Under the microscope: the Christmas Tree Galaxy Cluster

A blog by James DiFrancesco, Director, Research and Development of the Herzberg Astronomy and Astrophysics Research Centre (HAA) at the National Research Council Canada.

Just in time for the holidays!

A team of astronomers including Dr. Madeline Marshall, Herzberg Astronomy and Astrophysics Research Centre’s Plaskett Fellow, have published an analysis of combined James Webb Space Telescope (JWST) and Hubble Space Telescope observations of “The Christmas Tree Galaxy Cluster.” This cluster, whose real name is the rather unfestive MACS J0416.1-2403, is seen about 4.3 billion lightyears from the Sun. Its striking image is festooned with galaxies golden, red and blue. (The colour likely indicates if a particular galaxy is associated with the cluster, behind it, or in front of it, respectively.)

In my mind, a better name for this cluster would be “That Tangled String of Christmas Tree Lights You Should Have Packed Better Back in January Cluster,” but perhaps that name is too long. At any rate, the Christmas Tree analogy is actually rather apt because the team found the light from some galaxies blinking on and off like Christmas tree lights. In particular, they looked at the MACS J0416 field at 4 different times, finding 14 “transients” where the light increased and decreased in certain galaxies between observations.?The team interpreted 2 of these transients as being supernovae in their respective galaxies, that just happened to be caught at the time of observations. Indeed, when you have so many galaxies in 1 image, the chances that 1 has a supernova at any given time is higher than you might expect. The other 12 transients are flickering stars in their respective galaxies that have been highly magnified by gravitational lensing.

A careful look at the image will reveal the telltale arcs produced by gravitational lensing. These are background galaxies whose light has been bent by the mass of the foreground cluster, producing such distorted shapes. The gravitational lensing also magnifies the light, so oddly enough it is easier to see more distant galaxies behind the cluster than if the cluster weren’t there. Sometimes, particularly bright stars in background galaxies can be also spotted as a result of the lensing, and in a different paper the team report seeing in the MACS J0415 image an incredibly high mass star they have beautifully named “Mothra” after the giant kaiju from Japanese cinema. (If you’re wondering what Mothra has to do with Christmas, you obviously haven’t read my unproduced screenplay “A Christmas to Remember in Tokyo.”)

Mothra resides in a background galaxy seen when the Universe was only 3 billion years old and the star may actually be a pair of unresolved supergiants. Interestingly, the lensed observations of Mothra enabled the team to cleverly probe the substructure of mass in the cluster given the amount of mass on a particular scale needed to magnify the star. The team estimates that Mothra’s light was bent by a “perturber,” probably a clump of cold dark matter in the foreground cluster that is between 10,000 and 2.5 million times the mass of the Sun.

Congratulations to Dr. Madeline Marshall and the JWST Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) team for more exciting Webb results!

You can read the full Space.com story here: James Webb Space Telescope gets ready for the holidays with a cosmic Christmas Tree

You can also read the University of Missouri’s press release about the 14 transients here: Scientists find 14 new transient objects in space by peering through a galaxy “magnifying glass” and NASA’s press release here: NASA's Webb, Hubble Combine to Create Most Colorful View of Universe.

Cornell University also released this associated arXiv preprint: JWST’s PEARLS: Transients in the MACS J0416.1?2403 Field.

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