Lyman-alpha Emission from a Luminous z=8.68 Galaxy: Implications for Galaxies as Tracers of Cosmic Reionization (1507.02679v3)

Abstract: We report the discovery of Lyman-alpha emission (Ly$\alpha$) in the bright galaxy EGSY-2008532660 (hereafter EGSY8p7) using the MOSFIRE spectrograph at the Keck Observatory. First reported by Roberts-Borsani et al. (2015), it was selected for spectroscopic observations because of its photometric redshift ($z_{phot}=8.57^{+0.22}_{-0.43}$), apparent brightness (H${160}=25.26\pm0.09$) and red Spitzer/IRAC [3.6]-[4.5] color indicative of contamination by strong oxygen emission in the [4.5] band. With a total integration of $\sim$4.3 hours, our data reveal an emission line at $\simeq$11776 {\AA} which we argue is likely Ly$\alpha$ at a redshift $z{spec}=8.683^{+0.001}_{-0.004}$, in good agreement with the photometric estimate. The line was detected independently on two nights using different slit orientations and its detection significance is $\sim7.5\sigma$. An overlapping skyline contributes significantly to the uncertainty on the total line flux although the significance of the detected line is robust to a variety of skyline-masking procedures. By direct addition and a Gaussian fit, we estimate a 95\% confidence range of 1.0--2.5$\times10^{-17}$ erg s$^{-1}$ cm$^{-2}$, corresponding to a rest-frame equivalent width of 17--42 {\AA}. EGSY8p7 is the most distant galaxy confirmed spectroscopically to date, and the third luminous source in the EGS field beyond $z_{phot}\gtrsim7.5$ with detectable Ly$\alpha$ emission viewed at a time when the intergalactic medium is believed to be fairly neutral. Although the reionization process was probably patchy, we discuss whether luminous sources with prominent IRAC color excesses may harbor harder ionizing spectra than the dominant fainter population thereby creating earlier ionized bubbles. Further spectroscopic follow-up of such bright sources promises important insight into the early formation of galaxies.

Spectroscopic Measurement of a Redshift z=8.68 Galaxy: An Analytical Overview

The paper by Zitrin et al. presents significant findings in the field of high-redshift galaxy observations, focusing on the spectroscopic detection of Lyman-alpha (Lyα) emission from a galaxy at redshift z=8.68, designated as EGSY-2008532660 (EGSY8p7). Through detailed analysis utilizing the Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) on the Keck I Telescope, the authors have identified this as the most distant spectroscopically confirmed galaxy to date. This research offers critical insights into the role of such galaxies during the epoch of cosmic reionization.

Methodology and Findings

The primary methodology involved the use of MOSFIRE to obtain deep J-band spectroscopic data of EGSY8p7, located in the Extended Groth Strip. The object initially gained attention due to its photometric redshift estimate and distinctive red color in Spitzer/IRAC [3.6]-[4.5] indicating possible strong emission lines. The data revealed a significant emission line at 11776 Å, interpreted as Lyα at z=8.683. This detection was significant with a 7.5σ confidence level derived from 4.3 hours of observation, validating the presence of Lyα emission in this high-redshift galaxy.

The analysis of the emission line shows a flux range of 1.0–2.5 × 10^-17 erg s^-1 cm^-2, corresponding to a rest-frame equivalent width of 17–42 Å, suggesting active ongoing star formation. The systematic approach in data analysis, using both direct addition and Gaussian fitting methods, accounts for uncertainties like skyline interference, providing robustness to the results.

Implications for Cosmic Reionization

The detection of Lyα at z=8.68 occurs during a period believed to be dominated by a neutral intergalactic medium (IGM), contributing valuable data to understand regions where reionization might still be patchy. This finding raises questions about the homogeneity of the reionization process and the potential for such luminous galaxies to influence their local IGM environments significantly, possibly through the creation of ionized bubbles. The intense Lyα emission, along with the red IRAC color indicating strong [O III] and Hβ emissions, suggests these galaxies might possess harder ionizing spectra than their fainter counterparts, facilitating early reionization.

Future Directions and Theoretical Considerations

Given the significance of these observations, further investigation into similar high-redshift galaxies could provide deeper insights into the early stages of galaxy formation and their hierarchical structuring in the universe. Additional spectroscopic follow-up, especially targeting galaxies with notable IRAC color signatures, could refine our understanding of early cosmic epochs and the physics of galaxy formation.

The findings challenge some existing models and underscore the potential of luminous galaxies behaving as atypical samples within their environments. These results suggest that while such luminous objects might not be representative due to their rarity, they offer critical insights into periods of reionization and early galactic evolution.

Conclusion

This paper by Zitrin et al. significantly enriches our comprehension of high-redshift galaxies and their role in cosmic history, highlighting the importance of combining photometric and spectroscopic methods in pushing the boundaries of observational cosmology. These findings pave the way for future explorations and theoretical work, aimed at unraveling the complexities of the early universe and the influences of its most luminous constituents.