Keck Spectroscopy of Faint 3<z<7 Lyman Break Galaxies: - I. New constraints on cosmic reionisation from the luminosity and redshift-dependent fraction of Lyman-alpha emission (1003.5244v1)

Abstract: We present results from a new Keck spectroscopic survey of UV-faint LBGs in the redshift range 3<z<7. Combined with earlier Keck and published ESO VLT data, our sample contains more than 600 dropouts, offering new insight into the nature of sub-L* sources typical of those likely to dominate the cosmic reionisation process. Here we use this sample to characterise the fraction of strong Lya emitters within the continuum-selected dropouts. By quantifying how the "Lya fraction" varies with redshift, we seek to constrain changes in Lya transmission associated with reionisation. In order to distinguish the effects of reionisation from other factors which affect the Lya fraction (e.g. dust, ISM kinematics), we study the luminosity and redshift-dependence of the Lya fraction over 3<z<6, when the IGM is known to be ionised. These results reveal that low luminosity galaxies show strong Lya emission much more frequently than luminous systems, and that at fixed luminosity, the prevalence of strong Lya emission increases moderately with redshift over 3 < z < 6. Based on the correlation between blue UV slopes and strong Lya emitting galaxies in our dataset, we argue that the Lya fraction trends are governed by redshift and luminosity-dependent variations in the dust obscuration, with likely additional contributions from trends in the kinematics and covering fraction of neutral hydrogen. We find a tentative decrease in the Lya fraction at z~7 based on the limited IR spectra for candidate z~7 lensed LBGs, a result which, if confirmed with future surveys, would suggest an increase in the neutral fraction by this epoch. Given the supply of z and Y-drops now available from Hubble WFC3/IR surveys, we show it will soon be possible to significantly improve estimates of the Lya fraction using optical and near-IR spectrographs, thereby extending the study conducted in this paper to 7<z<8.

• The paper demonstrates that low-luminosity galaxies show a higher Lyα emission fraction, underlining their crucial role in cosmic reionization.
• It employs extensive Keck and VLT spectroscopy to reveal trends with redshift, noting an increase in Lyα emission from z=3 to z=6 and a tentative dip at z≈7.
• It highlights how both intrinsic characteristics and environmental factors, like dust and ISM kinematics, modulate Lyα photon escape and the interpretation of reionization.

Analyzing the Ly$\alpha$ Emitting Fraction at High Redshifts: Insights from Keck Spectroscopy

The paper conducted by Stark et al. presents a comprehensive examination of Lyman Break Galaxies (LBGs) over a redshift range from $3 < z < 7$, focusing particularly on the Ly$\alpha$ emission characteristics within this population. This work plays a crucial role in exploring the cosmic reionization epoch by leveraging spectroscopic data to infer the nature of typical sources likely dominating this process.

The authors assemble an extensive spectroscopic sample comprised of more than 600 LBGs, with data obtained from Keck and the Very Large Telescope (VLT), targeting UV-faint Lyman break galaxies. This approach permits a detailed investigation into the prevalence and variation of strong Ly$\alpha$ emitters over different redshifts and luminosities. By examining the distribution of Ly$\alpha$ emitters in this way, the researchers aim to tease apart the effects of different galactic and environmental factors influencing Ly$\alpha$ radiation visibility.

Key Findings and Claims

• Luminosity Dependency: The paper reveals a pronounced luminosity dependency in the fraction of galaxies exhibiting strong Ly$\alpha$ emission. Low luminosity galaxies, which are more frequent Ly$\alpha$ emitters, indicate a strong correlation between UV luminosity and the propensity to emit at the Ly$\alpha$ wavelength. For galaxies at $M_{\rm{UV}} \approx -19$, a significant portion emit Ly$\alpha$ strongly, suggesting these less massive galaxies play a critical role during reionization.
• Redshift Evolution: The fraction of strong Ly$\alpha$ emitters appears to increase with redshift from $z=3$ to $z=6$, yet exhibits a tentative decline at $z \approx 7$. This could suggest a rising neutral hydrogen fraction in the intergalactic medium (IGM) at $z=7$, affecting Ly$\alpha$ transmission and thus implicating cosmic reionization dynamics.
• Influence of Environmental and Galactic Factors: The relationship between Ly$\alpha$ emission and galaxy characteristics, such as dust content and neutral hydrogen covering fraction, suggests that Ly$\alpha$ transmission is not solely reliant on intrinsic galactic properties but is significantly influenced by the local and IGM environment. The paper discusses how dust reduction and ISM kinematics play roles in Ly$\alpha$ photon escape.

Implications and Future Directions

The findings propose that cosmic reionization is not a uniformly progressing field but nuanced by the evolving properties of galaxies and their environments. The decline in the Ly$\alpha$ fraction at $z \approx 7$ could offer a novel probe into the timeline and mechanics of cosmic reionization. However, the authors stress the necessity for deeper spectroscopic surveys to confirm these trends and for adequate modeling to interpret potential biases introduced by IGM density evolution.

The interplay between intrinsic and extrinsic factors affecting Ly$\alpha$ emission distribution opens intriguing questions around the relationship of star formation rates, ISM conditions, and overall cosmic evolution. Current and future advances in high-redshift galaxy detection with instruments like the Hubble Space Telescope’s WFC3/IR demonstrate promise in pushing the boundaries of this work, enabling enhanced detection of Ly$\alpha$ emission at greater redshifts. This will be critical in elucidating the evolution of early universe structures and understanding the pivotal epoch of reionization.

In essence, Stark et al.'s investigation furthers our understanding of galaxy formation and evolution processes during a formative period in the universe's history, and it sets a compelling foundation for upcoming explorations into high-redshift astronomy and reionization research.