JADES Ultra-red Flattened Objects: Morphologies and Spatial Gradients in Color and Stellar Populations

TL;DR

This study reveals a significant population of large, optically faint galaxies with diverse morphologies and high dust attenuation at redshifts 2-4, challenging previous notions of their compactness and structure.

Contribution

It extends the catalog of ultra-red flattened objects (UFOs) using JWST data, providing detailed morphological and stellar population analysis of these galaxies.

Findings

UFOs are large, optically faint galaxies with high stellar masses and star-formation rates.

Most UFOs have low Sersic indices, indicating disk-like structures.

UFOs exhibit high dust attenuation across their extents.

Abstract

One of the more surprising findings after the first year of JWST observations is the large number of spatially extended galaxies (ultra-red flattened objects, or UFOs) among the optically-faint galaxy population otherwise thought to be compact. Leveraging the depth and survey area of the JADES survey, we extend observations of the optically-faint galaxy population to an additional 112 objects, 56 of which are well-resolved in F444W with effective sizes, $R_e > 0.25''$, more than tripling previous UFO counts. These galaxies have redshifts around $2 < z < 4$, high stellar masses ($\mathrm{log(M_*/M_{\odot})} \sim 10-11$), and star-formation rates around $\sim 100-1000 \mathrm{M_{\odot}/yr}$. Surprisingly, UFOs are red across their entire extents which spatially resolved analysis of their stellar populations shows is due to large values of dust attenuation (typically $A_V > 2$ mag even at large radii). Morphologically, the majority of our UFO sample tends to have low S\'ersic indices ($n \sim 1$) suggesting these large, massive, optically faint galaxies have little contribution from a bulge in F444W. Further, a majority have axis-ratios between $0.2 < q < 0.4$, which Bayesian modeling suggests that their intrinsic shapes are consistent with being a mixture of inclined disks and prolate objects with little to no contribution from spheroids. While kinematic constraints will be needed to determine the true intrinsic shapes of UFOs, it is clear that an unexpected population of large, disky or prolate objects contributes significantly to the population of optically faint galaxies.