Bingjie Wang
The neutral intergalactic medium during the reionization of the universe precludes direct observation of ionizing Lyman-continuum (LyC) photons at these redshifts. In recent years, small samples of LyC emitters (LCEs) at low redshifts have been discovered, and clues have begun to emerge as to both the indirect signposts of leaking LyC and of the processes that enable its escape. We present results of a pilot program to test a new technique for finding LCEs, using the weakness of the [S II] nebular emission lines relative to typical star-forming galaxies as evidence that the interstellar medium (ISM) is optically thin to LyC. Significant emerging flux below the Lyman edge are detected in two out of three [S II]-weak star-forming galaxies at z~0.3 using the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. Statistical analyses are then performed with new data coming from the HST low-z LyC survey. We reaffirm the robustness of the [SII] diagnostic; moreover, preliminary results show a complex relation between optical emission line properties and escape fractions of LyC. Drawing on previous absorption and emission-line studies on starbursts, we will investigate the role of ISM geometry in LyC escape in a suite of models and compare to observations.
Naturally, direct observations of those ionizing Lyman-continuum (LyC) photons is precluded by the neutral intergalactic medium (IGM) at these redshifts.
We aim to understand how reionization happened through local galaxies that are LyC emitters (LCEs). The questions to be answered are:
How to find LCEs?
What is the escape fraction of LyC?
How do LyC photons escape into IGM?
I. A new technique for finding galaxies leaking LyC radiation: [S II]-deficiency (BW+ 2019)
The ionization potential for producing [S II] (10.4 eV) is less than that is needed for ionizing neutral hydrogen (13.6 eV). Much of the [S II] emission therefore arises in the warm partially ionized region near and just beyond the outer edge of the Stromgren sphere. In an HII region that is optically thin to ionizing radiation, this partially ionized [S II] zone is weak or even absent, and so the relative intensity of the [S II] emission lines drop significantly.
Results from a pilot study (HST-GO-15341; PI T. Heckman) show:
1. significant fluxes below the Lyman limit are detected in two of the three [S II]-weak-selected starburst galaxies at z ~ 0.3;
2. high relative escape fractions: 93.3% & 79.8% (photo-electric absorption of LyC due to hydrogen only);
3. low absolute escape fractions: 3.5% & 4.1% (high dust extinction).
II. Following up with the HST low-z LyC survey (BW+ 2020, in prep.)
With a larger sample from the survey (HST-GO-15626; PI A. Jaskot), we confirm the robustness of the [SII]-deficiency diagnostic. Preliminary results also reveal a complex relation between optical emission line properties and the escape fraction of LyC. Therefore, theories on what promotes LyC leakage remains to be tested. Knowledge of ISM/CGM conditions is essential. Little is known, however, of the spacial distribution of gas.
We will use absorption and emission lines to probe the geometry, building on our previous work (BW+ 2020).
III. A systematic study of galactic outflows via fluorescence emission: implications for their size and structure (BW+ 2020)
We observed a sample of starbursts (HST-GO-15340; PI T. Heckman), and found that their fluorescence emission lines are systematically weaker than the resonance absorption lines. We argue that the observed weakness is due to the missing emission arising on scales larger than those encompassed by HST/COS aperture (cf. Prochaska+ 2011; Scarlata & Panagia 2015; Zhu+ 2015).
Simplified analytical arguments imply shallow radial density profiles, and suggest that most of the observed absorbing material must be created/injected at radii much larger than that of the starburst. Therefore, existing estimates of outflow rates are likely to be systematically underestimated, and do not capture any radial dependence.
IV. Ongoing work: constraining gas geometry with advanced models
Taking a step further, we are using a modified version of RASCAS (Michel-Dansac+ 2020) to constrain the gas geometry in greater details. This work is done in collaborations with T. Heckman (JHU), T. Garel (Univ Lyon, Observatoire de Genève), & A. Henry (STScI)
V. Future work: shedding light on LyC radiative transfer
Emission and absorption-line profiles have been measured in some of the LCEs (e.g., Jaskot+ 2019). Additional data will be readily available from the HST low-z LyC survey. We will compare our models with observations to identify which diagnostics are the most reliable LyC predictors, and investigate the role of ISM/CGM geometry in LyC escape.