Image: The light collecting surface of the 18 segmented mirrors of 1.315 m side of the James Webb Space Telescope will make it possible to capture more photons and therefore to see further. We gain a factor of 100 between Hubble and James Webb Space Telescope (100 hours break from HST = 1 hour break from JWST).
NASA / JWST SCIENCE TEAM
JWST in the depths of space
James Webb Space Telescope (JWST) is the successor to Hubble Space Telescope (HST) and Spitzer or Space Infrared Telescope Facility (SIRTF). It was launched on December 25, 2021 and commissioning is scheduled for June 2022 because it requires the optical assembly located in the shadow of the heat shield to reach a temperature compatible (7 K) with infrared observations. .
Infrared radiation carries a considerable wealth of information, but the Earth's atmosphere diffuses or blocks some of the thermal radiation and prevents observation in most wavelengths of the infrared segment.
In infrared observation from space, it is necessary that the environment of the sensors is at a temperature lower than that of the radiation that is to be captured.
JWST was designed to exploit infrared sub-segments (PIR and MIR). For this it is particularly protected because it orbits around the point of Lagrange L2 located 1.5 million kilometers in the direction opposite to the Sun. It travels this large orbit in six months at a speed of about 1 km/s. Its distance from the Lagrange point L2 varies between 250,000 and 832,000 km. Its distance from the Earth oscillates between 1.5 and 1.8 million kilometers. Its maximum excursion above the ecliptic plane is 520,000 km. This orbit has been finely calculated so that the space telescope is never in the shadow of the Earth because its only source of energy comes from its solar panels. The orbit around L2 being unstable, the space telescope must activate its propulsion every 3 weeks.
One of the objectives of this $ 10 billion project is to capture the first lights of our deep universe, in order to understand how galaxies and stars appeared after the Big Bang.
By observing the depths of space and time in the infrared, astrophysicists hope to bring out protogalaxies from the deep black of space.
The world's most powerful telescope's first asset compared to Spitzer and Hubble is its mirror. Its hexagonal mirror in gold-plated beryllium (gold increases the reflectivity of infrared light) has 18 hexagonal segments of 1.315 m side (equivalent to a primary mirror of 6.5 meters). The mirror, protected by a thin layer of transparent amorphous glass, weighs only 705 kg thanks to its "honeycomb" structure which reduces its weight without affecting its resistance to expansion and contractions caused by temperature variations.
The other advantage of the JWST is its observation window which extends between 600 nm to 28000 nm that is to say in the near and medium infrared (PIR and MIR) including a part of the spectrum located in the visible. By comparison, that of HST ranges between 10 nm and 2500 nm, that is to say in the ultraviolet, visible and near infrared (NIR) range. The Spitzer observation window extends between 3600 and 160,000 nm in the medium and far infrared (MIR and LIR).
These two assets will enable it to search for primitive galaxies in deep space but also to analyze the biosignatures of exoplanets in close space.
NB: Infrared radiation is the segment of the electromagnetic spectrum located between the visible domain and that of the microwaves. Its wavelengths range from 0.78 µm (closest to the visible spectrum) to 5 mm (closest to microwaves). The infrared domain segment is divided into near infrared "PIR" (0.78 µm - 3 µm), mid infrared "MIR" (3 µm - 50 µm) and far infrared "LIR" (from 50 µm - 5 mm).
JWST in search of protogalaxies
Image: A great diversity of galaxies, some of which are 11 billion years old, dot the ultra-deep field of the cosmic tapestry. This composite image was created from photographs taken by the Hubble Telescope between 2003 and 2012. In this tiny hole in the southern celestial vault, we can count around 10,000 galaxies.
With JWST, by observing in the mid-infrared with the same exposure time, we could see 100 times more galaxies than in the HST image. Credit Nasa, ESA
With the Hubble Space Telescope, cosmologists were able to easily capture galaxies with a Redshift of 1, corresponding to galaxies 6 billion years old. With exposure times of 1 hour, HST captured galaxies with a Redshift of 4, corresponding to galaxies 1.5 billion years old. With exposure times of 15 days, HST captured near-infrared galaxies with a Redshift of 10, corresponding to galaxies 500 to 700 million years old after the Big Bang.
According to simulations, with JWST cosmologists hope to capture primitive galaxies with a Redshift of 20, which could reveal galaxies that are 200 to 400 million years old. It is possible that in tiny regions of the deep sky, lurk millions of very young glowing protogalaxies. It is precisely this process of formation of protogalaxies that interests cosmologists.
How did the first galaxies form after the Big Bang and how did they evolve?
Due to the expansion of the universe, galaxies move away from each other at a speed proportional to their distance (Hubble's law v = H0d - Hubble's constant H0 = 73 km/s/Mpc).
In fact, the greater the speed of recession, the further the galaxy is in space and time. By observing in the mid-infrared, JWST will provide us with images close to when the cosmos emerges from the dark age of its history.
To form a galaxy you have to collect a lot of baryonic matter (ordinary matter).
We now know that this accretion cannot be done without dark matter because without dark matter, the gravitational well is not deep enough. We also know that all galaxies are connected by dark matter.
During this early period of galactic formation, scientists hope to see turbulence, the presence or absence of a black hole, the shape of a protogalaxy, and the dynamics of the large Redshift gas. And why not understand the nature of dark matter!
NB: The redshift is a positive shift (z>0) towards the red, that is to say an increase in the wavelength, and a corresponding decrease in the frequency and of photon energy. Conversely, a decrease in wavelength and a simultaneous increase in frequency and energy, is known as negative redshift, or blueshift (z<0) (red and blue are the extremes of visible light spectrum).
The causes of the redshift:
- The radiation moves between objects which move away ("relativistic" redshift).
- The radiation moves towards an object in a lower gravitational potential ("gravitational" redshift).
- The radiation moves in an expanding space ("cosmological" redshift).
JWST in search of life
Image: JWST went in search of life.
Credit ESA David Sing
In January 2022, we know of 4884 confirmed exoplanets and 8288 candidates (ref. nasa catalog). Among the confirmed exoplanets some have properties that could be favorable to life.
Since the discovery of the first exoplanet (51 Pegasi b in October 1995), humanity's ultimate dream has been to find rare worlds like ours housing life.
If we can know the mass, radius and orbital parameters of an exoplanet, no space telescope has the ability to directly "see" the surface of an exoplanet. However JWST could analyze the composition of the gaseous envelope of an exoplanet.
During an exoplanet transit with its star, part of the light crosses the different heights of the exoplanet's atmosphere. This absorbed light (black absorption lines) corresponds to the chemical bar code of the composition of the atmosphere (water H2O, methane NH4, carbon dioxide CO2, ethane C2H6, etc.).
Despite the very small amount of light analyzed by the JWST spectrograph, it is possible to characterize the chemical composition of exoplanetary atmospheres. It is thanks to JWST's large mirror that scientists believe they can observe the absorption spectra of gas envelopes.
Depending on the wavelengths of the JWST observation spectrum, we will be able to obtain a kind of chemical identity card of the exoplanets and many signatures could appear.
In summary, with the JWST, observation in the field of infrared waves could revolutionize cosmology and exobiology. JWST has now gone into the depths of space and time in search of galaxies but also in the inner suburbs of our solar system in search of life.