COSMOS: The Cosmic Evolution Survey

This post is the second in a series of posts that tour the five CANDELS fields. Our previous post discussed the GOODS-North and -South fields. This one focuses on COSMOS and is a joint post written by Jeyhan Kartaltepe and Mara Salvato.

"It will be like being on the Moon and being able to recognize buildings in New York and trucks on Broadway."

It was with these words in 2003 at a conference in Venice that Nicholas Scoville, from the California Institute of Technology, Principal Investigator of COSMOS (the Cosmic Evolution Survey) informed the scientific community that the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) would image a contiguous two square degree area of the sky. Before CANDELS, COSMOS was the largest HST project ever approved and the COSMOS field remains the largest area of the sky ever surveyed by Hubble. To put this in context, two square degrees is 16 times the size of the full Moon and about 25 times the size of the GOODS fields. 

Illustration of the COSMOS field (right) relative to the full moon and the
GEMS, GOODS, and HUDF fields. Image credit: NASA, ESA, and Z. Levay

Since this was a brand new survey, without a history of observations from other telescopes, astronomers from all over the world began to point the eyes of every major astronomical facility on the ground (using telescopes in Hawaii, Arizona, New Mexico, and Chile), and from space (including the Galaxy Evolution Explorer (GALEX), the Spitzer Space Telescope, the Herschel Space Observatory, the Chandra X-ray Observatory, and the XMM-Newton satellite) in the direction of the COSMOS field to complement the exquisite HST imaging. This huge effort has resulted in COSMOS having one of the largest, deepest, and most uniform data sets across the entire electromagnetic spectrum, from the X-ray to the radio. Today the COSMOS field is one of the best-studied regions of the sky with an incredible archive of data available to the public. In addition, because of the location of the COSMOS field on the sky (it is equatorial), it will always be accessible by future facilities because it is visible from both the northern and southern hemispheres. This means that COSMOS will be one of the first places any new telescope will look -- indeed, plans are already in the works for surveys with ALMA, the J-VLA, and the LMT.

Detailed 3-dimensional map of the dark matter distribution in the
COSMOS field. Image Credit: Modified from NASA, ESA, R. Massey
(California Institute of Technology)

One of the main drivers of surveying such a large contiguous area of the sky with Hubble was to study the effects of large scale structure, or the cosmic web of galaxies and groups of galaxies called clusters. In many smaller area surveys, one can happen to land in a particularly dense area of the sky (on a galaxy group or cluster) or in an underdense area of the sky (on a void) and have a difficult time understanding how various galaxy properties relate to the environment in which they happen to be in. What properties are universal and what properties are affected by being in such close proximity to other galaxies? This structure can make it difficult for astronomers to determine which measurements they make are statistically representative of the entire universe, and which ones just happen to be the case in the small area they are looking at. This problem is known as cosmic variance and can be an issue for many astronomical surveys. By surveying a large area of the sky, COSMOS is able to probe a wide range of environments, from relatively empty areas, to galaxy groups and clusters, and thus mitigate the effects of cosmic variance. In addition, astronomers are able to use this data set to study how various galaxy properties relate to environment. One of the most exciting and widely publicized results from COSMOS was the ability to map the large scale structure of the COSMOS field, and thus the distribution of dark matter, in three dimensions using a technique called "weak gravitational lensing".

A large survey like COSMOS has another important advantage - numbers! There are a lot of galaxies in two square degrees of the sky - over two million identified from the optical data alone. Of these, we have accurate distance measurements (redshifts) to over 25,000 galaxies. We can use these to train tools used to estimate distances for the remaining objects. With this many galaxies, we can address many important questions in a statistical way with large samples of objects. This is especially important when studying the distant universe - at high redshift, COSMOS probes a volume of space that is comparable to that of surveys in the local universe, such as the Sloan Digital Sky Survey. This has enabled many robust galaxy evolution studies, such as understanding how frequent galaxy mergers and interactions have been over cosmic time. A large survey area also allows us to find very rare objects in the universe that would be unlikely to be found in a smaller area. For example, a candidate recoiling black hole was recently identified. This object was so unique and intriguing, it really captured the public's imagination!

Two Square Degrees: Piece by Karel Nel, inspired by the
COSMOS field. Credit: Karel Nel

One of the most unique aspects of COSMOS is that one of the team members, Karel Nel of the University of Witwatersrand in Johannesburgh, South Africa, is an artist who has learned about astronomy over the years through other members of the collaboration and by attending team meetings. This has inspired Karel's art work, and his pieces based on COSMOS have appeared in a number of art exhibits around the world, including the newly opened African Cosmos: Stellar Arts exhibit at the Smithsonian Museum of African Art in Washington, D.C.

COSMOS has produced many scientific papers and interesting science results, from members of the collaboration itself, as well as from other astronomers who made use of the publicly available data set. We have highlighted only a few of them here. All of these factors made the COSMOS field a clear choice for a survey with Wide Field Camera 3 (WFC3) as part of CANDELS. A portion of the COSMOS field is one of the key fields for the CANDELS wide area survey. The CANDELS observations of the COSMOS field now been completed and the analysis has just begun. Stay tuned for future blog posts about CANDELS science in the COSMOS field!