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Looking into (the) COSMOS with the James Webb Space Telescope

Looking into (the) COSMOS with the James Webb Space Telescope

With the imminent launch of JWST — we hope! — I am resurfacing an article I wrote earlier this year which appeared in a different form on the IAP front page. Next year I will be giving a public conference about this which will be streamed live.

Astronomers learned to read the patterns of stars in the starry night skies as clues to the shape of our own Milky Way. Powerful telescopes soon showed that galaxies themselves were spread out across the sky in a pattern that holds profound clues to the nature and eventual fate of the Universe itself. This is the history of astronomy: with each new instrument, scientists take a step towards a better understanding of our Universe. Very soon, using the James Webb Space Telescope (JWST), an international team of astronomers — my friends in the COSMOS project — will map out a vast unexplored region of the very distant Universe. A key prediction of current theories of the formation of the Universe is that stars and galaxies form hot ionised bubbles of gas in the early Universe. With JWST we will see for the first time the imprint of these bubbles on the large-scale distribution of the very first stars and galaxies. They will also help us to understand how mysterious dark matter helps galaxies to form at the earliest times.

Bubbles in the early Universe (from A. Loeb)

Over the past three decades, the Hubble Space Telescope has opened up vistas of the distant Universe only dimly glimpsed by ground-based observatories. However, charting out how galaxies are linked together across the sky seemed at first like an impossible task for Hubble. The telescope’s cameras can only see a tiny part of the sky in each exposure. But a team of astronomers ambitiously used Hubble to survey a region of the sky larger than the full moon — now known as the COSMOS field — making it the latest ever contiguous patch of the sky covered by Hubble. Since then, thousands of scientific papers have delved deep into the COSMOS field, drawing a detailed picture of how galaxies, dark matter, gas, and dust interact on large scales and how they have changed over cosmic time. Since those first Hubble observations, COSMOS has been covered by every major telescope and this data has been shared in a remarkably open and productive collaboration between all the world’s astronomers.

The original COSMOS Advanced Camera for Surveys HST mosaic

The COSMOS catalogues have become an essential reference point for the evolution of galaxies over cosmic time and an invaluable starting point to ask any new questions about the Universe. The papers describing them are among the most highly cited in astronomy. COSMOS catalogues have also played a crucial role in preparing for Esa’s Euclid mission.

Of course, astronomers would always like to see further. Data from COSMOS has hinted at another Universe of very distant, massive galaxies just out of reach of current surveys. The finite speed of light is a wonderful time-machine which means that these most distant objects lie, in fact, at the very beginnings of the Universe. But the expansion of the Universe means their light becomes redder and redder as it speeds towards us. To see so very far back, one needs a not only a telescope with a mirror large enough to collect these lonely photons arriving from the edge of the Universe — but also one sensitive to the infrared light bringing messages from these distant galaxies. An even greater challenge is that every ordinary object also emits infrared photons; these would drown out the faint signal from such distant galaxies. To detect this ancient light, the telescope must be cooled to a few degrees above absolute zero and placed far from terrestrial sources. The ten-billion-dollar James Webb Space Telescope (JWST), with its golden 6.5 meter foldable mirror, due to be launched and sent to one of the coldest places in the solar system, is just such a telescope. It is optimised for the study of these faint distant objects. A single exposure from JWST will see deeper than the decade-long ground-based infrared surveys that we in the COSMOS team have been carrying out.

Let’s hope that it will look like this in a few months…

JWST is a remarkable instrument, perhaps the most sensitive telescope ever constructed. Its cameras can record light from objects infinitely fainter than anything we have ever observed before. But where do you point such a beautiful and sensitive machine if it is looking deep into a country that no-one has ever visited before? The best thing to do, right at the start, is to survey a wide swath of this new territory to guide future scientist-explorers. But once again, such a survey seemed an impossible task with the tiny field-of-view of the JWST cameras. Nevertheless, we proposed a highly ambitious program — now called COSMOS-Web — to cover one third of the COSMOS field with JWST’s infrared and near-infrared cameras. Such a survey would require more than 200 hours of observations, an enormous amount for the highly competitive first year of Webb’s operations. Astronomers around the world requested more than four times the amount of time than were actually available. The results of this competition were announced earlier this year: the COSMOS-Web proposal was accepted, making it the largest survey to be carried out during the first year of JWST’s operations. We call it COSMOS-Web because it will map out the web of large-scale-structure of the Universe at the earliest epochs.

Simulated image of the JWST-COSMOS field

Our program comprises 206 hours with JWST’s NIRCAM instrument and 80 hours taken simultaneously with the longer-wavelength MIRI camera. Together, in COSMOS-Web these instrument will produce a new map of the very distant Universe. All data will be public. The new data will contain tens of thousands of very faint and distant galaxies, signposts for follow-up observations with JWST’s other very sensitive instruments. Astronomers will see, for the first time, the structure and form of galaxies in the early universe. And we will see, we hope, things we do not expect to see. Over the years observations in COSMOS has turned up countless interesting and peculiar objects and I am certain JWST observations will continue this tradition!

Olivier Le Fèvre

Olivier Le Fèvre

It was the winter of 1998. I was reaching the end of my thesis in Durham, England, and I knew it was time to start looking for a new city to live in, a new place to go. By this point in my life I’d already spent almost three years in North America (Socorro, New Mexico, and Victoria, BC) and six years in England (Durham and Manchester), and I knew that I really didn’t want to live in those kinds of of places any more, I knew that they weren’t for me. But was there any place in Europe that did the kind of science I wanted to do, what I had done in Canada and England? By that I meant surveys of the Universe with tens of thousands or hundreds of thousands of galaxies, studying what everything looked like on the largest scales, finding new objects and galaxies that no-one had ever gazed upon before. So I was more than just intrigued when I saw the job advertisement for a postdoctoral research assistant at the Laboratoire d’Astrophysique Spatiale (LAS), in Marseille. “Observational cosmology / VLT-VIRMOS deep redshift survey” it said. They were looking for someone to help out with a survey of the distant Unvierse that would be an order of magnitude larger than anything attempted before using a new instrument on one of the four European Very Large Telescopes, in Chile. And I although in my ignorance I had never heard of the LAS, I had certainly heard of the person offering the position — Olivier Le Fevre. He had already authored or co-authored many papers on distant galaxies, surveys, clustering, all the kinds of topics that I had been immersed in during my studies. In my anglocentric innocence I wasn’t sure if this science was being done in the Old World. It seemed like a wonderful opportunity to do some exciting work on the shores of the Meditteranean. I had never been to the south of France before and after long years in the North of England I was ready for sunshine. I applied, and was invited for an interview.

I remember very clearly that I had arrived early at the LAS and was waiting in the lobby for Olivier to arrive. The building dated from the 1970s; outside the facade was all lightly-tarnished metal and glass and inside there were narrow corridors with worn linoleum on the floor. It had been well lived in. Outside, the sun shone brightly even on that early in the morning in winter and from the lobby it was hard to see who was coming through the doors. I stood with Vincent Le Brun, waiting, and I saw the silhouette of a tall man walking towards me, ah there he is now … and I was surprised. He seemed to be only slighly older than me. He was tall, handsome, athletic and impeccably dressed. But from his impressive publication record I was expecting a greybeard and not this man I saw before me.

During my one-day stay at the LAS I was very well looked after. Olivier and Alain Mazure took me to lunch at a restaurant nearby surrounded by the rolling green fields of a golf course. I remembered Roger Davies’ advice and spoke slowly during my talk which detailed our painful efforts during my thesis in Durham to map a tiny part of the sky with hundreds of hours of telescope time. I soon learned that Olivier’s under-construction VIMOS instrument, combined with the new wide-field cameras coming online at the Canada-France-Hawaii-Telescope, planned to make all this instantly redundant and open up a completely new window on the Universe. Precise distance measurements would be possible for tens of thousands of galaxies and there would be photometry for millions more. The galaxy-counting skills I learned at Durham were what the team needed to help make the input catalogues for this new instrument. After my talk, I spent some time with Olivier in his office. I was immediately taken by the his motivation and the vast amount of data he intended to collect and the chance that it might answer all those hanging questions we’d had until now. It was very exciting and it was in France!

A few months later I had finished my thesis, and on the first week in January 1999, I started my postdoctoral position at the LAS with Olivier. Incredibly, they had offered me the position; I was certain that there must have been crowds of people banging on the doors of the LAS. Only later did I discover that there was only one other applicant. “Observational cosmology”, as Olivier’s job application promised, had yet to really come to the LAS which was not yet on the post-doc radar. A colleague from the UK even confided in me that he wouldn’t ring a telephone number in a French laboratory in case the person picking up the telephone spoke French to him! I soon discovered I was the only post-doc in the lab and in those early days almost the only person in the building after 19:00. The LAS had a certain charm: and there was a long table outside under the trees where you could eat lunch most days. After a few months there I got to know some wonderful people and my French steadily improved. However, the “observational cosmology group” promised in Olivier’s job announcement for the moment didn’t comprise more than five people, including myself and two or three students. I mention all this here only to insist that the LAM (the fusion of the LAS and Marseille Observatory) has become the great force that is today in surveys largely thanks to Olivier’s work.

In 1999, however, such a happy ending was far from certain. Data was steadily arriving from CFHT telescopes in Hawaii and the computer in my office had attached disks piling up to the ceiling. Could we keep with the data? Worse yet, VIMOS turned out to be a very challenging instrument to build and commission. It took all of Oliver’s skills in management and persuasion to get the instrument on the sky and keep to the schedule. The team worked very well together and although there was a very long period before the spectra arrived Olivier kept us motivated. At an observing run in Hawaii I met Yannick Mellier who put the resources of TERAPIX at our disposal which helped us a lot with the early data. In the end, despite these difficult early years, tens of thousands of spectra were collected and VIMOS has gone on to be one the most successful instruments at ESO.

Already, in the first few years of new century, the context was changing: in the space of a few short years observational cosmology was gaining in importance in the community. The group at LAS was growing. The skills I had learned were becoming increasingly important, important enough that in the summer of 2003 I was recruited as a staff astronomer at the Institute d’Astrophysique de Paris. Olivier played a very important role in that change through his tireless support of countless other projects and instruments. For example, the VIMOS spectrograph turned out to be a crucial instrument for spectroscopic follow-up of the COSMOS survey, one of the largest-ever allocations of Hubble Space Telescope time. As well as that, Olivier brought TERAPIX into the COSMOS project to help with initial imaging at CFHT. That was the origin of my own highly fulfilling involvement with the COSMOS project, a collaboration which is still going strong after a decade.

In the winter of 2017 at a meeting in Paris I told him that I had been diagnosed with thyroid cancer. The treatment was going well, I said. I am sure I told him (as I told everyone) that if you want to have a cancer, that’s the one you should get; treatment is straightforward. Unfortunately, only a week or so later, he would fall from his bicycle and be diagnosed with a brain tumor. It didn’t slow him down. He worked as tirelessly as ever to realise his countless projects even as his body weakened. He still came to meetings. He was there at at our COSMOS meeting in Copenhagen in the summer of 2018. One evening, Olivier, and Olivier Ilbert and myself ate together in a restaurant, outside on the terrace. Olivier was unfailingly positive even though he must have known his chances of survival were slim.

Today, more than twenty years after my first meeting with him in that distant winter of 1998, observational cosmology and survey astronomy is now firmly established in France. And this is in large part due to Olivier’s work.

52 photographs (2018) #25: In the round tower

52 photographs (2018) #25: In the round tower

Now, Copenhagen. Walking around on the city on a Saturday evening just after I arrived, I came across a round tower in the city centre. Inside, it looked like this:

In the round tower…

A smooth brick floor curves resolutely to the top of the tower. Some soft evening light shone through the windows.

Of course, I discovered once I got to the top that the Rundetaarn was once an astronomical observatory, built in 1642, thirty years before the Paris Observatory. Peering through a window near the top one can see instruments and telescopes. Today, the centre of Copenhagen, like the center of Paris is no longer an ideal location for observations, at least of celestial objects. 

The Strand magazine, 1896: a Howard Grubb Illustrated interview

The Strand magazine, 1896: a Howard Grubb Illustrated interview

After writing about telescopes in space in my last article, I was reminded of this “illustrated interview” of Howard Grubb, published in the Strand Magazine in 1896. It starts perhaps not very promisingly: “The poverty of Ireland is such that the superficial observers are apt to wonder whether any good thing can really come out of that distressful country”. It does improve from there! It was sent to me by a descendant of Grubb. It is very interesting, especially the part at the end about future large telescopes which, of course, will be floating in water. The image below is supposed be “casting the mirror for the great Melbourne telescope” but it doesn’t look like any kind of “astronomical” ceremony to me!

“Casting the mirror for the great Melbourne telescope”

Read the PDF here: