PEARLS: Low Stellar Density Galaxies in the El Gordo Cluster Observed with JWST

Abstract: A full understanding of how unusually large "Ultra Diffuse Galaxies" (UDGs) fit into our conventional understanding of dwarf galaxies remains elusive, despite the large number of objects identified locally. A natural extension of UDG research is the study of similar galaxies at higher redshift to establish how their properties may evolve over time. However, this has been a challenging task given how severely systematic effects and cosmological surface brightness dimming inhibit our ability to study low-surface brightness galaxies at high-$z$. Here, we present an identification of low stellar surface density galaxies (LDGs), likely the progenitors of local UDGs, at moderate redshift with deep near-IR observations of the El Gordo cluster at $z = 0.87$ with JWST. By stacking 8 NIRCAM filters, we are able to achieve an apparent surface brightness sensitivity of $24.59$ mag arcsec$<sup>{-2}$, faint enough to be complete to the bright end of the LDG population. Our analysis identifies significant differences between this population and local UDGs, such as their color and size distributions, which suggest that UDG progenitors are bluer and more extended at high-$z$ than at $z = 0$. This suggests that multiple mechanisms are responsible for UDG formation and that prolonged transformation of cluster dwarfs is not a primary UDG formation mechanism at high-$z$. Furthermore, we find a slight overabundance of LDGs in El Gordo, and, in contrast to findings in local clusters, our analysis does not show a deficit of LDGs in the center of El Gordo, implying that tidal destruction of LDGs is significant between $z = 0.87$ and $z = 0$.

• The paper identifies low stellar density galaxies in the El Gordo cluster using JWST NIRCAM, achieving a sensitivity of 24.59 mag arcsec² through stacked filter observations.
• The study reveals high-redshift UDG progenitors are bluer and more extended than local UDGs, indicating diverse formation mechanisms beyond passive evolution.
• The analysis uncovers a minor overabundance of LDGs in the cluster core, suggesting significant tidal destruction and environmental interactions influencing galaxy evolution.

Overview of "PEARLS: Low Stellar Density Galaxies in the El Gordo Cluster Observed with JWST"

The paper investigates low stellar surface density galaxies (LDGs) in the El Gordo cluster at a redshift of $z = 0.87$, utilizing deep infrared observations from the James Webb Space Telescope (JWST). This study concentrates on the potential progenitors of local Ultra Diffuse Galaxies (UDGs) by examining LDGs in a moderate redshift cluster environment. The researchers aim to elucidate the characteristics of UDGs in their earlier evolutionary stages, thereby providing insights into their formation mechanisms and spatial distribution.

Key Findings

1. Sample Identification and Surface Brightness Sensitivity:
   • The study identifies a sample of LDGs by employing JWST's NIRCAM observations. They achieved an apparent surface brightness sensitivity of 24.59 mag arcsec$^2$, which is sufficient to encompass the bright segment of the LDG population.
   • Through stacking eight NIRCAM filters, the researchers improve upon their detection limits for low-surface-brightness systems inherent to cosmological dimming challenges at high redshifts.
2. Differences Between Local UDGs and High-z UDG Progenitors:
   • Substantial differences concerning color and size distributions between the high-redshift LDGs and local UDGs were noted. High-redshift UDG progenitors exhibit bluer colors and more extended shapes compared to local UDGs at $z = 0$.
   • These findings suggest that multiple mechanisms contribute to the formation of UDGs and that the long-term transformation of cluster dwarfs doesn't adequately explain UDG formation at high redshift.
3. LDG Overabundance and Spatial Distribution:
   • The study reports a minor overabundance of LDGs in the El Gordo cluster compared to local densities. Unlike local clusters, there is no deficit of LDGs at the center of El Gordo, implying significant tidal destruction occurs from $z = 0.87$ to $z = 0$.
4. Observed Properties and Evolution:
   • LDGs at higher redshifts possess distinct stellar population characteristics in comparison to their local counterparts, hinting that the evolutionary pathways of these diffuse systems are likely diverse, potentially involving recent accretion into clusters rather than just passive evolution.

Implications and Future Directions

The paper's findings offer a crucial perspective on the early stages of UDG formation, revealing that evolutionary processes driven by environmental interactions may play a more prominent role than previously thought. The discrepancy between properties of high-redshift LDGs and their local descendants poses questions regarding the transformation mechanisms active within cluster environments across cosmic time.

Further investigations are warranted using JWST and other high-resolution instruments to confirm the prevalence and characteristics of LDGs in various cluster environments and to better understand the interaction between these galaxies and their surroundings. There is a compelling need to integrate these observations with simulations, providing insights into the mass assembly history and environmental effects that shape the LDGs, as it may lead to a deeper understanding of galaxy evolution on a cosmological scale.

Overall, the study significantly contributes to the niche yet dynamically evolving field of low-surface-brightness galaxy research, highlighting JWST's capabilities in uncovering features of previously challenging-to-study galactic populations.