New Webb images illuminate the formation of a galaxy cluster

New Webb images illuminate the formation of a galaxy cluster
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Separating different wavelengths of light allows us to track the movement of material toward and away from Earth.
expand / Separating different wavelengths of light allows us to track the movement of material toward and away from Earth.

A research team is publishing a paper based on new images taken by the Webb Space Telescope. The images reveal a dense concentration of matter in the early Universe, potentially showing early stages of the formation of a galaxy cluster. And thanks to the available spectrograph, Webb was able to confirm that several galaxies previously imaged by Hubble were also part of the cluster. It even tracked the gas stream ejected by the largest existing galaxy.

Graphing the spectrum

The key hardware for this job is NIRSpec. Near Infrared Spectrograph This is part of Webb’s suite of tools. While the instrument itself is quite complex, it works on principles that are important for things like your cell phone’s camera to work.

In these consumer cameras, sensors record the brightness of three different areas of the visible spectrum: red, green and blue. The resulting images are made by combining this information with different areas of the image with different intensities of each of these colors.

A spectrograph also works by monitoring the light intensity in a limited area of ​​the spectrum. The main difference is that the slices of the spectrum displayed are much smaller than the entire range of a color like blue. And in this case, they are not part of the colors – all wavelengths are in the infrared region of the spectrum. However, just like RGB images produced by a camera, each part of the spectrum can either be analyzed individually or combined into a full “color” image that includes a wide spectrum range.

Why is a spectrograph useful for looking at distant objects? There are two ways this is critical to work. The first is that light from the early Universe is redshifted as the Universe expands as it travels to Earth. Energetic photons at wavelengths such as UV are gradually stretched until they are recorded by Webb as infrared photons. Knowing exactly how stretched they are tells us the distance to objects, and to determine this we need to know the wavelengths available. A spectrograph provides this information.

The second key capability provided by a spectrograph is the tracking of motion materials. All elements have a set of specific wavelengths from which they emit light. However, if they are in motion relative to an observer, that wavelength is shifted red or blue by the Doppler effect, slightly changing the wavelength (this effect will be in addition to the redshift due to distance). Therefore, by identifying the emissions of a particular element and seeing how they change, we can follow the movement of these atoms even over great distances.

An active galaxy in a dense cluster

For the new study, Webb pointed to a quasar, or active galactic nucleus. These are incredibly bright because of all the light produced as swirls of matter around supermassive black holes at the center of galaxies. In this case, the color of the quasar, named J1652, was described as very red, suggesting that its light is strongly redshifted, and hence we see it as it was in the early Universe.

Webb imaging confirmed that J1652’s red color was due to significant redshift; The value of the redshift is z ≈ 3, which means that the galaxy is imaged as it existed 11 billion years ago. This is thought to be a critical time in galaxy evolution, when the enormous energies released by supermassive black holes begin to drive star-forming material out of the galaxy, placing a limit on star formation.

Another striking result of the spectrographic data is that at least three other objects detected in the same area in the Hubble images have the same redshift. This means they are additional galaxies close to J1652. Considering the entire imaged area is 85,000 light-years across, this is a very high concentration of galaxies. (For comparison, the Milky Way is over 100,000 light-years across, despite being significantly larger than these early galaxies.)

In addition to confirming the distances, the Webb data allowed the researchers to track ionized oxygen atoms emitting at an appropriate wavelength. The red and blueshifts seen in these data suggest that the quasar ejected material both roughly towards Earth and in the opposite direction, often consistent with two jets created by black holes. The large amount of material ejected is also consistent with the idea that quasar formation could eject raw materials, putting a limit on star formation.

But researchers seem to be most interested in the extremely high density of galaxies in the general field. Based on the amount of matter present, the researchers inferred the amount of dark matter and concluded that this is an area of ​​the Universe as dense as we’ve just imaged, suggesting that it was the product of the merging of two different matter. dark matter halos.

arXiv. Summary number: 2210.10074 (about arXiv). It will be published in The Astrophysical Journal Letters.

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