New observations of z~7 galaxies: evidence for a patchy reionization

Abstract: We present new results from our search for z~7 galaxies from deep spectroscopic observations of candidate z-dropouts in the CANDELS fields. Despite the extremely low flux limits achieved by our sensitive observations, only 2 galaxies have robust redshift identifications, one from its Lyalpha emission line at z=6.65, the other from its Lyman-break, i.e. the continuum discontinuity at the Lyalpha wavelength consistent with a redshift 6.42, but with no emission line. In addition, for 23 galaxies we present deep limits in the Lyalpha EW derived from the non detections in ultra-deep observations. Using this new data as well as previous samples, we assemble a total of 68 candidate z~7 galaxies with deep spectroscopic observations, of which 12 have a line detection. With this much enlarged sample we can place solid constraints on the declining fraction of Ly$\alpha$ emission in z~7 Lyman break galaxies compared to z~6, both for bright and faint galaxies. Applying a simple analytical model, we show that the present data favor a patchy reionization process rather than a smooth one.

Analysis of New Observations Revealing Patchy Reionization in z ∼ 7 Galaxies

The paper by Pentericci et al. focuses on spectroscopic observations of galaxies at redshift ( z \sim 7 ), aiming to elucidate the characteristics of the reionization epoch. Through comprehensive analysis, the paper presents evidence supporting the hypothesis of a patchy reionization process.

Methodology and Observations

The research employs deep spectroscopic observations targeting z-dropout candidates within the CANDELS fields. Despite achieving low flux limits with VLT's FORS2 spectrograph, robust redshift identifications were affirmed for merely two galaxies: one showcasing Ly(\alpha) emission at ( z=6.65 ) and another identified through Lyman-alpha break at ( z=6.42 ), albeit lacking emission line visibility. Additionally, the study establishes deep limits on Ly(\alpha) equivalent width (EW) for 23 galaxies from non-detections during extensive observation runs (15-27 hours).

Integrating new data with earlier samples yields a cumulative pool of 68 candidate galaxies at ( z \sim 7 ) with deep spectroscopic scrutiny. Within this larger collection, a mere 12 galaxies exhibit Ly(\alpha) line emission—highlighting a pronounced decline in the fraction of Ly(\alpha) emissive LBGs at ( z \sim 7 ) vis-à-vis ( z \sim 6 ).

Interpretation and Implications

Through semi-numeric simulations of reionization, the paper illustrates a high neutral hydrogen fraction ((\chi_{HI} \geq 0.51)) required to account for the observed drop in Ly(\alpha) emission amidst unchanged galaxy properties. This substantial fraction contradicts previous findings from quasar near-zone studies, positing complete reionization by ( z > 6.5 ).

Further extrapolation using Treu et al.'s phenomenological models unveils a preferential inclination towards patchy reionization. An attenuation factor (\epsilon) is derived, signifying substantial Ly(\alpha) quenching relative to ( z \sim 6 ). Importantly, evidence ratios favor the patchy model over the smooth alternative, implicating an inhomogeneous topology during early cosmic epochs.

Future Directions

To resolve the explored inconsistencies between model predictions and observational limits, deeper spectroscopic observations and refined simulations are essential. The paper suggests tackling cosmic variance by expanding observations across diversified sight-lines and consolidating data across extensive samples.

Conclusion

Pentericci et al. provide compelling evidence favoring a patchy reionization paradigm, as inferred from diminished Ly(\alpha) emission in high-redshift galaxies. Their findings beckon further investigations into the complex interaction between galactic evolution and cosmic medium properties during the elusive epoch of reionization, potentially steering future avenues in astrophysical research and theoretical amendments.