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is interested in the formation of the first structures and the reionisation of the universe. She is the project leader at MPA for LOFAR and is particularly involved in its studies of the Epoch of Reionisation.ContactMax Planck Institute for AstrophysicsKarl-Schwarzschild-Str. 1D-85748 GarchingRoom 101Phone +49-89-30000-2018Fax +49-89-30000-2235Email ciardi_at_mpa-garching.mpg.de

Benedetta Ciardi

is interested in the formation of the first structures and the reionisation of the universe. She is the project leader at MPA for LOFAR and is particularly involved in its studies of the Epoch of Reionisation.
Contact
Max Planck Institute for Astrophysics
Karl-Schwarzschild-Str. 1
D-85748 Garching
Room 101
Phone +49-89-30000-2018
Fax +49-89-30000-2235
Email ciardi_at_mpa-garching.mpg.de

Benedetta Ciardi's research interests

Early structure formation and feedback effects

Simulations like the one depicted in the Figure can be used to study the properties of the first galaxies. Zoom Image
Simulations like the one depicted in the Figure can be used to study the properties of the first galaxies.

The first structures to appear in the universe form the building blocks of present day galaxies and host the first stars, which initiate a complex interaction among different physical processes and deeply affect subsequent structure formation and evolution through feedback effects (Maio et al. 2007; Maio et al. 2009; Maio et al. 2010; Maio, Koopmans & Ciardi 2011; Maio et al. 2011; Dayal et al. 2013). The latter can be broadly classified as (i) chemical (associated to the metal enrichment which impacts the gas cooling), (ii) mechanical (through winds and supernovae explosions), and (iii) radiative (associated to ionization and heating of the gas from UV and X-ray photons). Despite the fundamental role played in shaping the universe we observe today, feedback effects are still poorly understood and are the subject of a continuous investigation (Ciardi & Ferrara 2005).

Reionization of the IGM

Maps of temperature (left) and HI fraction (right) at redshift 14, 9 and 7 (from top to bottom) as obtained in simulation of reionization run by our group. Zoom Image
Maps of temperature (left) and HI fraction (right) at redshift 14, 9 and 7 (from top to bottom) as obtained in simulation of reionization run by our group. [less]

The most relevant among the radiative feedback effects is the reionization of the IGM (Ciardi et al. 2000; Ciardi, Stoehr & White 2003; Ciardi, Ferrara & White 2003; Ciardi et al. 2006; Ciardi et al. 2012; Jeeson-Daniel, Ciardi & Graziani 2014) The radiation emitted by the first objects starts to reionize the surrounding gas. This epoch, when light returned after the so-called `Dark Ages', marks the beginning of the reionization process. While observations of absorption features in the spectra of high-z quasars show near complete absorption at epochs of 1 billion years after the Big Bang and have been interpreted as mapping the tail end of cosmic reionization, the analysis of the maps of temperature anisotropies in the cosmic microwave background (CMB) radiation from the WMAP and PLANCK satellites suggests that reionization must have begun at much earlier times, and so must have had a complex history. A theoretical modeling of the process is paramount to the understanding of the present day universe due to its impact on structure formation.

Radiative transfer

HeII distribution around a quasar as produced by CRASH Zoom Image
HeII distribution around a quasar as produced by CRASH

While semi-analytic approaches are a useful tool for a parameter exploration in reionization studies, a more correct modeling of the reionization process requires following the full radiative transfer of ionizing photons. CRASH (Ciardi et al. 2001; Maselli, Ferrara & Ciardi 2003; Iliev et al. 2006; Maselli, Ciardi & Kanekar 2009; Pierleoni, Maselli & Ciardi 2009; Partl et al. 2011; Graziani, Maselli & Ciardi 2013) is a 3D Monte-Carlo radiative transfer code which follows the propagation of UV and X-ray photons emitted by an arbitrary number of point sources through an inhomogeneous gas distribution. It calculates self-consistently the evolution of hydrogen, helium, gas temperature and some metal species.

Observational probes

Surface brightness maps for a Lya emitter as seen along six different lines of sight (Jeeson-Daniel et al. 2012) Zoom Image
Surface brightness maps for a Lya emitter as seen along six different lines of sight (Jeeson-Daniel et al. 2012)

A theoretical modeling of the high-z universe should be guided by observations. For this reason, it is important to use the outcome of such modeling to compare to observational data, when available, or to propose observational strategies for planned/future facilities. Among possible probes of the high-z universe we can mention e.g. gravitational wave emission (Schneider et al. 2000), infrared molecular line radiation from the first stars (Ciardi & Ferrara 2001), the detection of gamma-ray bursts through the intergalactic gas (Ciardi & Loeb 2000; Inoue, Omukai & Ciardi 2007; Campisi et al. 2011; Salvaterra et al. 2013; Ma et al. 2015; Ciardi et al. 2015), the imprint that reionization should leave on the temperature anisotropies in the CMB (Bruscoli et al. 2000; Salvaterra et al. 2005), high-z Lyalpha Emitters (Kurk et al. 2004; Jeeson-Daniel et al. 2012).

21cm line from neutral hydrogen

Maps of 21cm line emission (in terms of the so-called differential antenna temperature and in units of log K) at an observed frequency of (from top to bottom and left to right) 98, 103, 110, 116, 123, 131, 139, 147 and 157 MHz (Ciardi & Madau 2003) Zoom Image
Maps of 21cm line emission (in terms of the so-called differential antenna temperature and in units of log K) at an observed frequency of (from top to bottom and left to right) 98, 103, 110, 116, 123, 131, 139, 147 and 157 MHz (Ciardi & Madau 2003) [less]

Although since the late 1950s it has been recognized that neutral hydrogen in the IGM may be directly detectable in emission or absorption against the CMB radiation at the frequency corresponding to the redshifted neutral hydrogen 21 cm line providing a direct probe of the era of cosmological reionization, only now radio interferometers such as LOFAR, MWA and PAPER are delivering the first data. At present we only have available observations of the last stages of the process and of the global amount of electrons produced during reionization, but we are lacking any information on the reionization history and its sources. The 21cm line from neutral hydrogen in the IGM would provide invaluable information on the distribution and evolution of HI and would allow, for the first time, to map the temporal evolution of the reionization process (Ciardi & Madau 2003; Di Matteo, Ciardi & Salvaterra 2004; Salvaterra et al. 2005; Valdes et al. 2006; Ciardi & Salvaterra 2007).

I am a core member of the LOFAR Epoch of Reionization Key Science Project (e.g. Jelic et al. 2008; Harker et al. 2010; Zaroubi et al. 2012; Ciardi et al. 2013; Jensen et al. 2013; Yatawatta et al. 2013; Wiersma et al. 2013; Patil et al. 2014; Ciardi et al. 2015; Vedantham et al. 2015), and MPA has also contributed to the LOFAR project building one of the international stations.

 
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