Understanding Color Gradients in Galaxies

The colors, or relative brightness at different wavelengths, of stellar populations give us insight into their physical properties, like the age, dust content or metallicity. In the local universe it has been known for decades that galaxies have radial gradients in their color, indicating complex, multi-component formation histories. During my thesis I have studied color gradients out to a redshift of 2 to understand the growth of galaxies at this epoch.

Hubble measures optical color gradients at z=1-2

. In a recent study (arXiv link here), we used imcasacade (a flexible method we developed to measure galaxy profiles, see more info here) to measure the color gradients in galaxies out to z=2. We find both star-forming and queiscent galaxies contain negative color gradients, mirroring results in the local universe. These gradients vary as a function of stellar mass and redshift.

However, with Hubble, we can only observe the rest-frame optical colors of galaxies at this epoch. This limits the ability to interpret the physical cause as both dust and age (also metallicity) can equally affect the optical colors of galaxies.

JWST observations reveals the cause of color gradients

With the longer wavelength capabilities of the new James Webb Space Telescope, we can finally investigate the causes of color gradients at z=2. These infrared measurement help differentiate between the effects of dust attenuation and age on color of a galaxy.

In a new study we use JWST to study the color gradients in galaxies at z=2 (arXiv link here). For a sample of roughly 40 star-forming galaxies in early images from the JWST we find that dust is the main cause of color gradients, not age. This is different that the local universe, where spiral galaxies, like the milky way, have an old red bulge and younger blue disk. At this early epoch the galaxies are still building up their components on their way to becoming the spiral galaxies of the local universe.