SO Webloquio: Empirical and physical properties of Lyman continuum emitters

Lyman continuum emitters are galaxies showing escaping ionizing radiation, which thus contributes to ionizing the intergalactic medium. They may be the dominant source of cosmic reionization.

I will present an overview of the observations and modeling of low-z analogs of the sources of cosmic reionisation recently discovered. HST observations, including UV spectroscopy with COS and rest-UV imaging with the WFC3, combined with ground-based spectroscopy have provided essential discoveries of Lyman continuum (LyC) emitting galaxies both a low (z~0.3) and high redshift (z~2-3). After decade-long searches, we have now significant samples of galaxies showing clear signs of escaping LyC radiation, as required to explain cosmic reionization by galaxies, as most commonly accepted.

The initial studies of Izotov et al. (2016, 2018) have identified eleven z~0.3-0.4 compact star-forming galaxies selected for strong emission lines and a high [OIII]/[OII] ratio. HST has shown that they are all confirmed Lyman continuum emitters with escape fractions ranging from 2 to 72% and a strong production of LyC photons. From all their measurable properties they are excellent analogs for the sources of cosmic reionization. Now the Low-z Lyman Continuum Survey (LzLCS), a HST Large Programme, has recently been completed (Flury et al. 2022), providing a unique low-z reference sample for Lyman continuum studies. The low-z sample allows us not only to measure the LyC escape fraction, but also to constrain their stellar populations, ionizing radiation field, dust properties (extinction and attenuation law), and to establish the presence of a porous ISM geometry, which explains the LyC escape.

Finally, I will show how the observations and state-of-the-art spectral modeling of UV and optical absorption and emission lines allow us to establish different methods to indirectly measure Lyman continuum escape. These will be further tested with JWST cycle 1 observations, and are key to interpret upcoming observations of galaxies in the epoch of reionization with the JWST and 30m class telescopes in the near future.

31/05/2022 - 12:30
Dr. Daniel Schaerer
Université de Genève, Switzerland