Disc galaxies: thick and thin

Present-day disc galaxies often contain a thick, star-filled outer disc and an embedded thin disc of stars. Three major theoretical scenarios have been proposed by astronomers to explain how this dual-disc structure comes to be. Using archival data from the NASA/ESA/CSA James Webb Space Telescope, a team of astronomers is closer to understanding disc galaxies’ origins, and the stellar thick- and thin-disc formation process. The team carefully identified, visually verified, and analysed a statistical sample of more than 100 edge-on disc galaxies at various periods — up to 11 billion years ago (or approximately 2.8 billion years after the Big Bang). The results of their analysis suggest that galaxies form a thick disc first, followed by a thin disc. The timing of this process depends on a galaxy’s mass: high-mass, single-disc galaxies transitioned to two-disc structures around 8 billion years ago, while low-mass, single-disc galaxies formed their thin discs about 4 billion years ago.

This image was created with Webb data from proposals: 1180 (D. Eisenstein), 1895 (P. Oesch), 2079 (S. Finkelstein), 1181 (Eisenstein), 2514 (C. Williams), 3577 (E. Egami), 1345 (S. Finkelstein), and 1837 (J. Dunlop).

[Image description: Two mosaics of edge-on disc galaxies observed by Webb. Each mosaic has eight images, split in two rows. The mosaic at the top is titled “thin and thick disc galaxies.” The mosaic at the bottom is titled “thick disc only galaxies.” Each disc galaxy is centered within a square frame and lies against the black background of space. They appear as thin lines with a slight bulge in their centers. A few of the galaxies are horizontal or vertical, but many are angled diagonally. The thin and thick disc galaxies are overall whiter and brighter compared to the thick disc only galaxies, which are fainter and brown-orange. Text in the bottom right of each box lists the galaxy’s redshift.]