JWST takes a peek at the first ever galaxies – Astrobites

Title: Panic! At the Disks: First Rest-frame Optical Observations of Galaxy Structure at z>3 with JWST in the SMACS 0723 Field

Authors: Leonardo Ferreira, Nathan Adams, Christopher J. Conselice, Elizaveta Sazonova, Duncan Austin, Joseph Caruana, Fabricio Ferrari, Aprajita Verma, James Trussler, Tom Broadhurst, Jose Diego, Brenda L. Frye, Massimo Pascale, Stephen M. Wilkins, Rogier A. Windhorst, Adi Zitrin

First Authors Institution: University of Nottingham, Nottingham, UK

Status: Accepted to The Astrophysical Journal Letters, available on arXiv

Ever since the first data release of the James Webb Space Telescope (JWST) in July, it has become clear that this telescope is going to completely transform our view of the distant Universe. Galaxies that looked like featureless blobs when viewed through the Hubble Space Telescope can now be resolved in incredible detail (see Figure 1), despite the fact that Hubble has been one of the worlds leading telescopes for the past 30 years.

Figure 1: Four galaxies from the SMACS 0723 field (the focus of todays paper), as seen by the Hubble Space Telescope (left) and the James Webb Space Telescope (right). Each one displays features that were undetected with Hubble, but can easily be seen with JWST. Credit: NASA/ESA/STScI.

Being able to measure the shapes of galaxies (known as their morphology) is vital if we want to understand how galaxies, including our own, were formed. Galaxies typically come in two shapes: thin, delicate disk-shaped galaxies, and spheroid-shaped elliptical galaxies, but it is still not really clear how and when these different galactic structures emerged. Todays paper uses early JWST observations of a large galaxy cluster, called SMACS 0723, to measure the shapes of very distant galaxies. With this exciting new data, the authors hope to expand our knowledge of galaxy evolution all the way to the very dawn of our Universe.

This photo of SMACS 0723 is one of the first images to be released from JWST. The cluster is located about four billion light years away at a redshift of 0.4, but todays paper actually looks at even more distant galaxies, in the background of this image many of these have been magnified by the gravitational lensing of the cluster. Specifically, it looks at 280 background galaxies at redshifts between 1.5 and 8, meaning we are seeing them just 1-4 billion years after the beginning of the Universe.

The authors firstly measure galaxy shapes using quantitative properties of galaxies, such as their concentration and asymmetry. Their really exciting findings, however, come from classifying these galaxies by eye, splitting them into three categories: disks, spheroids, and peculiars.

Galaxies in this third class have an irregular shape, which can be caused by processes such as starbursts or tidal interactions. Alternatively, collisions between galaxies (known as galaxy mergers) that are currently in-progress can lead to these peculiar galaxies. These violent events are thought to play a major role in galaxy evolution: in the early Universe mergers allow large amounts of mass to clump together, which can later form a galactic disk. Later on, they can destroy these fragile disk structures, turning disk galaxies into featureless ellipticals.

It turns out that at high redshifts (between 3 and 6), about half of galaxies have a disk shape (Figure 2). This is much higher than we previously thought the data from the Hubble telescope shows that it found a disk fraction of less than 10% at similar redshifts! Interestingly, according to JWST, the disk fraction also stays roughly constant across the whole range of redshifts.

Figure 2: Fraction of spheroid, disk, and peculiar galaxies at different redshifts, measured with JWST in todays paper, and with Hubble (HST) in previous work. The trends found by Hubble had predicted the number of disks would decrease at redshifts greater than three, and that most galaxies would be peculiar. JWST shows that this is not the case. Figure 4 in todays paper.

Our current idea that mergers assemble galaxies in the early Universe means that we would expect to find lots of peculiar galaxies and few disks at high redshift, as these disks are still in the process of forming. However, the near-constant disk fraction found in this study indicates that disk galaxies (like the Milky Way) have existed in a fairly stable state for more than 10 billion years, seemingly contradicting our old ideas.

So whats going on? There are several ways to interpret these results. It could be that almost all mergers occur extremely early in the Universe, quickly forming disk galaxies, and that these disks survive until the present day because recent mergers are far less common than our current theories suggest. Alternatively, it could be that only some classes of galaxies are built up by mergers, or even that mergers are simply far less likely to destroy disk structures than we previously thought.

Whatever the case, it indicates that we may need to refine current theoretical ideas about how galaxies assemble and evolve through mergers, which is one of the key predictions of our widely-accepted model of the Universe (the Lambda cold dark matter, or CDM, model). Some articles based on this work have gone a step further, stating that this research disproves CDM, or even the Big Bang. However, despite the homage to noughties emo-pop in the title of this paper, theres really no reason to panic. Tuning and re-tuning theories to fit new data is a normal part of the scientific process. In fact, this paper is exciting: it tells us that we still do not truly know where galactic structure came from, but that new science carried out using this new telescope will finally give us a chance to understand the origins and lives of galaxies.

Astrobite edited by Aldo Panfichi

Featured image credit: NASA/ESA/STScI

About Roan HaggarI'm a PhD student at the University of Nottingham, working with hydrodynamical simulations of galaxy clusters to study the evolution of infalling galaxies. I also co-manage a portable planetarium that we take round to schools in the local area. My more terrestrial hobbies include rock climbing and going to music venues that I've not been to before.

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JWST takes a peek at the first ever galaxies - Astrobites