Science images: what the James-Webb telescope tells us about colliding galaxies
Here is one of first images of the James-Webb telescope, revealed on July 12, 2022. It represents five galaxies whose grouping is called "Stephan's quintet".
The first images from the new telescopes must respond to the legitimate curiosity of the public and the cravings of researchers who have been waiting for them for many years… and thus be both sensational and scientifically instructive.
Among the preferred targets are therefore galactic collisions. When galaxies collide, tidal forces tear them apart and shape spectacular tails. The induced processes – shocks, formation of stars and clusters, accretion and ejection of gas around active nuclei – are of particular interest to astrophysicists.
Now the "Stephan's quintet" seen in this image offers not one, but five examples of colliding galaxies! This exceptional system had already fed the anthology of the Hubble Space Telescope and the calendar of the terrestrial telescope Canada-France-Hawaii.
Stephan's quintet photographed by Hubble.
NASA, ESA and the Hubble SM4 ERO Team
The image taken by the James-Webb is the result of a complex combination of different monochromatic images, obtained with two space telescope instruments, NIR Cam et MIRI. Individual images reveal different facets of galaxies: young or old stars, ionized or molecular gas. The combined image illustrates with false colors the spatial distribution of each component.
How does this image taken with the James-Webb differ from the iconic one obtained with Hubble? First of all, whereas Hubble observed in the ultraviolet and the visible range, this one was taken in the infrared, or rather “the” infrared.
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James-Webb's specialty: observing in the infrared
The so-called "near" infrared, down to a wavelength of a few micrometers, opens a window on stars as old as our Sun which are the most numerous in galaxies, but not the most luminous, at least in the visible where they are dazzled by younger stars. In our image, the pale yellow color discloses the mass map of stellar populations of galactic halos as well as tidal tails.
Moreover, the near infrared is relatively insensitive to the extinction of light by dust grains and therefore makes it possible to reveal the presence of stars where they are concentrated, for example in stellar nurseries. Thus the dark bands due to dust, very present on the Hubble image, disappear with the eyes of the James-Webb.
Beyond 5 micrometers and the investigation range of the NIRCam instrument, we enter the flowerbeds of MIRI and the “medium” infrared domain, which extends down to 30 micrometers. The sources of emission in the “medium” infrared are multiple and the interpretation of this light complex… as are the phenomena present in Stéphan's quintet.
For example, in the quintet, the gas clouds were rammed by one of the group's galaxies arriving at high speed. This dust has been heated by the shocks and radiates (it would also do so by heating during outbreaks of star formation for example), which explains the very visible red streaks between the galaxies.
Other filaments seem to escape from the galaxy located at the top of the image. They bear witness to a significant revival of activity generated by the collisions at the heart of the galaxy, where an ultra-massive black hole lurks.
Better resolution
But if the James-Webb telescope takes over Hubble, it is above all through its gain in resolution.
Like any space telescope that overcomes atmospheric turbulence, Hubble already shone with the finesse of its images. The James Webb excels thanks to the large size of its mirror. With it, diffuse emission zones split into multiple stellar clusters.
This is why the light from one of the Quintet galaxies, NGC 7320 (on the left in the image) has a different dotted texture than the others: it is resolved into individual stars. However, even observed with the exceptional acuity of the James-Webb telescope, at the distance of the quintet, the light should be diffuse.
In fact, NGC 7320 is located in the foreground and therefore does not belong to the group… but the quintet is not a quartet for all that! Indeed, theimage obtained with the Canada-France-Hawaii telescope had revealed the presence of a fifth galaxy, located outside the field of view of the James-Webb.
This projection effect has been known for a long time, but it is illustrated in this image in a remarkable way. The James-Webb therefore not only provides spectacular images for the public and valuable data for scientists: it also has educational virtues.
Pierre-Alain Duke, Research Director at the CNRS, Director of the Strasbourg Astronomical Observatory, University of Strasbourg
This article is republished from The Conversation under Creative Commons license. Read theoriginal article.
