Euclid: ERO -- NISP-only sources and the search for luminous $z=6-8$ galaxies

Abstract: This paper presents a search for high redshift galaxies from the Euclid Early Release Observations program "Magnifying Lens." The 1.5 deg$^2$ area covered by the twin Abell lensing cluster fields is comparable in size to the few other deep near-infrared surveys such as COSMOS, and so provides an opportunity to significantly increase known samples of rare UV-bright galaxies at $z\approx6-8$ ($M_{\rm UV}\lesssim-22$). Beyond their still uncertain role in reionisation, these UV-bright galaxies are ideal laboratories from which to study galaxy formation and constrain the bright-end of the UV luminosity function. Of the 501994 sources detected from a combined $Y_{\rm E}$, $J_{\rm E}$, and $H_{\rm E}$ NISP detection image, 168 do not have any appreciable VIS/$I_{\rm E}$ flux. These objects span a range in spectral colours, separated into two classes: 139 extremely red sources; and 29 Lyman-break galaxy candidates. Best-fit redshifts and spectral templates suggest the former is composed of both $z\gtrsim5$ dusty star-forming galaxies and $z\approx1-3$ quiescent systems. The latter is composed of more homogeneous Lyman break galaxies at $z\approx6-8$. In both cases, contamination by L- and T-type dwarfs cannot be ruled out with Euclid images alone. Additional contamination from instrumental persistence is investigated using a novel time series analysis. This work lays the foundation for future searches within the Euclid Deep Fields, where thousands more $z\gtrsim6$ Lyman break systems and extremely red sources will be identified.

• The paper demonstrates the use of Euclid’s NISP-only data to identify 168 potential high-redshift galaxies from a sample of over 500,000 sources.
• It employs a 1.5 sq deg survey and classifies sources into 139 extremely red objects and 29 Lyman-break candidates at z=6–8.
• The study offers crucial insights into early galaxy formation and the ultraviolet luminosity function during cosmic reionization.

Overview of Euclid Early Release Observations and Search for High-Redshift Galaxies

The academic paper titled "Euclid: Early Release Observations -- NISP-only sources and the search for luminous $z=6$--8 galaxies" provides an analysis of the initial findings from the Euclid mission, mainly focusing on high-redshift galaxies. Using data from the Near Infrared Spectrometer and Photometer (NISP) of the Euclid space telescope, the study aims to catalog and characterize sources that are detected only in the infrared, potentially identifying galaxies formed during the reionization epoch.

Key Findings

• High-Redshift Galaxy Detection: The survey spans a significant area of 1.5 square degrees, using the gravitational lensing capabilities of the Abell galaxy clusters fields. This vast coverage enhances the capability to identify ultra-violet bright galaxies at redshifts $z=6$ to 8.
• Sample Size and Photometric Data: From a robust sample of 501,994 detected sources, the study identifies 168 objects that do not emit significant visible light, indicative of high-redshift galaxies or extremely red sources.
• Classification: The sources are categorized into 139 extremely red sources—likely high-redshift dusty star-forming galaxies or low-redshift quiescent systems—and 29 Lyman-break galaxy candidates, which are typically more homogeneous and found at higher redshifts ($z=6$--8).
• Astrophysical Implications: The identified galaxies can act as laboratories to study the physics of early galaxy formation. Understanding the properties and distribution of these galaxies provides insight into the bright end of the ultraviolet luminosity function, contributing to the broader narrative of cosmic reionization.

Methodology

The paper details the observational techniques and data processing methods applied in the Euclid mission. It includes the calibration of photometric measurements and the use of model spectral energy distributions to interpret the data for selected candidates.

Challenges and Considerations

• Contamination Concerns: The potential contamination by brown dwarfs and remnants of instrumental persistence from detector readings pose significant challenges in accurately identifying true high-redshift candidates.
• Morphological Distinction: The resolution offered by NISP is constrained when distinguishing between compact galaxies and star-like objects, which may limit the ability to validate galaxy candidates purely through morphological data.

Future Directions

• Utility of Euclid Deep Fields: With Euclid poised to cover even more extensive and deeper fields, the prospect of cataloging thousands more of these high-redshift galaxies is anticipated. This expansion will purportedly refine our understanding of the evolution and distribution of early galaxies.
• Complementary Observations: Integration with existing deep fields from auxiliary optical and infrared telescopes, such as the James Webb Space Telescope and Spitzer Space Telescope, will further validate these findings and expand our understanding of the cosmic dawn.

Conclusion

Overall, the study successfully demonstrates the potential of the Euclid mission to significantly enhance our catalog of high-redshift galaxies. By combining both spacious coverage and deep optical and infrared imaging, Euclid has begun contributing valuable data toward understanding galaxy formation and the nature of cosmic reionization. With continuous improvements and more extensive surveys in the pipeline, Euclid's role in mapping the early universe is poised to become invaluable.