For astrophysicist Sylvie Vauclair: “James Webb exceeds all expectations!”

What do the JWST images released by NASA since July tell us?

The images revealed by the James Webb Space Telescope (JWST) are breathtaking, even for professional astrophysicists who had good reason to expect exceptional results. But JWST, as they say in some questionnaires, “exceeds expectations”! Everything worked more than perfectly in orbiting and installing the various parts of the telescope, including its heat shield and shields. It is a huge success. It is not insignificant here to recall that the launch of this excellent instrument, developed by NASA for many years, with the participation of the European Space Agency (ESA) and the Canadian Space Agency (CSA), has been carried out by a French Ariane 5 launcher from the Kourou base in French Guiana. A great responsibility for the French scientists, who have confirmed their capabilities in the space field with this flawless launch.

Unlike the Hubble Space Telescope (HST), which orbits Earth at an altitude of approximately 600 km, JWST orbits the Sun at the same time as Earth, 1.5 million km from our planet. That is, the Earth is always between the Sun and it. This particular point in space is called the Lagrange point L2. In fact, the JWST describes a kind of figure eight curve around the L2 point for somewhat complex reasons of spatial stability.

People looking at an image of JWST for the first time are usually surprised by its shape. Most publicly known telescopes look like tubes. At the bottom of the tube there is a “primary mirror”, which has a parabolic shape and which concentrates the radiation coming from the room onto another mirror, called “secondary”. This is where observations can be made, except that the radiation received by the secondary mirror is generally sent back to another location by a system of other mirrors, to an eyepiece if you are looking directly, or to analyzer instruments lights if we are doing scientific analysis. The tube only serves to protect the instrument, especially to avoid stray light. The JWST includes all of this except for the tube it doesn’t need in space. On the other hand, he needs strong protection against solar radiation, the famous “shield”, which gives the impression of a wizard on his magic carpet!

So from this particular spatial location, with a 6.5 m primary mirror (compared to 2.4 m for HST), JWST sends us images of extraordinary resolution with enormous detail. This is where the biggest surprise comes from. We discover the intimate structure of known celestial objects that do not necessarily resemble what digital simulations allow us to imagine. So yes, in addition to the beauty of the images received, JWST provides a wealth of information to professional astronomers who have years of work ahead of them to better understand the universe to which we belong…

What contribution compared to those of the mythical Hubble?

Thanks to its location in space, as well as the size and special structure of its primary mirror, JWST can capture images similar to those of the HST in much less time. The famous “deep field” of the HST (Hubble Deep Field), which excited so many people, scientists and the public by showing thousands of very distant galaxies observed in the same direction of the sky, required many days of exposure, while a similar image is obtained by the JWST in a few hours.

Another very important difference between JWST and HST, absolutely fundamental to the study of the distant universe in particular, is that they do not observe the radiation from celestial bodies in the same wavelengths, although their observation intervals overlap. While the HST observes the visible light of stars as well as the ultraviolet light and the part of the infrared closest to the visible, the JWST focuses its observations on the infrared with some visible light but no ultraviolet. Its primary mirror also consists of 18 hexagonal beryllium elements, highly resistant to extreme cold, covered with a thin layer of gold, a metal chosen for its great power of reflection in the infrared.

Astrophysicists expect a lot from far-infrared observations. In fact, the universe is expanding, that is, the galaxies are moving away from us, the faster they are far away, and this movement away is manifested by a shift towards the red and l infrared light they emit. So the longer we observe in the infrared, the better we can distinguish very distant galaxies. And as we see these galaxies in the past, at the time when light left them to reach us, these infrared observations lead us to the direct view of the first galaxies to have existed in the universe.

There are also many other reasons to wait for new discoveries thanks to infrared observation. The birth of planetary systems, for example, takes place in a cocoon of gas and dust that hides visible light but allows infrared to pass…

Which images from JWST surprised you the most?

The first image of JWST, unveiled to the public on July 11, 2022 by Joe Biden, is absolutely stunning. We can distinguish very distant galaxies, and above all, in all its beauty, the well-known phenomenon of gravitational arcs (Einstein’s arcs). These bright figures are distorted, arcuate views of other galaxies far beyond those visible directly in the image. The rays that reach us from these hyper-distant galaxies are actually deflected by the matter they encounter on their way, hence the distortion of the images.

Other images from JWST particularly impressed me, the very detailed ones of interstellar matter, and also the images of the Cartwheel Galaxy, surrounded by a brilliant ring of very young stars, formed during collisions with d other galaxies still visible nearby.

But most amazing to me was the very sharp image of the central regions of the galaxy Messier 74 (NGC 628), also known as the “Phantom Galaxy”. It appears as a kind of canvas everywhere perforated with circular shapes. Be careful, make no mistake about it, in reality these are not holes, they are large dense clouds which look dark in the galaxy precisely because of their density!

When and how will the analyzes and scientific interpretations of these images develop?

The radiation collected by JWST has already been analyzed in situ by four instruments located behind the mirror: the NIRCam camera, the NIRSpec spectrograph and the NIRISS spectrocamera operating in the near-infrared and the MIRI spectrocamera operating in the mid-infrared . There are therefore not only cameras that provide images, but also spectrographs that transmit spectral analyzes of the radiation, processed according to the wavelength.

Analyzes and interpretations of these data began immediately. It must be said that astrophysicists worldwide have been preparing for this for many years. Hundreds of projects had been submitted in areas as diverse as exoplanets, galaxies, the intergalactic medium, large structures in the universe, the solar system, stellar physics, the interstellar medium, black holes… The Commission’s international programs selected more than 200 for the first year.

A few privileged international teams, distinguished for their special involvement in JWST, were admitted to work as soon as the telescope was commissioned, in an “early” framework. One of these teams, specialized in the study of the formation of planetary systems, especially in the Orion Nebula, is coordinated by the Toulouse astrophysicist Olivier Berné. This French researcher was therefore one of the first scientists in the world to access JWST images!

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