Astronomy departmental seminar - Alexandra Le Reste (SU)

Title: Neutral gas properties of high-redshift analog galaxies: 21cm observations of Lyα and LyC-emitting galaxies

Neutral hydrogen (HI) is the most abundant baryonic element and a crucial component of galaxies. In the early universe, the interaction between neutral gas and the UV light produced by galaxies is thought to have given rise to the last major phase transition of the Universe: cosmological reionization. During reionization the bulk of HI within the intergalactic medium (IGM) was ionized. However, observational constraints limit our understanding of the interplay between the radiation produced by galaxies and their neutral gas, and that of the physical processes that caused this important cosmological period. Another poorly understood mechanism is the one that drives the escape of Lyman-α (Lyα) emission from star-forming galaxies and shapes the line profile of this well-used tracer of galaxies at high redshift. To fully solve these problems, direct observations of
the neutral gas content and distribution of Lyα and ionizing radiation emitters are needed.
In this talk, I will present 21cm observations of the neutral gas reservoirs of rare local galaxies that are analogous to objects in the early universe. We have observed the neutral gas of Haro 11, the closest known ionizing radiation (Lyman Continuum, LyC) leaking galaxy. This 21cm observation was the first to successfully map the neutral gas distribution of a confirmed LyC emitter. We observed a strongly asymmetric neutral gas distribution resulting from a merger event, with the bulk of the HI mass offset from the regions producing LyC radiation in the galaxy. By decreasing the column density of gas on large scales, this HI distribution has facilitated the escape of ionizing radiation from the center of the galaxy to the IGM.
We have also observed the neutral gas content and distribution of local Lyα-emitting galaxies on a variety of angular scales. We found that global HI properties did not correlate well with any metric quantifying Lyα emission, but resolved 21cm observations show tentative trends between Lyα and HI properties. This indicates that if neutral gas regulates the escape of Lyα emission, it does so on small scales. Furthermore, we have found clear evidence of interaction in most of the galaxies in the sample (>60%), indicating that mergers also play an important role in the Lyα escape from galaxies. According to cosmological models, galaxy mergers occurred more frequently in the early universe. Assessing the contribution of environment and galaxy interactions at high redshift will be essential to fully understand the first billion years of our universe.