by Ray A. Lucas
We recently finished taking data for the Hubble Frontier Fields project, and we’ve learned many very useful and exciting things, both about the universe and the objects in the images. We’ve also learned much in terms of technical issues in the images as well. The recent STScI Newsletter article by Jennifer Mack and Norman Grogin recounts much of the latter. For me, however, the most fun in projects like this is usually the beginning, with the back and forth of discussions both scientific and technical about exactly what to do and how to do it. I think it is the speculative nature and the ideas flowing back and forth in the developmental phase of programs such as this that most whets my appetite for finding what we ultimately see in the images.
Having been a member of all of STScI’s internal working groups for the various community service deep field projects we have done here since the very first one, and even one of its important predecessors, it has been very interesting to me, and the most fun, to be a part of the debate, discussions, and activity involved in designing and planning these observations, and the way that process has taken place over those years. So, in this blog article, I’d like to share my own longer, and more personal context for the Frontier Fields program and those which went before.
In the beginning, the Servicing Mission 1 Early Release Observations (SM1 ERO) were a demonstration of the power of the new, optically corrected Wide Field and Planetary Camera 2 (WFPC2) and Hubble combination. One of the major goals of the particular SM1 ERO program in which I was involved was simply to go as deeply as we could in 10 orbits in a redder WFPC2 filter in what, for then, was viewed as a very deep—perhaps the deepest ever—detailed, high-resolution image of the night sky. This was in the area of a cluster at a redshift of ~0.4, and a quasar beyond it in the same field of view with a possible more distant cluster around that at a redshift of ~2.055. (Quasars are incredibly bright objects thought to be powered by supermassive black holes.) This image of galaxy cluster Abell 851, or CL0939+4713, showed that Hubble and the then-new WFPC2 were superb tools for revealing the shapes of very distant galaxies in the early universe.
Even with the success and revelatory power of that image, it was still viewed as a very risky thing of possibly dubious value to commit many more HST orbits and staff time and effort to try a significantly even deeper field. STScI’s then-Director Bob Williams convened a panel of community experts who debated whether such a thing should be attempted, and if so, what type of field should be targeted. The idea that something should initially be tried in a generic, nominally empty deep field eventually came to the fore, but it was still seen as a possibly big gamble that might not live up to its potential for the great amount of time required. It took a courageous decision by Williams to go ahead with the project, committing a significant portion of his Director’s Discretionary time to the project.
A number of people rightly felt that they already had significant work of their own which needed pursuing and finishing and, when asked if they would be willing to take part in this original HDF experiment, declined. However, there were still some relatively few of us who had been discussing the possibilities of this informally. In my own case, having helped design and set up the SM1 ERO observations of CL0939+4713, I was eventually asked if it was technically feasible for us to even attempt such deep field observations.
We performed many experiments helping to define what kinds of possibilities existed. Our experiments were, fortunately, successful, and the ultimate success of the original Hubble Deep Field ushered in a new sociological phenomenon in the field: [professional astronomical] community-service projects with high-level science products quickly released to the astronomical community, with prohibitions on internal staff use of those data and catalogs for their own scientific use for some pre-determined time.
In 1998, the Hubble Deep Field-South targeted a quasar with both imaging and spectroscopy, and included many more flanking fields and much deeper parallel observations—all in multiple cameras spanning wavelengths from long UV to infrared, including both the newer Space Telescope Imaging Spectrograph (STIS) and Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) instruments, as well as WFPC2. Even today, I think these HDF-South observations have been underutilized, although they have now been targeted by, for example, the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT) of the European Southern Observatory. This underutilization came as the community gravitated more to observations of another southern-hemisphere field, the Chandra Deep Field-South, which by then had deeper X-ray observations. Hopefully the HDF-South and its Flanking Fields will still be exploited more fully in the future.
Astronauts installed the Advanced Camera for Surveys (ACS) in 2002. Under then-Director Steve Beckwith, we designed the Hubble Ultra-Deep Field (in the middle of the Chandra Deep Field-South) around use of the ACS and using WFPC2 and NICMOS in parallel, creatively making the pure parallel operational system give us the then-deepest-ever detailed UV and infrared observations.
Subsequent observations with ACS and the even newer WFC3 camera have given us even greater depth and wavelength coverage at higher resolution, particularly in the infrared channel of WFC3. This led to the GO program—not an STScI community service program, but done by external observers adding to the HUDF via approval by the international peer-review committees which review proposals and recommend observations to be done—called the Extreme Ultra-Deep Field. This program combined all existing archival imaging with still more new, deep infrared observations to try to look even farther back in time.
All of this gives context to the observations which we have just recently finished: the Hubble Frontier Fields. Again convening a panel of community experts, and building on the success of large observing programs such as CLASH and CANDELS, STScI’s then-Director Matt Mountain explored a brilliant idea to use the gravitational lensing effect of massive galaxy clusters to magnify galaxies in the early universe beyond them, and to also provide a baseline for searches for higher-redshift supernovae. The plan was to come as close as is possible for Hubble to come to the bread-and-butter observations of the much-anticipated, soon-to-be launched James Webb Space Telescope in searching for some of the earliest galaxies in the distant, early universe.
The panel recommended that a group of six galaxy clusters and six adjacent parallel fields be targeted. That was a very important development, because it also addressed in a major way a phenomenon known as cosmic variance. In this phenomenon, the large-scale structure of the universe affects observations, so that a measurement of any region of sky may differ from a measurement of a different region of sky by a considerable amount. Because the size of the fields of view of Hubble’s cameras are roughly the size of a grain of sand held at arm’s length, we’re talking about deep line-of-sight “pencil beams” in the sky when we talk about these deep fields. With the superb resolution of Hubble’s cameras, incredible detail is attained, and we can see thousands of galaxies in unprecedented detail all across their fields of view.
But given what we now know about the larger-scale structure of the universe, when it comes to the matter which we can detect, at least, there are longer filaments and areas where they intersect, and voids in between. Sometimes, the small field of view of a camera may land on a filament of galaxies, and other times in a void between filaments, or partly on a filament and partly off. Therefore, the more deep fields we observe in various different places around the sky, the more we statistically beat down the perhaps unusual or anomalous statistical effects of any one particular local environment in the area of that particular deep field as we attempt to identify the more general nature of the universe across filaments and voids, etc.
A major feature of the Hubble Frontier Fields program is the use of two fields in parallel, on-cluster and off-cluster, for each of the galaxy clusters targeted in the program, giving us both a cluster-centric and a generic parallel field at some much larger distance away from the cluster, for each cluster. So, in effect, we get 12 fields for the price of six. Six on-cluster fields are dominated by each galaxy cluster’s environment—something very different from a traditional deep field in terms of the physics and dynamics affecting its galaxies, and also somewhat peculiar to that cluster— and six are off-cluster, parallel fields that contain thousands of field galaxies not particularly in any cluster environment. Given the relatively small angular size of each individual parallel field, this larger number of parallel fields especially helps to minimize the effects of cosmic variance when measurements from all other similar deep fields are combined or considered together.
When Matt Mountain’s committee recommended a study of six galaxy clusters and six parallel fields, we still had to work out which clusters to observe. Under the overall leadership of Jennifer Lotz, we conducted a trade study, a common tactic in situations such as this. Various factors about each potential cluster and their advantages and disadvantages as potential targets were examined in greater detail. We tried to keep in mind anything which might bias our selections in various ways. The number of clusters was gradually winnowed down as we discussed each of them, until we had our final six. After that, we prioritized them, planning to do an initial set, and then the remainder if a mid-course review by the external panel felt that it was warranted to continue and complete the program based on results to that time.
A professional astronomical community program of improving gravitational lensing models was also put in place, with competitive proposals for grant funding to do the work and share the improved resulting models with the community. Also, having seen the power of public outreach in our other efforts, we involved those at STScI who are best at bringing our work to both the wider astronomical community and the public to allow them to help more meaningfully and widely bring our efforts to light. We also reviewed our prior experiences and policies and precedents from the various earlier deep field programs and debated whether any needed adjustment.
So, now, we can also say that we’ve been lucky. The Hubble Space Telescope and the science instruments have performed well, getting us all the data we had hoped and planned to get. As we continue to work on the data, we’re now seeing the more refined versions of the Frontier Fields images which we will release to the community soon. They are indeed beautiful and interesting, and they will help the community—all of us—to better prepare for the soon-to-come James Webb Space Telescope observations.
Webb’s much greater size than Hubble, and the much greater sensitivity of the new telescope and its detectors, will mean that Webb can make the faster exploration of more deep fields a reality. The resulting statistical advantages will give us greater confidence in the answers we find in our ongoing community studies of galaxy origins, and their formation and evolution to the forms we see in galaxies much nearer by us in space and time. It has taken a lot of work from everyone involved in our various teams of people designing and planning, implementing and scheduling the observations, and processing the data, but it has been, as with the earlier programs, a joy to see the impact spread into the community.
For me, it has been one of the great privileges and honors of my many years here at STScI to have been a part of all of our various extragalactic, deep-field, community-service programs since the original Hubble Deep Field, and to have worked with so many exceptional people who have helped to conduct these programs and produce these science products for the use of the world-wide astronomical community and the public in general.