[Paper Review] LAGER Ly$\alpha$ Luminosity Function at $z\sim7$, Implications for Reionization

Isak Wold, Sangeeta Malhotra+16 more|arXiv (Cornell University)|May 25, 2021

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study presents a new Ly$α$ luminosity function at $z\sim7$ using 6.1×10$^6$ Mpc$^3$ of survey volume from four LAGER fields, finding moderate evolution from $z=5.7$ that supports a fully or largely ionized intergalactic medium by $z\sim7$, with a volume-averaged neutral hydrogen fraction $x_{\text{HI}} < 0.33$ at 1σ confidence.

ABSTRACT

We present a new measurement of the Ly$\alpha$ luminosity function at redshift $z=6.9$, finding moderate evolution from $z=5.7$ that is consistent with a fully or largely ionized $z\sim7$ intergalactic medium. Our result is based on four fields of the LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) project. Our survey volume of $6.1 imes10^{6}$ Mpc$^{3}$ is double that of the next largest $z\sim 7$ survey. We combine two new LAGER fields (WIDE12 and GAMA15A) with two previously reported LAGER fields (COSMOS and CDFS). In the new fields, we identify $N=95$ new $z=6.9$ Ly$\alpha$ emitters (LAEs); characterize our survey's completeness and reliability; and compute Ly$\alpha$ luminosity functions. The best-fit Schechter luminosity function parameters for all four LAGER fields are in good general agreement. Two fields (COSMOS and WIDE12) show evidence for a bright-end excess above the Schechter function fit. We find that the Ly$\alpha$ luminosity density declines at the same rate as the UV continuum LF from $z=5.7$ to $z=6.9$. This is consistent with an intergalactic medium that was fully ionized as early as redshift $z\sim 7$, or with a volume-averaged neutral hydrogen fraction of $x_{HI} < 0.33$ at $1\sigma$.

Motivation & Objective

• To measure the Ly$α$ luminosity function at $z=6.9$ with improved survey volume and completeness.
• To assess the evolution of Ly$α$ emission from $z=5.7$ to $z=6.9$ to constrain the ionization state of the intergalactic medium (IGM).
• To evaluate the reliability and completeness of Ly$α$ emitter (LAE) detection in deep, wide-field surveys.
• To determine whether the observed luminosity density decline is consistent with a fully ionized IGM at $z\sim7$.

Proposed method

• Combining data from four LAGER fields: COSMOS, CDFS, WIDE12, and GAMA15A, to achieve a total survey volume of 6.1×10$^6$ Mpc$^3$.
• Identifying $N=95$ new $z=6.9$ Ly$α$ emitters (LAEs) in the two new fields (WIDE12 and GAMA15A) using deep narrowband imaging.
• Characterizing survey completeness and reliability through detailed source detection and photometric redshift analysis.
• Fitting the observed Ly$α$ luminosity function with a Schechter function to derive best-fit parameters.
• Comparing the evolution of Ly$α$ luminosity density with that of the UV continuum luminosity function from $z=5.7$ to $z=6.9$.
• Using the luminosity density evolution to constrain the volume-averaged neutral hydrogen fraction $x_{\text{HI}}$ at $z\sim7$.

Experimental results

Research questions

• RQ1 Does the Ly$α$ luminosity function at $z=6.9$ show significant evolution from $z=5.7$, and what does this imply for the ionization state of the intergalactic medium?
• RQ2 What is the volume-averaged neutral hydrogen fraction $x_{\text{HI}}$ at $z\sim7$, based on the observed luminosity density evolution?
• RQ3 Are the Ly$α$ luminosity function parameters consistent across multiple independent fields, and do any fields show deviations such as a bright-end excess?
• RQ4 How does the Ly$α$ luminosity density evolution compare to that of the UV continuum luminosity function at these redshifts?
• RQ5 To what extent do the new LAE detections in WIDE12 and GAMA15A improve constraints on the high-redshift Ly$α$ emissivity?

Key findings

• The Ly$α$ luminosity function at $z=6.9$ shows moderate evolution from $z=5.7$, consistent with a fully or largely ionized intergalactic medium.
• The best-fit Schechter function parameters for the combined LAGER fields are in good general agreement across all four fields.
• Two fields, COSMOS and WIDE12, show a bright-end excess in the Ly$α$ luminosity function above the Schechter fit, suggesting possible overdensities or selection effects.
• The Ly$α$ luminosity density declines at the same rate as the UV continuum luminosity function from $z=5.7$ to $z=6.9$, indicating similar evolutionary trends.
• This luminosity density evolution implies a volume-averaged neutral hydrogen fraction $x_{\text{HI}} < 0.33$ at $z\sim7$ at 1σ confidence, supporting early reionization.