Exchange report by Enrico Garaldi

SFB 956 sub-project: C4 
Supervisor: Prof. Dr. Cristiano Porciani, Argelander-Institut fuer Astronomie, Bonn (Germany) 
Hosting institution: University of California at Santa Cruz, Astronomy and Astrophysics department, Santa Cruz (USA) 
Supervisor at the hosting institution: Prof. Dr. Piero Madau 
Duration of stay abroad: 22/01/18 - 06/04/18

Introduction

How did the first stars transform our Universe? How does the cosmic web affect the formation of galaxies? These are the two big questions I am trying to answer in my doctoral thesis (within sub-project C4) using cosmological simulations that capture gravitational, hydrodynamical and radiative interactions. The first question concerns the Epoch of Reionization (EoR), a period of time – roughly 13 billions years ago – when the neutral gas left behind by the Big Bang (and the subsequent recombination) was transformed in a highly-ionized plasma by the intense radiation of the first stars and quasars. This process radically changed the environment in which galaxies form and evolve, producing lasting effects that we still witness today. In the first part of my doctoral project I employed numerical simulations to disentangle the contributions from different sources of photons, showing that quasar can only play a minor role in such early epoch. Thus, the next step is to investigate the properties of the other main actor: galaxies. However, the very large distances of these objects render their detection and characterization extremely challenging. An alternative route to understand their properties is to study the inter-galactic medium (IGM) – the diluted baryonic material residing between galaxies – that imprints absorption features on the spectrum of bright background sources. During the EoR, the IGM was not sufficiently ionized to allow significant transmission of the background photons, but at the tail-end of reionization active sources produced pockets of highly-ionized gas in their vicinity. These regions generate small transmission spikes in the otherwise-completely-absorbed background spectrum that have been recently observed for the first time.

The exchange experience

Science
Recent observations of transmission spikes during the ending phases of the EoR open a new window on the properties of the IGM and of galaxies living in that period of time. However, their full exploitation requires numerical modeling to bridge the gap between observed spectra and the physical state of the IGM. In order to pursue this task, I joined the Astronomy and Astrophysics department at the University of California at Santa Cruz (UCSC), USA and teamed up with Prof. Dr. Piero Madau, one of the world leaders in theoretical and numerical studies of the IGM. We employed state-of-the-art hydrodynamical and radiative-transfer simulations run by Prof. Dr. Nickolay Gnedin at the University of Chicago and investigated the optimal way to retrieve the underlying properties of the IGM regions producing the transmission spikes. These are directly related to the emission properties of sources responsible for the production of ionizing photons. During the 2.5 months I spent at UCSC I familiarized with the simulation suite and determined the relation between the detected number of transmission spikes and the global neutral fraction of the Universe. Then, I investigated the physical link between the shape of transmission spikes and the underlying IGM state. Finally, thanks to a fortunate scientific interaction with another guest of UCSC, I am now exploring a statistical way to constrain the properties of the first stars and galaxies using the transmission spies. All these results will be presented in a scientific publication that I started writing in the last two weeks of my stay.

Life
Santa Cruz is a small but vibrant town, proudly progressive and even-more-proudly home of the first surfing beach outside Hawai’i. The first word I will always associate with this city is ‘relaxed’, which is by no means synonym to ‘inactive’! It is a very enjoyable city, with Californian weather, relaxed atmospheres and friendly people (I was welcomed by a local with the motto “We don’t care about what you wear, what you believe in or what you like, you will always be welcome to Santa Cruz!”). As if this was not enough, UCSC is located undoubtedly in the most beautiful campus I have ever seen: when Santa Cruz decided to host a University, the city bought a huge chunk of forest and squeezed the campus in it, rather than clearing space for buildings. The amazing result is that entering UCSC translates to walk into a forest where departments, classrooms and libraries grow alongside the tall redwoods. Do I need to say that it is full of wildlife? All these, together with famous surfing beaches, attracts a lot of people and rises the living costs to remarkably high ends (totally worth it, if you ask me!).

Networking
Changing continent not only forces you in a different mindset and environment (which is good!) but also brings you closer to institutions and researchers that are usually hard to visit because of their distance. During my time in Santa Cruz I had the opportunity to discuss and exchange ideas with scientists I had never met and to learn a lot about research topics that are rarely pursued in my home institution. Similarly, I feel I gave back to them by sharing my knowledge and experience in fields that are not explored there. I also got the opportunity to visit the group of Prof. Dr. Joseph Hennawi at the University of California at Santa Barbara, where I had very productive discussions that helped me in advertising my work and improving parts of the research project I carried out at UCSC.

Conclusions

My experience has been extremely positive from every point of view, and I recommend it to any doctoral candidate who may read these lines. However, such an enjoyable and fruitful exchange would have not been possible without many people I wish to thank: the SFB 956 for providing fundings for this amazing experience, my supervisor for being very supportive of this adventure since the beginning, UCSC for the friendly environment and lively scientific activity and finally my overseas host Prof. Dr. Piero Madau for being always available, supportive and most of all for his contagious energy. I truly believe this experience greatly contributed to my knowledge and, most of all, to my growth as a scientist and as a person.