The longer a telescope spends looking at a
target, the more sensitive the observations
become, and the deeper we can look into space.
But to get the full picture of what's happening
in the Universe, astronomers also need observations
at a range of different wavelengths, requiring
different telescopes. These are the key ideas
behind the Great Observatories Origins Deep
Survey, or GOODS for short.
The GOODS project unites the world's most
advanced observatories. These include ESO's
Very Large Telescope, the NASA/ESA Hubble
Space Telescope, the Spitzer Space Telescope,
the Chandra X-ray Observatory and many more,
each making extremely deep observations of
the distant Universe, across the electromagnetic
spectrum. By combining their powers and observing
the same piece of the sky, the GOODS observatories
are giving us a unique view of the formation
and evolution of galaxies across cosmic time,
and mapping the history of the expansion of
Now, this is not the first time that telescopes
have been used to give us extremely deep views
of the cosmos. For example, the Hubble Deep
Field is a very deep image of a small piece
of sky in the northern constellation of Ursa
Major. This revealed thousands of distant
galaxies despite the fact that the whole field
is actually only a tiny speck of the sky,
about the size of a grain of sand held at
Now, with GOODS, many different observatories
have brought their powers to bear on two larger
targets, one centered on the original Hubble
Deep Field in the northern sky, and one centered
on a different deep target, the Chandra Deep
Field South, in the southern sky.
The main GOODS fields are each 30 times larger
than the Hubble Deep Field, and additional
observations cover an area the size of the
These areas of the sky were already some of
the most extensively explored, and so the
combination of existing archival data and
many new, dedicated observations gives us
an unprecedented view of the history of galaxies.
At ESO's Very Large Telescope on Cerro Paranal,
the 8.2-metre diameter giants were used for
a total of almost 100 nights of dedicated
observations. The telescopes made images of
the region both in near-infrared light, and
on the boundary between visible light and
ultraviolet light. At these short wavelengths,
only telescopes on exceptional sites such
as the VLT's Cerro Paranal have a chance to
observe through the Earth's atmosphere.
The NASA/ESA Hubble Space Telescope observed
the GOODS regions at optical and near-infrared
wavelengths, in order to detect distant star-
forming galaxies among other things. Now,
Hubble spent a total of 5 days observing the
fields, spread over five repeat visits. Each
of these was separated from the previous one
by about 45 days. Now, by spreading out the
observations like this, Hubble was able to
watch out for new supernovae appearing over
the months, providing key information for
studying the expansion and acceleration of
the Universe due to the mysterious dark energy.
But it wasn't just Hubble making space-based
observations for GOODS.
NASA's Spitzer Space Telescope imaged the
GOODS regions in near- and mid-infrared light
for 5 days, at wavelengths up to 30 times
longer than the Hubble observations. These
longer wavelengths are important for revealing
distant galaxies whose light may be obscured
by cosmic dust, or stretched by the expansion
of the Universe, making them invisible to
These multi-wavelength observations identified
tens of thousands of galaxies.
To get a full understanding of the history
and development of galaxies over the vast
stretch of the Universe's history, we need
to be able to pin down their distances more
precisely, to fix them in cosmic time.
As these galaxies are so far away, the light
waves we see from them today started their
journey up to about 13 billion years ago,
and because the Universe has been expanding
since the Big Bang, back then the Universe
was less than one seventh of its current size.
During the billions of years of the light's
journey, its wavelength has been stretched
as the fabric of space has expanded. This
effect is known as "redshift" because, for
example, light that was originally blue or
ultraviolet in color is shifted to longer,
Back on the ground, astronomers used spectrographs
on ESO's Very Large Telescope to capture the
spectra of galaxies, spreading out their light
like the colors of a rainbow.
Now, the spectra allow astronomers to measure
the redshifts of the galaxies, and hence,
An extensive campaign produced redshifts for
almost 3000 galaxies in the GOODS fields.
Now, with this knowledge, we can place the
galaxies at their distances along a vast cone
of space, stretching out from our own vantage
point like a searchlight beam into the cosmos.
We can take an amazing journey through kind
of a tunnel towards the edge of the Universe.
In some places, the galaxies cluster together,
forming structures which are up to tens of
millions of light years in scale.
dThese projects are an excellent example of
how great observatories are joining together,
across the electromagnetic spectrum, to give
us a more complete view of galaxies over the
history of the Universe. Already, astronomers
have written over 400 papers based on these
data, with even more in the pipeline! And
on top of that, the observations of the GOODS
fields will continue in the future.