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

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.

• The paper identifies and characterizes a new population of 56 Ultra-red Flattened Objects (UFOs) in the JWST JADES survey, challenging prior assumptions about optically faint galaxy morphology.
• These UFOs are massive and highly star-forming but surprisingly exhibit low S r sic indices and disk-like structures dominated by high dust attenuation (Av > 2).
• The findings suggest that widespread dust attenuation drives the extreme redness, impacting models of dusty galaxy evolution at high redshifts and highlighting the need for future spectroscopic study.

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

The paper "JADES Ultra-red Flattened Objects: Morphologies and Spatial Gradients in Color and Stellar Populations" presents an empirical investigation centered on a newly identified population of ultra-red flattened objects (UFOs) within the JWST Advanced Deep Extragalactic Survey (JADES). These UFOs are an unexpected subset of the optically faint galaxy population primarily characterized by their extended structure and distinct spectral properties.

Key Findings

1. Observational Overview: The study identifies 112 optically faint galaxies, among which 56 qualify as UFOs. These galaxies were detected leveraging the depth and survey area of the JADES, which allowed a significant extension of the known ultra-red galaxy sample.
2. Characteristics of UFOs: This population is marked by its red color, substantial stellar masses ($\mathrm{log(M_*/M_{\odot})} \sim 10-11$), and considerable star-formation rates (SFRs between $\sim 100-1000 \mathrm{M_{\odot}/yr}$).
3. Morphology and Structure: Surprisingly, morphological analyses reveal that the majority of UFOs possess low Sérsic indices ($n \sim 1$), suggesting dominance by a disk-like, rather than a bulge-centric, structure. Moreover, their axis ratios ($0.2 < q < 0.4$) imply that their intrinsic shapes might be a mixture of inclined disks and prolate objects with minimal spheroidal contributions.
4. Spatial Gradients: UFOs exhibit radial color gradients, where high dust attenuation plays a critical role in their observed redness. Dust attenuation values of $A_V > 2$ magnitudes are prevalent even at substantial galactic radii.
5. Astrophysical Implications and Hypotheses: The novelty of a large, robustly-red galaxy population challenges prior assumptions that the majority of optically faint galaxies are compact. This paper posits that the observed structural and spectral peculiarities likely influence current understanding of dusty star-forming galaxy evolution at $z \sim 2-4$.

Implications and Future Directions

The study contributes significantly to understanding the structures and evolution of dusty galaxies at high redshifts by highlighting a large, previously unclassified population. The revelation that UFOs are not predominantly compact forms but rather extended and red due to widespread dust attenuation has implications for models of galaxy evolution and the cosmic star-formation history.

For future investigations, this study underscores the need for further spectroscopic follow-up to confirm the inclination assumptions and unravel the specific kinematics of these galaxies. Additionally, obtaining spatially-resolved spectroscopic data could refine the understanding of their intrinsic shapes and help in understanding their evolution into the present-day massive elliptical population.

Given its extensive dataset and robust methodologies, this research holds potential to redefine gaps in the high-redshift universe's galaxy distribution models and star formation paradigms. Furthermore, the findings necessitate an expanded exploration of morphologically similar galaxies to ascertain the full impact of dusty ultra-red galaxies within the cosmic framework.