Three Indirect Probes of Lyman continuum LEakage from galaxies

Project Information

TRIPLE

Grant agreement ID: 757258

DOI

10.3030/757258

Project closed

EC signature date 9 October 2017

Start date 1 March 2018

End date 28 February 2023

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 500 000,00

EU contribution

€ 1 500 000,00

Coordinated by

UNIVERSITE DE GENEVE
Switzerland

Periodic Reporting for period 4 - TRIPLE (Three Indirect Probes of Lyman continuum LEakage from galaxies)

Reporting period: 2022-09-01 to 2023-02-28

Summary of the context and overall objectives of the project

Cosmic reionization corresponds to the period in the history of the Universe during which the predominantly neutral intergalactic medium was ionised by the emergence of the first luminous sources.
Young stars in primeval galaxies may be the sources of reionization, if the ionising radiation, called Lyman continuum (LyC), that they produce can escape their interstellar medium: the escape fraction of LyC photons from galaxies is one of the main unknowns of reionization studies.
This ERC project contributes to answer this question, by computing from simulated galaxies indirect diagnostics of LyC leakage that were recently reported in the literature, and comparing the virtual observables with the direct escape of LyC photons from simulated galaxies, and with observations.
The first diagnostic for LyC leakage relates the escape of the strongly resonant Lyman-alpha radiation from galaxies to the LyC escape. It was proposed by the PI (Verhamme et al. 2015), and recently validated by observations (Verhamme et al. 2017, Izotov et al. 2018).
The second diagnostic proposes that the strength of Oxygen lines ratios can trace density-bounded interstellar regions. It was the selection criterion for the successful detection of 5 strong Lyman Continuum Emitters from our team (Izotov 2016a,b).
The third diagnostic relates the metallic absorption line strengths to the porosity of the absorbing interstellar gas in front of the stars.
The increasing opacity of the intergalactic medium with redshift renders direct LyC detections impossible during reionisation.
Indirect methods are the only probes of LyC leakage in the distant Universe, and the proposed diagnostics will soon become observables at the epoch of interest with the James Webb Space Telecscope.
They have passed the validation tests, it is now urgent to calibrate these indicators on state-of-the art simulations of galaxy formation. This is the main objective of the project.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The tool that we are using to realise this project has been developed by the team: the radiation transfer code RASCAS, for RAdiation SCAttering in Simulations, is publicly available (http://rascas.univ-lyon1.fr) and is described in Michel-Dansac et al. 2020.
We have been intensively working on two out of the three indirect probes of the escape of ionising radiation from galaxies that we proposed to investigate: Lyman-alpha (Garel et al. 2021; Maji et al. 2022) and the ISM absorption lines (Mauerhofer et al. 2021), since the Oxygen lines ratio was recently ruled out by observational data (e.g. Izotov et al. 2018), as well as simulations (Katz et al. 2020). On the other hand, we propose a new, unforeseen, indirect probe of the escape of ionising radiation from galaxies, which is the MgII resonant doublet. Our main research achievements so far are summarised below:

A. How to use Lyman-Alpha (Lya) to trace LyC escape from virtual galaxies ?
We find a strong correlation between the Lyman-alpha and LyC luminosities of a statistical sample of ~2000 virtual galaxies from the SPHINX project (https://sphinx.univ-lyon1.fr) opening a new path to search for LCEs, by selecting Lya-bright galaxies.
We propose formulas to predict the LyC outputs from a galaxy, knowing its Lyman-alpha outputs, and a few galaxy properties.

B. Can we use UV absorption lines to trace LyC escape from virtual galaxies ?
We produced mock UV absorption spectra of a galaxy observed in thousands of directions, to study the link between the escape of LyC from virtual galaxies and observational properties of metallic LIS absorption lines (Equivalent Widths in absorption and fluorescent emission, residual fluxes, velocities).
We find that directions where these lines are well saturated always correspond to lines of sight with no leakage of ionising radiation, but the contrary is not true: there are lines of sights with weak absorption lines but weak LyC escape fractions, leading to a lot of scatter in the correlations between the observational quantities, and the LyC leakage.

C. Can we use MgII spectral properties to trace LyC escape from galaxies ?
As the Lymna-alpha line of Hydrogen, the MgII doublet is resonant, and tracing the neutral phase of the InterStellar Medium (ISM).
Thanks to a grid of around 1000 idealised geometries, we are investigating the relation between MgII peak ratio and peak shift with the escape fraction of ionising radiation (Verhamme et al in prep.).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The UV absorption lines are not great predictors for the escape of ionising radiation from galaxies, but can we use them to infer the amount and the kinematics of the absorbing gas ?
This is the aim of the follow-up study that we recently started (Mauerhofer et al. in prep).

The Lya predictions made by the team so far are global quantities, but in order to allow for a fair comparison with observations, we are now investigating the variation of the Lya and LyC properties of our galaxies with directions (Blaizot et al in prep).

MgII has been recently proposed in the literature as the the new LyC tracer (Henry et al. 2018, Chisholm et al. 2020), we are working on making quantitative theoretical expectations (Verhamme et al. in prep).

Finally, we are reporting the discovery of the cosmic web in emission in the deepest observation ever done with MUSE, Multi Unit Spectrograph Explorer at ESO/VLT (>140 hours of integration, Bacon et al 2021).
Detecting this cosmic web in Lya emission has been a long standing quest, and the most sticking result from the MUSE eXtra Deep Field (MXDF) is that the level of emission cannot be explained by fluorescence of the UV background only, but we are probably detecting the cumulative light from faint, dwarf, galaxies, for the first time.
In order to test our interpretation for this "strong" signal, we plan to compute the Lya emissivity of each cell, and run Lya radiation transfer experiments in the full SPHINX simulation volume, and compare with the detected signal.
We are working on solving technical issues (memory footprint in particular), and are expecting original results before the end of the project (Garel et al in prep).

detection of a galaxy in extreme ionising radiation, for the first time.