What James-Webb tells us about colliding galaxies

The first images from the new telescopes must respond to the legitimate curiosity of the public and the urge of researchers who have been waiting for them for many years… and thus be both sensational and scientifically instructive.

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Among the preferred targets are therefore galactic collisions. When galaxies collide, tidal forces tear them apart, forming 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.

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But the “Stephan quintet” that we see in this image offers not one, but five examples of colliding galaxies! This extraordinary system had already fueled the Hubble Space Telescope anthology and the Canada-France-Hawaii Terrestrial Telescope calendar.

Stephan’s Quintet photographed by Hubble. NASA, ESA and the Hubble SM4 ERO Team

The image taken by James-Webb is the result of a complex combination of different monochromatic images, obtained with two space telescope instruments, NIRCam and 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.

Scientists measure the expansion rate of the universe thanks to collisions between black holes

How does this James-Webb image differ from the iconic Hubble image? First of all, where 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 thesis: observation 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 is dazzled by younger stars. In our image, the light yellow color reveals the mass map of stellar populations of galactic haloes as well as tidal tails.

JamesFurthermore, 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 star nurseries. Thus, the dark bands due to dust, very present in the Hubble image, disappear with James-Webb’s eyes.

Beyond 5 micrometers and the survey area 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 many and the interpretation of this light complex… like the phenomena present in Stéphan’s quintet.

For example, in the quintet, the gas clouds were hit 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 bursts of star formation, for example), which explains the highly visible red streaks between the galaxies.

Other filaments appear to escape from the galaxy located at the top of the image. They testify to a significant revival of activity generated by collisions in the heart of the galaxy, where an ultramassive black hole lurks.

Better resolution

But if the James-Webb telescope overtakes 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. James Webb stands out thanks to the large size of its mirror. With it, diffuse emission zones are divided into several star clusters.

This is why the light from one of the Quintet galaxies, NGC 7320 (on the left of the image) has a different dotted texture than the others: it dissolves into individual stars. Even observed with the exceptional sharpness of the James-Webb telescope, at a distance from the quintet, however, 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! In fact, the image obtained with the Canada-France-Hawaii telescope had revealed the presence of a fifth galaxy, located outside the field of view of James-Webb.

This projection effect has been known for a long time, but it is illustrated in this image in a remarkable way. James-Webb therefore not only provides spectacular images for the public and valuable data for scientists: it also has educational virtues.

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