An uncontaminated measurement of the escaping Lyman continuum at z ∼ 3

Abstract

Observations of reionization-era analogs at z ∼ 3 are a powerful tool for constraining reionization. Rest-ultraviolet observations are particularly useful, in which both direct and indirect tracers of ionizing-photon production and escape can be observed. We analyse a sample of 124 z ∼ 3 galaxies from the Keck Lyman Continuum Spectroscopic Survey, with sensitive spectroscopic measurements of the Lyman continuum region. We present a method of removing foreground contamination from our sample using high-resolution, multi-band Hubble Space Telescope imaging. We re-measure the global properties of the cleaned sample of 13 individually-detected Lyman continuum sources and 107 individually-undetected sources, including a sample-averaged absolute escape fraction of fesc, abs = 0.06 ± 0.01 and a sample-averaged ratio of ionizing to non-ionizing ultraviolet flux density of 〈f900/f1500〉out = 0.040 ± 0.006, corrected for attenuation from the intergalactic and circumgalactic media. Based on composite spectra, we also recover a strong positive correlation between 〈f900/f1500〉out and Lyα equivalent width (Wλ(Ly$\rm \alpha$)) and a negative correlation between 〈f900/f1500〉out and UV luminosity. As in previous work, we interpret the relationship between 〈f900/f1500〉out and Wλ(Ly$\rm \alpha$) in terms of the modulation of the escape of ionizing radiation from star-forming galaxies based on the covering fraction of neutral gas. We also use a Wλ(Ly$\rm \alpha$)-weighted 〈f900/f1500〉out to estimate an ionizing emissivity from star-forming galaxies at z ∼ 3 as εLyC ≃ 5.5 × 1024 erg s−1 Hz−1 Mpc−3. This estimate, evaluated using the uncontaminated sample of this work, affirms that the contribution of galaxies to the ionizing background at z ∼ 3 is comparable to that of active galactic nuclei.

The space density of ultra-luminous QSOs at the end of reionization epoch by the QUBRICS Survey and the AGN contribution to the hydrogen ionizing background

“…Such a value for f esc is much higher than what is directly measured in galaxies at lower redshifts (z = 3 − 4; e.g. Grazian et al 2016Grazian et al , 2017Pahl et al 2021). This tension would require rapid evolution in the physical properties of the global galaxy population (Davies et al 2021) or relaxing somewhat the constraints derived from the MFP measurements (Cain et al 2021).…”

Binary evolution pathways of blue large-amplitude pulsators

“…BPASS is extensively used for interpretation of massive star populations (e.g. Pahl et al 2021;Cullen et al 2021;Vijayan et al 2021;Neugent 2021;Marshall et al 2020;Stevance et al 2020b;Neugent et al 2020;Shivaei et al 2020;Topping et al 2020;Dorn-Wallenstein & Levesque 2020;Wilkins et al 2020;Chrimes et al 2020;Sanders et al 2020). Its accuracy and capabilities in the low mass regime have been explored through the white dwarf progenitor initial to final mass ratio (Eldridge et al 2017), the impact of low mass binary stars on understanding ages of globular clusters and elliptical galaxies (Stanway & Eldridge 2018) and consideration of the type Ia supernova rate (Tang et al 2020), producing good matches to the data in each case.…”

The [OIII]$\lambda5007$ equivalent width distribution at z $\sim2$: The redshift evolution of the extreme emission line galaxies

“…To do so, we use population-average estimates of the ionising photon production efficiency and escape fraction. For the escape fraction, we use the recently-derived value from Pahl et al (2021), indicating an average of 𝑓 esc ∼ 6% for z 3 UV-selected galaxies (see also Steidel et al 2018;Bassett et al 2021). Here we assume that z 2 galaxies have a similar value.…”

“…The O32 values that appear required for large 𝑓 esc (O32>6) are uniquely found in EELGs, in particular those systems with [OIII]𝜆5007 EW > 750 Å (e.g., Tang et al 2019;Sanders et al 2020;Onodera et al 2020). At z ∼ 3, Pahl et al (2021) measure an average 𝑓 esc for SFGs to be ∼ 6%. In contrast, individual EELGs with [OIII]𝜆5007 EW well above 1000 Å have been observed to exhibit escape fractions up to an order of magnitude greater than these typical systems (Vanzella et al 2016;Rivera-Thorsen et al 2017;Izotov et al 2018;Fletcher et al 2019).…”