Sometimes an image is so compelling that we can ignore what it is telling us about its subject and just enjoy the brilliance. This is certainly true of this image of NGC 5068 released by ESA. But Universe Today readers are curious, and after enjoying the galactic portrait for a while, they want to know more.
NGC 5068 is close enough to be a galaxy. It is about 17 million light-years away and is frontal from our perspective, making it an optimal object for scientific observations. It is over 45,000 light-years in diameter and is a field galaxy, meaning it is not associated with a group or cluster of galaxies. NGC 5068 is a bit lonely out there.
The JWST’s view of the galaxy comes from two of its instruments: MIRI and NIRCam. Each of these two instruments captured NGC 5068, and the main image is a combination of the two.
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The image shows a myriad of individual stars and gas clouds illuminated by the stars embedded in them. While those are jaw-dropping, and the JWST’s ability to resolve so many stars is extraordinary, something else catches our attention: the galaxy bar, seen in the upper left half.
JWST captured the images as part of the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) program, which studies the multiscale processes of galaxy evolution, star formation and stellar feedback. PHANGS combines high-resolution observations with the latest theoretical models. The JWST is the latest contributor, but other telescopes such as Hubble and ALMA have already made major contributions to PHANGS.
The idea behind PHANGS is to measure the early stages of star formation and feedback. They also want to understand how dust-influenced starlight can track gas and star formation in different galactic environments. In short, star formation affects many things and a better understanding of the whole process will lead to breakthroughs in other areas of astrophysics and astronomy.
It’s ambitious, but the PHANGS team has made a lot of progress, including the recent publication of its 100th paper. The JWST is the latest telescope to contribute, and as these JWST images make clear, PHANGS has more progress and papers in its future.
PHANGS already has a large collection of images and data of things like star clusters, regions of active star formation, clouds and molecular complexes, and star-forming emission nebulae. But the JWST is taking PHANGS to the next level. Its infrared observing power can peer through the clouds of gas and dust that hide the star formation process from other telescopes. One of the explicit scientific goals of the JWST is to peer into these hidden regions more effectively than its predecessors.
Astronomers are interested in barred spiral galaxies like NGC 5068 for a variety of reasons, but their bars are one area of particular interest. The bars affect the flow of star-forming gas in the galaxy, and astronomers want to better understand the role they play.
Bars have an important task. They funnel gas from spiral arms into the galactic center and trigger the birth of stars. They also mix material in the inner regions of a galaxy and stimulate radial migration. Astronomers think the bars are a recurring rather than a permanent feature. On a scale of about two billion years, a galaxy oscillates between having a bar and not having one.
There is some evidence that bars affect what is called the metallicity gradient, but it is not conclusive. Metallicity refers to the presence of elements heavier than hydrogen and helium. Everything heavier than hydrogen and helium was created inside stars, while hydrogen and helium have been around since the Big Bang. So metallicity tells astronomers a lot about the nature, age, and composition of a galaxy.
Typically, the stars furthest from the galactic center, in the galactic halo, are older and have the lowest metallicity. Near the galactic center, where the rod channels gas, the stars are younger and more metallic. The metallicity gradient of a galaxy describes how the metallicity changes from the galactic nucleus to the central bulge, disk, spiral arms, and finally the halo. Globular clusters on the outskirts of a galaxy contain the oldest stars with the least metallicity.
The metallicity gradient is an important feature of galaxies such as NGC 5068. It is a measure of how rich a galaxy is in elements heavier than hydrogen and helium and how these elements diffuse through the galaxy. The early stages of a galaxy’s evolution leave their mark on the gradient. When astronomers can trace the metallicity of stars from the galactic center outward, it creates a gradient.
In the Milky Way, the gradient shows decreasing metallicity from the nucleus to the halo. It’s called a negative gradient. As observing technologies improved, astronomers found more detail in the gradients. The barred spirals have a break in the slope, indicating that something has shifted as the galaxy has evolved. The slope break can be shallow then steep or steep then shallow.
NGC 5068 has a “shallow” radial metallicity profile, meaning that the inner stars have similar metallicity, hence a shallow slope. But outer stars have more variation in their metallicity, meaning they have a shallow slope in the gradient.
A 2023 paper showed how NGC 5068’s bar induces gas inflows into the central region. The inflows mix the interstellar medium within the rod region, which flattens the metallicity gradient within the break radius (0.82 kpc). “The nearly flat internal metallicity gradient (?0.005 dex/kpc) is strong evidence of efficient radial migration and rod-driven material mixing,” the paper states.
Although a galaxy’s bar affects the metallicity gradient, there is no single way this happens. Different barred spiral galaxies exhibit different gradients, so there is no single common mechanism. There’s a lot going on in the center of a galaxy, and while a rod can funnel new star-forming gas into the region and drive star formation, there are also outflows that have their effect.
These are the kinds of details astronomers are trying to better understand with the JWST. But the picture is stunning no matter how deeply we want to dig into it.
Astronomy is about understanding the cosmos and how we are a part of it. But it is also about open-hearted wonder and awe at the beauty and grandeur of nature.
The JWST is feeding us a balanced diet of both.
A high resolution TIFF version of this stunning image is below for download.
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