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Deep Investigation on Molecular Processes At early Cosmic Times

Periodic Reporting for period 1 - DeepIMPACT (Deep Investigation on Molecular Processes At early Cosmic Times)

Reporting period: 2018-02-01 to 2020-01-31

The discovery of galaxies formed when the Universe was less than 1 Gyr old has been a major breakthrough of extragalactic astronomy. During this period, called Epoch of Reionization (EoR), the Universe experienced a dramatic change: from being a dark “sea” of neutral hydrogen and helium, it got completely reionized and polluted with elements heavier than helium. Thus, characterising the bulk of galaxy population at these early epochs is pivotal because of their strong impact on the cosmic evolution. The most distant galaxies have been detected searching for their Lyman-α emission (Lyman Alpha Emitters; LAEs), and/or for the “step”, introduced by the blanketing effect of neutral hydrogen into their UV continuum emission (Lyman Break Galaxies; LBGs). LAEs/LBGs are quite “normal” objects, therefore they are thought to be fairly representative of the bulk of galaxies at the end of the EoR. Our knowledge about these sources regards their stellar component (star formation rates, stellar age and mass). However, we have few clues about their interstellar medium, the properties of the giant molecular clouds (GMCs) where stars form, and, most importantly, the possible presence of faint Active Galactic Nuclei (AGN).

With this project called DeepIMPACT (Deep Investigation on Molecular Processes At early Cosmic Times) we study the properties of faint AGNs through their signature in the molecular emission spectrum.
This is of utmost importance because only extremely luminous AGN, in which the gas is accreting onto a Super Massive Black Hole (SMBH), have been detected at z>6.
Their formation is still puzzling, and the better strategy to look for SMBH seeds, and in general for lower luminosity AGN, is matter of ongoing debate in the astrophysical community.

The overall objectives of DeepIMPACT can be summarised as follows:
1) Model the chemical, physical, and radiative processes in giant molecular clouds illuminated by both young stars and AGN.
2) Quantify the impact of X-ray photons on the clumpy structure of molecular clouds,
3) Clarify whether the AGN signature can be observed in the molecular emission and pinpoint galaxy candidates for observational follow up targeting the molecular gas.
"From the beginning of the project I have been working along several lines. First of all, I focussed the attention on the clarification of the impact of X-ray photons on the clumpy structure and molecular emission, expecially that of Carbon Oxide (CO) from molecular clouds. I developed a program that allow to compute the CO emission assuming different densities, level of turbulence, and illumination conditions for the clouds. Such early version of the model has been successfully used to interpret CO observation in high-z AGN and devise observational proposal (some of which have been accepted, and we are now waiting for the data from the telescopes).

After that, I developed an analytical model that approximates the distribution of clouds in galaxies and I tested the results against local observations. At the same time, the results of this work have been presented in international conferences and invited seminars in several EU institutes (e.g. Bologna, Geneva, Oslo). The global results from the whole implementation of DeepIMPACT has been published in Monthly Notices of the Royal Astronomical Society

The results from the implementation of the DeepIMPACT project have been valuable in order to interpret observations of line emission from high redshift galaxies obtained by collaborators (both in Leiden and around the world).

Finally, I also implemented the effect of shock-heating in molecular clouds, and provided my calculations to the SPICA Collaboration for the writing of the White Paper entitled ""Probing the high-redshift universe with SPICA: Toward the epoch of Reionisation and beyond"". SPICA is a proposed space mission that has been selected in March 2018 by ESA’s fifth medium class mission in its Cosmic Vision science programme, with a planned launch date in 2032s."
DeepIMPACT has certainly produced several advanced beyond the state of the art. These can be summarised as follows:
First of all we have, for the first time, developed a stand-alone model able to catch the internal structure of molecular clouds and the effect of illumination of X-ray.
All the previous X-ray models describing the molecular gas emission in galaxies did not account for the clumpy structure of clouds.
This is a fundamental breakthrough as the high density clumps present in such structures are the most affected by presence (or absence) of X-rays and a proper description is mandatory in order to interpret state-of-art observations.
Moreover, thanks to DeepIMPACT, I contributed to the planning of the next generation infrared satellite satellite (SPICA). This mission is currently selected by ESA for the M5 mission in the Cosmic Vision programme.
Finally, we have discovered that the compactness of galaxies in the early Universe might strongly affect their observability, this paves a new exciting way for future observations of their molecular gas.
The effect of the increased X-ray luminosity on the CO emission as resulting from DEEP Impact