galaxies

Galaxies Evolved Surprisingly Quickly in the Early Universe

Anyone familiar with astronomy will know that galaxies come in a fairly limited range of shapes, typically; spiral, elliptical, barred-spiral and irregular. The barred-spiral galaxy has been known to be a feature of the modern universe but a study from astronomers using the Hubble Space Telescope has recently challenged that view. Following on observations using the James Webb Space Telescope has found the bar feature in some spiral galaxies as early as 11 billion years ago suggesting galaxies evolved faster in the early Universe than previously expected. 

Our own Galaxy, the Milky Way is a spiral galaxy with a central nucleus and spiral arms emanating out from the centre. Our Solar System lies about 25,000 light years from the centre. Look at the galaxies in the sky though and you will see a real mix but generally they fall under the four main categories. Edwin Hubble tried to bring some structure to the different shapes by developing his galaxy classification scheme to articulate not only the shape but also the sub categories within them. 

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It has been known for some time that galaxies aren’t static. They move and they evolve and change. Spiral galaxies for example, as they age, they often develop a bar feature. The bar joins up the spiral arms instead of a nucleus connecting them and it is believed they are temporary, forming when a build of gas creates a burst of star formation. 

The existence of a bar in a spiral galaxy suggests that the galaxy is fairly stable. Understanding just how the bar feature forms is key to understanding the evolutionary process of the galaxy itself. All previous observations showed that the appearance of the bar significantly reduces from the nearby Universe to redshifts near a value of one. This tells us that the bar seemed to be a modern feature and not present in the early Universe. 

The barred spiral galaxy NGC 1300. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

In a new paper by lead author Zoe A Le Conte, observations from the more sensitive James Webb Space Telescope report that galaxies to greater redshift are studied for bar features. Data is used from the Cosmic Evolution Early Release Science Survey and the observations from the Public Release Imaging for Extragalactic Research studies. Only the galaxies that also appear in the Cosmic Assembly Near Infra Red Deep Extragalactic Legacy Survey are used giving a sample of 368 face on galaxies. 

The team visually searched through the 368 galaxy selection to classify and identify those with bars between redshifts 1 and 2 and then repeated the exercise for those between redshift 2 and 3. As expected, the fraction of bars reduced from around 17.8% between a red shift of 1 and 2 down to 13.8% at the greater red shift of 2 to 3. 

The study revealed that JWST’s infra-red sensitivity picked up twice as many barred-spiral galaxies than the HST’s more blue sensitive imaging platform. Le Conte and her team conclude that the evolution of bars in spiral galaxies began to appear at a much earlier epoch, around 11 billion years ago. 

Source : A JWST investigation into the bar fraction at redshifts 1 ? z ? 3

Mark Thompson

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