Journey to the Middle of Webb's Deep Field

Astrophysicist André Grandchamps of the Rio Tinto Alcan Planetarium analyzes the image of the first deep field of the James Webb telescope.

The first deep field of the James Webb Telescope.

Beyond its celestial beauty, what can be seen in this image described as the deepest of the Universe captured to date?

The answer is short, but evocative: thousands of galaxies – including some of the most distant ones that appear to us for the first time thanks to the NIRCam and MIRI instruments of the James Webb Telescope. These two imagers, supported by the Canadian precision guidance detector, which makes it possible to point to a target with extreme precision, observe the Universe in the near and mid-infrared. These wavelengths pass through dust clouds and reveal celestial objects that remained invisible to other telescopes such as Hubble.

The galaxy cluster SMACS 0723 as seen by Webb (left) and Hubble (right).

Let's start by recalling that the photograph corresponds to a very small part of the sky in the southern hemisphere located in the constellation of the Flying Fish. It's like holding a small grain of sand between two fingers towards the sky, recalls André Grandchamps.

The snapshot shows the galaxy cluster SMACS 0723. The galaxies that form this cluster lie towards the center of the image and take on the appearance of hazy whitish circles with a little of nebulosity around in shades of gray, notes the astrophysicist.

The cluster galaxies correspond to the fuzzy white spots, some of the brightest of which are in the center of the cluster. 'image.

Not to be confused with the stars, which are in the foreground of the image.

The celestial bodies which appear with six bluish lines around them are stars of our galaxy which are in our field of vision, between the cluster and us, notes Mr. Grandchamps.

Stars of our galaxy are in our field of vision, between the cluster and us.

“While these Milky Way stars are a few tens of thousands of light-years away, the galaxies in the cluster are much further away at about 4.6 billion light-years away. Earth. »

— André Grandchamps, astrophysicist at the Planétarium Rio Tinto Alcan

The light that these galaxies reflect was therefore emitted shortly before the formation of our planet.

< p class="e-p">The large mass of the cluster distorts the space-time around it. This gravitational lensing effect amplifies the light emitted by the more distant galaxies behind it. They appear in the image as darker, often stretched orange spots around the cluster.

Close-up of a distant galaxy surrounded by two galaxies in the SMACS 0723 cluster.

“You have a nice example of this to the right as you go to the top of the image. The two whitish dots are two cluster galaxies. The sort of flattened orange ridge in between is a distant galaxy. »

— André Grandchamps, astrophysicist at the Planétarium RioTinto Alcan

The distance to this orange ridge is not yet known, but several galaxies that lie behind the cluster lie between 11 and 13 billion light-years from Earth.

Most small orange galaxies are very far away. The distance of one of them, which is hardly visible in the full image, is estimated at 13.1 billion light-years.

The orange dot at the bottom left of the center corresponds to a galaxy located 13.1 billion light-years away.

Mr. Grandchamps adds that the very small orange dots are not necessarily the most distant. They can also belong to objects that are inherently small. To know if an object is very far away, it must be evaluated.

The Canadian NIRISS instrument (for imager and spectrograph without slit in the near infrared) makes it possible to study celestial objects in order to establish their composition, but also to measure their distance.

By breaking down the light emitted by a celestial object using spectroscopy, it is possible to know its composition and distance. It's that every chemical element in the Universe has different characteristics, a kind of chemical fingerprint. For example, the characteristics of hydrogen are different from those of helium and lithium, explains Grandchamps.

The lens effect can sometimes create a mirror effect. The phenomenon is observable to the left and to the right of the central cluster. It is possible to see each of the galaxies twice, which forms luminous arcs. Furthermore, Webb's image distinctly reveals their bright, star-filled cores, as well as the orange star clusters along their edges.

These arcs are mirror images of the same galaxy. The arc light to the left of the center blue line (initial image) was emitted 9.3 billion years ago.

The two flattened orange lines that are above each other at the bottom of the image have the same spectral signature, so have the same chemical compositions. They probably belong to the same galaxy, notes André Grandchamps

It is also possible to see a galaxy speckled with stellar clusters; it's near the end of the center star's vertical blue line, to the right of a long orange arc. You can see pockets of stars in formation reflected from top to bottom.

The spotted galaxy appears in the center of the image.

The deep field also reveals many types of galaxies, such as a galaxy spiral, visible to the right of the original image.

This galaxy is of the spiral type, like our Way milk.

• It was US President Joe Biden who unveiled the now historic image at a July 11 event at the White House, six months after James Webb was launched into orbit.
• The James Webb Deep Field Telescope is the eighth of its kind published since 1995. The Hubble Telescope is the first-ever deep field feat. The instrument picked up four others, the last of which (extremely deep field) dates back to 2012. The other deep fields were taken using the Chandra telescopes (1999) and the European Southern Observatory's Very Large Telescope ( 2012).

Webb's Deep Field shows the same star cluster as seen with the Hubble Telescope.

“It's totally voluntary because we wanted to compare the two telescopes. With just 12 hours of exposure, Webb got about as good a picture of very distant objects as Hubble could in 100 hours. We can now dream of the results that the Webb telescope will obtain with the same exposure time! »

— André Grandchamps, astrophysicist at the Planétarium RioTinto Alcan

Webb's technical abilities would make it possible to go back in time and, perhaps, to see the first galaxies and stars that were formed between 500,000 years and a billion years after the great formative boom of the Universe.

We had never had instruments that allowed us to hope to see them, it is now possible. We know they appeared one day because we are here today, enthuses André Grandchamps.

He uses this parallel to illustrate the work that scientists wish to accomplish: Imagine aliens arriving on Earth and encountering only adult humans. They will wonder where we come from! They will eventually find a daycare and a nursery and understand our evolution. Finding the (primordial) nursery of stars is kind of what Webb is working on. Certainly we see more mature stars and galaxies because they spend more of their evolutionary time in their "adult life".