GOODS, Under Astronomers’ AEGIS, Produce GEMS

No, not really (but I got all three key words into the title in a way that sorta makes sense).

Astronomers, like most scientists, just love acronyms; unfortunately, like most acronyms, on their own the ones astronomers use make no sense to non-astronomers.

And sometimes not even when written in full:
GOODS = Great Observatories Origins Deep Survey; OK that’s vaguely comprehensible (but what ‘origins’ is it about?)
AEGIS = All-wavelength Extended Groth strip International Survey; hmm, what’s a ‘Groth’?
GEMS = Galaxy Evolution from Morphology and SEDs; is Morphology the study of Morpheus’ behavior? And did you guess that the ‘S’ stood for ‘SEDs’ (not ‘Survey’)?

But, given that these all involve a ginormous amount of the ‘telescope time’ of the world’s truly great observatories, to produce such visually stunning images as the one below (NOT!), why do astronomers do it?

GEMS tile#58 (MPIfA)

Astronomy has made tremendous progress in the last century, when it comes to understanding the nature of the universe in which we live.

As late as the 1920s there was still debate about the (mostly faint) fuzzy patches that seemed to be everywhere in the sky; were the spiral-shaped ones separate ‘island universes’, or just funny blobs of gas and dust like the Orion nebula (‘galaxy’ hadn’t been invented then)?

Today we have a powerful, coherent account of everything we see in the night sky, no matter whether we use x-ray eyes, night vision (infrared), or radio telescopes, an account that incorporates the two fundamental theories of modern physics, general relativity and quantum theory. We say that all the stars, emission and absorption nebulae, planets, galaxies, supermassive black holes (SMBHs), gas and plasma clouds, etc formed, directly or indirectly, from a nearly uniform, tenuous sea of hydrogen and helium gas about 13.4 billion years ago (well, maybe the SMBHs didn’t). This is the ‘concordance LCDM cosmological model’, known popularly as ‘the Big Bang Theory’.

But how? How did the first stars form? How did they come together to form galaxies? Why did some galaxies’ nuclei ‘light up’ to form quasars (and others didn’t)? How did the galaxies come to have the shapes we see? … and a thousand other questions, questions which astronomers hope to answer, with projects like GOODS, AEGIS, and GEMS.

The basic idea is simple: pick a random, representative patch of sky and stare at it, for a very, very long time. And do so with every kind of eye you have (but most especially the very sharp ones).

By staring across as much of the electromagnetic spectrum as possible, you can make a chart (or graph) of the amount of energy is coming to us from each part of that spectrum, for each of the separate objects you see; this is called the spectral energy distribution, or SED for short.

By breaking the light of each object into its rainbow of colors – taking a spectrum, using a spectrograph – you can find the tell-tale lines of various elements (and from this work out a great deal about the physical conditions of the material which emitted, or absorbed, the light); “light” here is shorthand for electromagnetic radiation, though mostly ultraviolet, visible light (which astronomers call ‘optical’), and infrared (near, mid, and far).

By taking really, really sharp images of the objects you can classify, categorize, and count them by their shape, morphology in astronomer-speak.

And because the Hubble relationship gives you an object’s distance once you know its redshift, and as distance = time, sorting everything by redshift gives you a picture of how things have changed over time, ‘evolution’ as astronomers say (not to be confused with the evolution Darwin made famous, which is a very different thing).


The great observatories are Chandra, XMM-Newton, Hubble, Spitzer, and Herschel (space-based), ESO-VLT (European Southern Observatory Very Large Telescope), Keck, Gemini, Subaru, APEX (Atacama Pathfinder Experiment), JCMT (James Clerk Maxwell Telescope), and the VLA. Some of the observing commitments are impressive, for example over 2 million seconds using the ISAAC instrument (doubly impressive considering that ground-based facilities, unlike space-based ones, can only observe the sky at night, and only when there is no Moon).

There are two GOODS fields, called GOODS-North and GOODS-South. Each is a mere 150 square arcminutes in size, which is tiny, tiny, tiny (you need five fields this size to completely cover the Moon)! Of course, some of the observations extend beyond the two core 150 square arcminutes fields, but every observatory covered every square arcsecond of either field (or, for space-based observatories, both).


GOODS-N is centered on the Hubble Deep Field (North is understood; this is the first HDF), at 12h 36m 49.4000s +62d 12′ 58.000″ J2000.

GOODS-S is centered on the Chandra Deep Field-South (CDFS), at 3h 32m 28.0s -27d 48′ 30″ J2000.

The Hubble observations were taken using the ACS (Advanced Camera for Surveys), in four wavebands (bandpasses, filters), which are approximately the astronomers’ B, V, i, and z.

Extended Groth Strip fields (AEGIS)


The ‘Groth’ refers to Edward J. Groth who is currently at the Physics Department of Princeton University. In 1995 he presented a ‘poster paper’ at the 185th meeting of the American Astronomical Society entitled “A Survey with the HST“. The Groth strip is the 28 pointings of the Hubble’s WFPC2 camera in 1994, centered on 14h 17m +52d 30′. The Extended Groth Strip (EGS) is considerably bigger than the GOODS fields, combined. The observatories which have covered the EGS include Chandra, GALEX, the Hubble (both NICMOS and ACS, in addition to WFPC2), CFHT, MMT, Subaru, Palomar, Spitzer, JCMT, and the VLA. The total area covered is 0.5 to 1 square degree, though the Hubble observations cover only ~0.2 square degrees (and only 0.0128 for the NICMOS ones). Only two filters were used for the ACS observations (approximately V and I).

I guess you, dear reader, can work out why this is called an ‘All wavelength’ and ‘International Survey’, can’t you?

GEMS' ACS fields (MPIfA)


GEMS is centered on the CDFS (Chandra Deep Field-South, remember?), but covers a much bigger area than GOODS-S, 900 square arcminutes (the largest contiguous field so far imaged by the Hubble at the time, circa 2004; the COSMOS field is certainly larger, but most of it is monochromatic – I band only – so the GEMS field is the largest contiguous color one, to date). It is a mosaic of 81 ACS pointings, using two filters (approximately V and z).

Its SEDs component comes largely from the results of a previous large project covering the same area, called COMBO-17 (Classifying Objects by Medium-Band Observations – a spectrophotometric 17-band survey).

Special thanks to reader nedwright for catching the error re GEMS (and thanks to to readers who have emailed me with your comments and suggestions; much appreciated)

7 Replies to “GOODS, Under Astronomers’ AEGIS, Produce GEMS”

  1. Very comprehensive article!

    “GOODS = Great Observatories Origins Survey”

    I was wondering what the “D” stands for, please.

  2. @Buxtehude

    “D” stands for Deep, as in the scope of the article 🙂 Nice one Jean.

  3. Thanks Buxtehude, AndyInv. 🙂

    Yes, I had left out the word “Deep”; fixed now (double thanks Buxtehude).

  4. Thanks nedwright.

    I had a suspicion that the GEMS webpage probably wasn’t correct (that’s where I got the info from; I’ve emailed them on this), but ran out of time to track it down. It seems the COSMOS observations were done after the GEMS ones (at least the bulk of them) – do you know if that’s so? Certainly the COSMOS results, of the general public kind, appear later than the GEMS ones.

    Fixed (but further edits may be necessary)

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