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
the Universe.
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 arm's length.
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 full Moon.
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
Hubble.
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,
redder wavelengths.
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,
their distances.
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.