Research Topics

Dark Ages is the term used to refer to the cosmic times extending from the recombination epoch, when free protons and electrons in the Universe combined into hydrogen atoms (300.000 years after the Big Bang) to the reionization epoch, which marks a transition back to a fully ionized intergalactic plasma. The end of the Dark Ages hosts a number of fundamental processes: the birth of the first stars, supernovae and black holes; mini-galaxies start to form which later on will constitute the building blocks of larger galaxies; UV/ionizing photons starting the process of cosmic reionization are first emitted; a large number of radiative, mechanical and chemical feedback processes shaped the evolution of cosmic structures. At the end of this fascinating period, the Universe witnesses a Cosmic Dawn, the return of light after the Big Bang.

The study of the Dark Ages lies at the forefront of research. Only recently it has started to be seriously investigated thanks to greatly improved computational and observational tools. Members of DAVID are active researches working in many different but related aspects of this field using state-of-the art supercomputer numerical simulations, semi-analytic models and observations.

DAVID team members are currently involved in two ERC-funded research programs:

FIRST: the first stars and galaxies, PI R. Schneider, based in INAF/OARoma, is an ERC/Starting Grant funded for 5 years (2012-2017);

AIDA: An Illumination of the Dark Ages: modeling reionization and interpreting observations, PI A. Mesinger, based in Scuola Normale di Pisa, is an ERC/Starting Grant funded for 5 years (2015–2020);

A brief description of the main research topics is given below. More information can be obtained through the publication list and the members personal web pages.

The Nature of the First Stars

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The first stars are the first sources of light, heavy elements (metals) and dust after the Big Bang. They are predicted to form from the collapse of gas of primordial composition at redshift 20 - 30 in the first mini-galaxies with typical masses of 106 Msun. Recent studies have started to tackle the formation of the first stars through numerical simulations based on hierarchical scenarios of structure formation. The movie shows the results of a 3D simulation by Abel, Bryan & Norman (2000) zooming in from the large scale structure down to the forming protostars. The simulations indicate that primordial gas cools and fragments into clumps with masses of order of 1000 Msun which are direct progenitors of the stars that will form in their interiors. The final stellar masses depend on the complex physics governing mass accretion onto the protostellar core, and the most recent models show that are likely to be 30 Msun < M < 100 Msun, much larger than what observed in present-day stellar populations. You can read more on this in the corresponding FIRST project web page.

Credits: Matteo de Bennassuti, Stefania Salvadori

The First Metals and Dust

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At the end of their short lifetime (3-5 Myrs) the first massive stars might explode as very energetic supernovae, ejecting metals synthesized in their interiors in the surrounding gas. During the explosions, a significant fraction of the gas-phase metals can be condensed into solid dust grains, making supernovae the main dust factories at high redshift. The metals and dust now present in the gas have dramatic consequences for the evolution of pre-enriched collapsing star forming gas clouds. As can be seen from the image, metals and dust represent the major drivers of the transition in fragmentation scales, and therefore in stellar masses. Thus, we expect that above a critical metallicity threshold, gas collapse leads to the formation of stars in the conventional mass range, similar to those observed in the nearby Universe. Searching for fossils of these second-generation stars represents a promising way to constrain the nature of the first stars, which so far have never been directly detected. Interesting results on this topic can be also found in the corresponding FIRST web page.

Credits: Stefania Marassi, Ruben Salvaterra

First Galaxies

Credits: Livia Vallini, Pratika Dayal

First Black Holes

Credits: Fabio Pacucci, Rosa Valiante

Feedback Processes

Credits: Manuela Bischetti & Chiara Feruglio

Cosmic Reionization and 21cm

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At z~1100 the intergalactic medium (IGM) is expected to recombine and remain neutral until the first sources of ionizing radiation form and reionize it. The application of the Gunn-Peterson test to QSOs absorption spectra suggests that the HI reionization is complete by z~6, but the epoch of complete reionization has yet to be firmly established. Several authors claim that the known population of quasars and galaxies provides ~10 times fewer ionizing photons than are necessary to keep the observed IGM ionization level; thus, additional sources of ionizing photons are required at high redshift, the most promising being early galaxies and quasars. An alternative attractive mechanism is represented by decaying/annihilating dark matter particles, which might provide a way to test the nature of dark matter via its effects on reionization. The figure shows the complex structures of ionized (in black) and neutral (in red/green) regions of the IGM obtained through a numerical simulation.

Numerical tools

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A crucial ingredient in studying the reionization process is accurately following the radiative transfer of ionizing photons from their production site into the IGM. The code CRASH ( Ciardi et al 2001; Maselli et al 2003) has been designed to this aim and it is able to account for the simultaneous presence of multiple point sources and of a UV background radiation. The code has been tested on a wide range of values for density, luminosity and spectral type of the sources and does not require any symmetry for the density field in the simulation box. Thanks to the extensive use of Monte Carlo techniques, CRASH has a great versatility and has already been applied to a number of astrophysical problems ranging from the proximity effect around high-z quasars to the Lya forest and the size of HII regions around quasars as estimated from their spectra. The code has partecipated to the Cosmological radiative transfer codes comparison project, an extensive comparison between radiative transfer codes based on different algorithms.

Credits: Andrea Pallottini, Luca Graziani

Topic revision: r6 - 20 Oct 2015, RaffaellaSchneider
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