James Webb facing a cosmic wall: why see the very first galaxies is so difficult

The quest for the very first galaxies in the universe, born only a few million years after the Big Bang, is one of the reasons for being in the James-Webb space telescope. A team of astronomers has just determined that it will be very difficult to observe galaxies further, therefore earlier, than those recently found. Explanations …

The further we see, the earlier we see in the universe

Shortly after its launch on Christmas day 2021, the NASA infrared telescope had imaged the Galaxy closest to the Big Bang, Jades-GS-Z14, seen in the universe 14 times younger than today, about 300 million years on the current 13.78 billion.

Recall that because the light emitted by these galaxies takes time to arrive to us (at 300,000 km/s), we see them as they were millions, even billions of years. This is of course the case for all these primitive galaxies, and that is why we can practically scrutinize the first ages of the universe.

This one being expandingthe more we go back in time, the more you see it young and little. The light that runs through the space to us must therefore get rid of distances which go by increasing due to the expansion, which has the effect of stretching its wavelength and making it more and more red. This effect is logically named “Red offset” (REDSHIFT in English) and scientific popularizers often associate it with the sound becoming more serious with an ambulance which is moving away. Astronomers symbolize it by a small “Z”, hence the Z14 of the Jades-GS-Z14 galaxy. Her REDSHIFT is 14, which is also the enlargement factor of the universe between the moment when this galaxy sends us its light and now.

And the very first galaxies born just after the Big Bang?

Logically, the very first galaxies formed in the universe of around 100 to 200 million years must have their light excessively offset towards red. A few months ago, a team had proposed candidates at Z = 19 approximatelybut they have not yet been confirmed to our knowledge. The problem is that these truly primitive galaxies are surrounded by dust which also tends to make their light red!

Indeed, the clouds of dust block the “visible” light and above all let pass infrared waves. It is therefore very difficult when astronomers find a galaxy supposedly very very far from knowing whether its distance is real or artificially increased by said dust.

The barrier of the universe 15 times smaller (Z = 15)

Astronomers have therefore used surveys from the sky to look for galaxies candidates for a red time (Z) greater than 15. They found it nine to a “Z” possibly higher than Jades-GS-Z14, therefore potentially older than the latter. They nevertheless determined that some were likely to be intruders, ie galaxies pretending to be older than they are. This last point, which one could judge thorny, can however constitute a real advance. Indeed, if a profile of false candidates can be drawn up, this will help future observations to save time. In the end, there are three galaxies that resist their selection and can claim to be older than the famous Jades-GS-Z14.

No galaxy to a red to red greater than 20 was found in the surveys studied. The team concludes in particular that if some galaxies to a “Z” greater than 15 were confirmed, this would require a revision of the theoretical models according to which they should be less numerous.

In conclusion of their study, astronomers offer a methodology likely to help find these elusive galaxies born in the universe of only 150 million years.