reality of cosmic dark energy by measuring the brightness of a special class of exploding supernovae called Type Ia. By spotting these objects out to farther distances, CANDELS will establish whether these objects are in fact precision "standard candles" for probing the geometry of spacetime. This test is essential to ratify the discovery of dark energy, the most important breakthrough in cosmology since Hubble's discovery of the expanding Universe in 1929.
What will CANDELS measure?
CANDELS is designed to focus on two critical epochs in cosmic evolution.
At "Cosmic Dawn", less than 1 billion years after the Big Bang, the first seeds of cosmic structure began to take shape. Here CANDELS will:
- Compile the first definitive census of "infant" galaxies. Galaxies like the Milky Way were accreting their first large quantities of gas at this epoch but had formed fewer than 1% of their total complement of stars. CANDELS will reveal hundreds of such systems and measure key properties such as size, star formation rate, and stellar mass.
- Map the positions of infant galaxies in space to assess their clustering. The amount of clustering is one of the best ways to measure their dark-matter halo masses, which are otherwise invisible to telescopes.
At "Cosmic High Noon", 2-4 billion years after the Big Bang, galaxies went on a growth splurge as huge gravity-driven rivers of gas flowed into them along the "cosmic web". Here CANDELS will:
- Uncover the blueprints of future galaxy structure. Galaxies at "high noon" look profoundly different from today -- they are patchy, chaotic, and peppered with giant blue blobs of newly-formed stars, far different from the stately pinwheels of today's mature spirals. Underlying this apparent chaos, we suspect is a "scaffolding" of older, redder stars that prefigure the future structure to form. This scaffolding jumps out in CANDELS WFC3 images, where these redder stars dominate the light.
- Snapshot the birth of cosmic black holes. Thanks to Hubble, we have learned that every large nearby galaxy harbors a massive black hole at its center, weighing up to a billion solar masses. These black holes are dim, fossil relics today -- but shone brightly as quasars at the peak epoch of mass accretion near cosmic high noon. Current theories suggest that super-massive black holes grow primarily as a result of galaxy mergers, which force vast quantities of gas into galaxy centers, triggering star formation and black-hole growth. The process becomes self-limiting, with the energy released by gas falling into the black hole heating or blowing out the rest of the gas and removing the fuel for subsequent star formation. Combining the CANDELS WFC3 images with data from X-rays to the far-infrared will help to test these theories and pin down the timescale for the various stages of the process.
- Extend the collection of measured Type Ia supernovae brightnesses to within 2.5 billion years of the Big Bang. This leaps over intervening epochs to a time when the effect of dark energy on cosmic acceleration was negligible and ordinary matter dominated the geometry of spacetime. This simpler era will permit the first clean test of the constancy of Type Ia standard candles independent of dark energy. Extraordinary accuracies of a few percent in brightness will be required, testing the limits of both the observatory and the ingenuity of her human observers.
Throughout the three-year cosmic odyssey of CANDELS, our team of over 100 astronomers from 12 countries will be reporting regularly on our important findings. Return here to see pictures and read our news.