A tour of the five CANDELS fields. Last stop: EGS (Extended Groth Strip)

This post is the last in a series of posts that tour the five CANDELS fields. Our previous posts discussed the GOODS-North and -South fields, COSMOS, and UDS.

I hope it is true that the best is saved for last! But what does "best" mean? If you want the deepest data from X-ray to radio energies, the two GOODS fields win hands down! But each of the fields are small, so if, instead, you want the largest field, COSMOS is a no-brainer. On the other hand, since the near-infrared is where the action is to reach farthest back in time, UDS , with its panoply of some of the deepest near-infrared pictures ever taken from the ground, is also a contender for "best". But for a balance of area against depth from X-ray to radio and for sheer richness of spectral and imaging data, the Extended Groth Strip (EGS) is hard to beat. Prior to CANDELS, EGS was already well endowed and organized under an umbrella project known as the All-wavelength Extended Groth strip International Survey (AEGIS) -- check it out --it is a lovely site and has plenty of details on why the EGS is such a cool area for astronomers to explore! EGS was a natural to be one of the CANDELS fields.

Here is an example of what you will see with Google Sky

for the AEGIS field when choosing the Spitzer Space Telescope 

mid-infrared images. Note the markings of interesting objects 

such as a gravitational lens and a supermassive black hole. All 

the objects with circles can be clicked to reveal a wealth of data 

such as the redshift measured from spectra taken with the

Keck Telescope and even links to more information at

 NASA Extragalactic Database.(Image credit; Google Earth)

AEGIS is also a great area for non-professionals to explore. It is especially appealing to the eye with its relatively large region (twice the area of the two GOODS fields), all in color (using two filters). Not only can you enjoy "trekking through the field" comprised of 63 tiles of the HST Advanced Camera for Surveys (ACS), but also you can explore the region as if you had eyes sensitive from the X-ray through ultra-violet and all the way to the sub-mm energies. AEGIS was selected to be the first guinea pig for GOOGLE SKY to implement a feature that enables visualizing, simultaneously, a region with many, over-lapping, multi-wavelength surveys. The figure on the right is a glimpse of what you can do and see with this feature -- check here for details of how to implement this tool.

An interesting aspect of EGS is its origins. While the last in this series for CANDELS, its birth is actually the oldest and thus first of the CANDELS fields. The story began 18 years ago in 1994 when Professor Ed Groth of Princeton University, the namesake of this CANDELS field, led a new survey with the recently repaired Hubble Space Telescope (HST). HST had been refurbished with a new "set of glasses", the Wide Field Planetary Camera (WFPC2), that helped it see clearly and sharply, despite the spherical aberration problem in its primary mirror -- check out Groth's website under "Some Goodies" to read more about the vast improvement in the quality of images, as shown below:

This image illustrates a comparison of a ground based image, an HST image
before the spherical aberration was fixed, and an image after the fix -
all on the same star field at the same scale. You can see more and fainter stars
with the fixed HST for two reasons: first, the star images are smaller, so there's less
overlap; second, the smaller images can be detected against a smaller patch of
background light. Image credit: Ed Groth

This survey was a single long chain of 28 pointings, totaling about 140 square arc minutes in area, and known as the Groth Strip or Groth Strip Survey (GSS). Each pointing, except one, took roughly two hours of exposure time with the new camera and reached depths nearly 1,000,000,000 (1 billion) times fainter than seen with the naked eye. The exception was an "ultra-deep" pointing that had exposures nearly 7 times longer and thus reached about 2.5 times yet fainter. Groth designed the combination of area and depth of the survey to enable astronomers to answer our most profound questions in cosmology (the shape, size, and age of the universe) and about the birth, assemblage, and evolution of galaxies through the clustering distributions, counts, colors, sizes, and morphologies of the faintest, most distant galaxies.

GSS is noteworthy for the use of two filters in separate images. Such pairs of images provided a color in the optical, not only making it visually interesting, but also endowing the image with a vast potential of new information for astronomers to glean such information as the approximate distances of the galaxies, the youth of the galaxies, and even the masses in stars of the galaxies. Another noteworthy aspect was the intensive follow-up surveys for years to come, not only from the ground via images through additional filters or via spectroscopy using the world's largest optical telescopes (check out the Deep Extragalactic Evolutionary Probe or DEEP survey that used the 10-meter Keck Telescopes) but also from space, e.g., with 5 pointings of 200,000 seconds and 3 pointings of 800,000 seconds with an X-ray camera aboard the Chandra Space Telescope .

This is a map of the Extended Groth Strip (EGS) region of the 

sky, showing the sky coverage for several of the AEGIS data 

sets and the complexity of their shapes, sizes, and relative 

orientations. For reference, the full moon is shown to scale. 

Shown next to the moon at the 4:30 position is the size of 

the original Hubble Deep Field (pink shape). The Hubble 

Space Telescope (HST) images taken as part of AEGIS using 

the Advanced Camera for Surveys (ACS) are shown in grey in 

the center of the EGS. The new CANDELS WFC3 images and 

ACS images are imbedded within the central region of the original 

HST images. (Image credit: Christopher Willmer & Dale Kocevski)

Thirteen years later in 2007, after the next (3rd) generation instrument, the Advanced Camera for Surveys (ACS) became available, the original strip was extended in length and area. These Extended Groth Strip (EGS) images were acquired and served as the core of the AEGIS survey. Like its predecessor, two filters in the optical were used, but the area of sky covered was expanded by over a factor of 4 to about 600 square arc minutes, the largest, deep, contiguous, color mosaic image in the optical with HST of the distant universe. CANDELS, with its addition of more ACS optical images and exciting, new, WFC3 near-infrared images, is the natural next generation.

As with all the other CANDELS fields, AEGIS was a fertile hunting ground for astronomers worldwide to make new science discoveries, some resulting in "paradigm shifts" from an older, commonly-accepted view or paradigm to a new one. One example was the common view of the dominance of "major mergers", i.e., cosmic collisions of two hefty galaxies, and their resultant strong bursts of star formation to explain the rapid increase of star formation in galaxies back in time. But new data from AEGIS showed otherwise. Exploiting the rich, multi-wavelength data to estimate the star formation activity and amount of stars ("stellar masses"), Kai Noeske and his colleagues discovered that galaxies had a "single-track mind" during most of their lives while forming stars. They would lie on a narrow path in a plot of the amount of their star formation activity versus their mass in stars, meaning that few took large detours. Yet, such excursions should have been frequently seen if bursts of star formation activity, induced by major mergers, commonly dominated their lives. More recently, this view was reinforced by the exquisite HST images from four of the five CANDELS fields that were used to find close pairs or highly disturbed galaxies that were "major mergers" as well as "minor mergers" between dwarfs and hefty galaxies by Jennifer Lotz and her colleagues. They discovered that major mergers were less frequent while the minor mergers were dominant at earlier times.

As another example of a "paradigm shift", Kirpal Nandra and his colleagues discovered that distant galaxies which hosted active supermassive black holes (active galactic nuclei or AGN) were not, as commonly expected, blue. Such blue colors are the expected consequences of active births of new stars that should concurrently accompany the infusion of gas needed to fuel an AGN. These galaxies hosting AGN, chosen by being bright in X-rays, were, instead, surprisingly more passive and redder. This finding supports the alternative view that, while AGN may, through still-uncertain physical processes, help galaxies transition from their blue, active stage to their redder, quiescent stage, the actively growing stage of the supermassive black hole may last for an extended period, many 100's of millions of years, long after the galaxy has been "quenched".

In conclusion, while each of the CANDELS fields have their pros and cons, and proponents of each field may legitimately argue why their field may be "best" for solving this or that science problem, we can all clink our beer mugs together in agreement that the full set of five of the truly outstanding regions of the sky form a cohesive whole that is greater than any combination of its parts. The science originally proposed by Groth are as relevant today as when he envisioned them for his far humbler survey two HST generations ago. We all have no doubt that CANDELS will continue to serve as the premier real estate for the deepest, richest astronomical surveys for new generations of instruments and telescopes to come.