JWST/NIRCam color gradients reveal signs of inside-out quenching in the lensed dusty star-forming galaxy El Anzuelo (z=2.3)

The size of a galaxy is a fundamental observable for calibrating our understanding of how galaxies evolve with time. Yet, the distribution of a galaxy’s mass can differ significantly from its UV, optical, and infrared light profiles. Recent work has shown that the far-IR continuum of dusty star-forming galaxies—extreme objects forming stars at rates of up to 100-1000 Msun/yr—originates from a substantially more compact region than that traced by rest-frame UV/optical. At these shorter wavelengths, mass-to-light ratios vary dramatically, due to gradients in dust attenuation or stellar age/metallicity. For the first time, JWST is now enabling high-resolution mapping of rest-frame NIR continuum at z > 1, meaning that we can directly probe the distribution of stellar mass in an unbiased manner.

To test this, we focus on a gravitationally-lensed dusty starburst at Cosmic Noon behind the El Gordo cluster (z=0.87), named El Anzuelo (SFR~80 Msun/yr, A_V~1.6). Thanks to recent JWST/NIRCam imaging in 8 filters as part of PEARLS, combined with archival imaging with ALMA and with 9 filters from Hubble, there is extensive photometric sampling from rest-frame 200 – 1300 nm. With careful lens modeling, we reconstruct the source-plane distribution at sub-kpc scales (especially the highly-magnified nuclear region). We find that the galaxy center is notably redder than at outer radii, but is also spatially coincident with the dust emission peak, suggesting that elevated nuclear attenuation may be responsible for the color gradient. The specific star formation rates in the galaxy’s central kiloparsec are three times smaller than those of the outer disk, in line with a scenario where bulge formation quenches the galaxy in an inside-out manner. Yet, the effective radius of the galaxy is remarkably consistent from the near-UV to the near-IR. Given the central concentration of dust, one would expect that the attenuation of nuclear UV emission would lead to broader UV sizes. Since this is not the case, we may instead be capturing spatially-distinct UV-bright and NIR-bright components, as expected for an ongoing merger, such as the luminous IR object VV 114 in the local Universe. This extraordinary lensed object uncovered by JWST offers a glimpse of the new window into stellar mass assembly during the peak of the Universe’s star formation history that is now enabled in particular by JWST, ALMA, and HST working in tandem.