Spectroscopic Confirmation of Three z-Dropout Galaxies at z = 6.844 - 7.213: Demographics of Lyman-Alpha Emission in z ~ 7 Galaxies

Abstract: We present the results of our ultra-deep Keck/DEIMOS spectroscopy of z-dropout galaxies in the SDF and GOODS-N. For 3 out of 11 objects, we detect an emission line at ~ 1um with a signal-to-noise ratio of ~ 10. The lines show asymmetric profiles with high weighted skewness values, consistent with being Lya, yielding redshifts of z=7.213, 6.965, and 6.844. Specifically, we confirm the z=7.213 object in two independent DEIMOS runs with different spectroscopic configurations. The z=6.965 object is a known Lya emitter, IOK-1, for which our improved spectrum at a higher resolution yields a robust skewness measurement. The three z-dropouts have Lya fluxes of 3 x 10^-17 erg s^-1 cm^-2 and rest-frame equivalent widths EW_0^Lya = 33-43A. Based on the largest spectroscopic sample of 43 z-dropouts that is the combination of our and previous data, we find that the fraction of Lya-emitting galaxies (EW_0^Lya > 25A) is low at z ~ 7; 17 +- 10% and 24 +- 12% for bright (Muv ~= -21) and faint (Muv ~= -19.5) galaxies, respectively. The fractions of Lya-emitting galaxies drop from z ~ 6 to 7 and the amplitude of the drop is larger for faint galaxies than for bright galaxies. These two pieces of evidence would indicate that the neutral hydrogen fraction of the IGM increases from z ~ 6 to 7, and that the reionization proceeds from high- to low-density environments, as suggested by an inside-out reionization model.

• The paper confirms three z-dropout galaxies at redshifts 7.213, 6.965, and 6.844 through robust detection of asymmetric Lyα emission profiles.
• It quantifies Lyα fluxes and rest-frame equivalent widths, showing a decline in the Lyα emitter fraction compared to z~6, with values as low as 17±10%.
• The results imply an increased neutral hydrogen fraction in the IGM, underscoring the need for enhanced spectroscopic techniques to probe early cosmic reionization.

Spectroscopic Study of Lyman-Alpha Emission in Galaxies at Redshift z ∼ 7

The study conducted by Ono et al. provides an in-depth examination of the Lyman-alpha (Ly$\alpha$) emission characteristics within z-dropout galaxies at $z \sim 7$ using ultra-deep Keck/DEIMOS spectroscopy. With cosmic reionization acting as a critical epoch in the universe’s evolutionary timeline, understanding the properties of these high redshift galaxies is pivotal in advancing our knowledge on galaxy formation and cosmological phenomena.

The researchers spectroscopically confirmed three z-dropout galaxies at redshifts $z=7.213$, $6.965$, and $6.844$, identifying the $z=7.213$ galaxy in two independent DEIMOS runs using distinct spectroscopic configurations. The Ly$\alpha$ signal, with asymmetric profile characteristics, was robustly detected, yielding significant signal-to-noise ratios. The redshifts were determined from Ly$\alpha$ emission lines that exhibited typical profiles consistent with theoretical models of high-redshift LAEs.

Their comprehensive analysis revealed the Ly$\alpha$ emission's Ly$\alpha$ fluxes and rest-frame equivalent widths. These findings contribute to the cumulative data on high-redshift Ly$\alpha$ emitters, indicating lower fractions of such emitters at $z \sim 7$, specifically $17 \pm 10${\%} and $24 \pm 12${\%} for galaxies at different UV luminosities. This was substantiated by a comparison with $z \sim 6$ data series, highlighting a pronounced decline in the fraction of Ly$\alpha$-emitting galaxies, suggesting an increased neutral hydrogen fraction in the IGM from $z \sim 6$ to $z \sim 7$. This supports an inside-out reionization model.

Furthermore, the brightness of these galaxies, a significant focus of the study, suggests that current models might underestimate the bright end of the $z \sim 7$ Ly$\alpha$ luminosity function. This speculation is corroborated by the study’s findings of GN-108036, a remarkably luminous galaxy, displaying surprisingly strong nebular contributions in its spectral energy distribution (SED) fitting. This reveals insights into the galaxy's age, stellar mass, and star-formation rates, aligning with ongoing shifts in theoretical models of early universe luminosity functions and the demands these shifts place on our understanding of cosmic variance and observational limits.

In terms of implications, these results inherently stress the importance of improving spectroscopic techniques and instrumental capabilities to probe deeper and more accurately into these formative periods of cosmic history. By understanding the composition and distribution of the early IGM and the intrinsic properties of these high-redshift galaxies, we can gain further understanding of the processes driving cosmic reionization and subsequent galaxy evolution.

Looking forward, the advancements in understanding the neutral hydrogen fraction of the IGM and the environmental proceeding of reionization from dense to sparse regions open the door for refinement in models and subsequent simulations, essential in testing the large scale effect of high-redshift emitters on the cosmos. Together, these methodologies and results suggest a pathway towards unraveling the complexities of early cosmic epochs and their genomic and galactogenetic narratives. Future spectroscopic endeavors, enriched by more in-depth data on Ly$\alpha$ incidence and evolution, could further elucidate the dynamic processes characterizing the Epoch of Reionization. Such endeavors will be vital for progressing theoretical predictions and guiding empirical insights into reionization and galaxy evolution.