The Keck Lyman Continuum Spectroscopic Survey (KLCS): The Emergent Ionizing Spectrum of Galaxies at $z\sim3$ (1805.06071v2)

Abstract: We present results of a deep spectroscopic survey designed to quantify the statistics of the escape of ionizing photons from star-forming galaxies at z~3. We measure the ratio of ionizing to non-ionizing UV flux density <f900/f1500>obs, where f900 is the mean flux density evaluated over the range [880,910] A. We quantify the emergent ratio of ionizing to non-ionizing UV flux density by analyzing high-S/N composite spectra formed from sub-samples with common observed properties and numbers sufficient to reduce the statistical uncertainty in the modeled IGM+CGM correction to obtain precise values of <f900/f1500>_out, including a full-sample average <f900/f1500>_out=$0.057\pm0.006$. We further show that <f900/f1500>_out increases monotonically with Ly$\alpha$ rest equivalent width, inducing an inverse correlation with UV luminosity as a by-product. We fit the composite spectra using stellar spectral synthesis together with models of the ISM in which a fraction f_c of the stellar continuum is covered by gas with column density N(HI). We show that the composite spectra simultaneously constrain the intrinsic properties of the stars (L900/L1500)_int along with f_c, N(HI), E(B-V), and $f{esc,abs}$, the absolute escape fraction of ionizing photons. We find a sample-averaged $f_{esc,abs} =0.09\pm0.01$, and that subsamples fall along a linear relation $\langle f_{esc,abs}\rangle \sim 0.75[W(Ly\alpha)/110 A]$. We use the FUV luminosity function, the distribution function $n[W(Ly\alpha)]$, and the relationship between $W(Ly\alpha)$ and <f900/f1500>out to estimate the total ionizing emissivity of $z\sim3$ star-forming galaxies with Muv < -19.5: $\epsilon{LyC}\sim 6\times10^{24}$ ergs/s/Hz/Mpc$^3$, exceeding the contribution of QSOs by a factor of $\sim 3$, and accounting for $\sim50$% of the total $\epsilon_{LyC}$ at $z\sim3$ estimated using indirect methods.

Overview of the Keck Lyman Continuum Spectroscopic Survey

The Keck Lyman Continuum Spectroscopic Survey (KLCS) offers a significant contribution to our understanding of the escape of hydrogen-ionizing photons from star-forming galaxies at a redshift of approximately $z \sim 3$. Utilizing data from the W.M. Keck Observatory, the authors present a comprehensive spectroscopic survey encompassing 124 galaxies with a mean redshift of $\langle z \rangle = 3.05\pm0.18$. This survey specifically targets the emergent ionizing spectrum from galaxies, analyzing the crucial balance between ionizing and non-ionizing UV flux densities, which has direct implications for the ionizing emissivity that contributes to cosmic reionization.

Key Findings

The KLCS presents a range of findings regarding the ionizing properties of galaxies at high redshift:

1. Ionizing to Non-Ionizing UV Flux Density Ratio: The paper measures the emergent ratio of ionizing to non-ionizing UV flux density, $f_{900}$, finding a sample-averaged $f_{900}/f_{1500} = 0.057\pm0.006$. This ratio increases with decreasing UV luminosity and is inversely correlated with rest equivalent width W$_{\lambda}$, suggesting that galaxies with lower luminosities and larger equivalent widths are more efficient at leaking ionizing photons.
2. Empirical Models and Simulations: Utilizing Monte Carlo simulations, the paper presents models of the opacity due to H I in the intergalactic and circumgalactic mediums. These models help quantify how ionizing flux is affected by intervening materials, which is critical for extrapolating observations to intrinsic properties.
3. Escape Fraction and Galaxy Properties: The escape fraction of ionizing photons, $f_{\rm esc}$, is connected to the intrinsic properties of the galaxies, including the stellar population synthesis and interstellar medium characteristics. The paper uses these models to constrain the escape fraction and other intrinsic properties of the galaxies. The derived average escape fraction for the sample is $f_{\rm esc} = 0.09\pm0.01$, with specific linear relations found for subsets of data.
4. Ionizing Emissivity: The total ionizing emissivity of $z \sim 3$ galaxies is estimated to exceed contributions from QSOs by a factor of $1.2$ to $3.7$, accounting for approximately 50% of the total ionizing background. This finding underscores the significant role that ordinary star-forming galaxies, as opposed to quasars, play in the ionizing budget of the Universe at these epochs.

Implications

The implications of the KLCS are profound both theoretically and observationally. The survey's findings enable better modeling of the early Universe's ionizing radiation budget, which is essential for simulations of cosmic reionization. The derived escape fractions and emissivities provide constraints that are critical for adjusting theories related to galaxy formation and evolution under the influence of a pervading ionizing background.

Future Developments

The research sets the stage for further exploration into the connections between intrinsic stellar properties and the details of the interstellar media that regulate the escape of ionizing photons. With more sophisticated models and future observations, particularly from facilities like the James Webb Space Telescope (JWST), researchers can refine these measurements, extend them to higher redshifts, and better understand the transition from the "dark ages" to a fully reionized Universe.

In conclusion, the KLCS provides crucial insights into the escape processes of ionizing photons from high-redshift galaxies, enriching our understanding of cosmic reionization and the interplay between galaxy evolution and the intergalactic medium.