Cosmic gem: James-Webb explores an early spiral galaxy

Not long ago, we shared with you the discovery of a primitive galaxy presenting the mass of stars that the Milky Way must have had at the same period. Called Firefly Sparkle, it is not, however, a spiral galaxy. The one that James-Webb has just discovered has only blown out 1.5 billion candles and yet has a beautiful, well-defined spiral shape with two fairly clear arms. So what? Well, this is very rare this early in the history of the Universe. For the moment, this galaxy only bears the rather unpoetic code name A2744-GDSp-z4; so we will call it Spira Aurea (golden spiral) for the purposes of this article.

This cadet of the Milky Way is located in the magnificent galaxy cluster Abell 2744, also called Pandora Cluster:

Spira Aurea already “weighs” 14 billion solar masses!

Spira Aurea already has very good measurements for its very young age with a stellar disk of approximately 32,000 light years for 14 billion solar masses, as well as a central bulge, this very luminous zone located at the center of galaxies and where the oldest stars are concentrated.

A galaxy is obviously made up of stars, and what astronomers have found is very interesting. Indeed, in its early youth – as James-Webb observes it today – Spira Aurea formed 58 stars per year (or rather the equivalent of 58 solar masses to be more rigorous), i.e…. 58 times more than our galaxy. But this is expected, because galaxies have very high star formation rates early in their existence.

The average age of its stars, determined from the light spectrum and the famous HR diagram (which we will undoubtedly explain in detail soon), is estimated to be 228 million years old. Spira Aurea thus formed a majority of its suns just 839 million years after the Big Bang. As a result, it must have formed 10 billion solar masses in just a few hundred million years, which is a really high rate!

The authors of the study note that these results call into question the classic scenarios of galaxy formation and evolution. They hope for new observations with the James-Webb in order to bring new elements to cosmology.

But in fact, what exactly are these famous spiral arms?

We thank you for this excellent question, because cosmologists are not absolutely sure. The most likely thing is that these arms are some sort of density waves, a bit like traffic jams on the highway. To understand this, we must understand that the Sun, which rotates in the Milky Way, has not always been in the galactic arm in which we are currently. It will even come out one day: we enter and exit the density waves that are these arms, a bit like on the highway we extricate ourselves from a traffic jam to find another a few kilometers further on…

In fact, if the arms of galaxies were real material arms, with the stars still remaining in them, the difference in speed of the stars near the center and at the end of the arms would cause them to roll up and scatter.