Research
Being especially fascinated by our Universe when it was young, I am interested in any aspects of this remote era. My research to date spans a wide range of times, from the period when the initial conditions for cosmic structure formation were created (during inflation) to the epoch when these seeds grew into the first stars.
During my Ph.D. I focused on the following projects:
Fluctuations in the 21-cm signal from the first stars
(read more).
Impact of the relative motion between the dark matter and the baryons on the first stars
(read more).
Pre-inflationary relics and their cosmological imprints
(read more).
Fluctuations in the 21-cm signal from the first stars
Observations of the redshifted 21-cm line of neutral hydrogen, scheduled for the next decade, are expected to lead us in a new era of direct observations of the epoch of the first stars. The first stars are expected to be highly clustered due to strong fluctuations in local density and the relative velocity (read more). The inhomogeneity in the first stars are mirrored by the radiation of these stars and by the redshifted 21-cm background. The first stars are believed to form from molecular hydrogen and their radiation plays an important role in the history of the Universe. Three types of radiative backgrounds are usually considered: X-rays heat the gas; Lyman-Werner (L-W) photons dissociate H2 and thus serve as negative feedback for star formation, eventually ending the epoch of the first stars; and Lyα photons couple the 21-cm line to the gas temperature and therefore make possible observations of this era via the 21-cm line. The relative timing of these three backgrounds is not fully determined. It is believed that the Lyα coupling happens relatively early (around z ~ 30) whereas heating fluctuations are important much later (z ~ 20). The L-W background is uncertain and is discussed here and in the references within.
(Left) with velocity, (right) without velocity. The illustration is taken from
Visbal, Barkana, Fialkov, Tseliakhovich & Hirata, Nature (2012).
In Visbal, Barkana, Fialkov, Tseliakhovich & Hirata, Nature (2012) we explore the impact of the relative motion on the 21-cm signature of the first stars at z ~ 20, around the redshift of heating transition, i.e., the moment when the gas temperature equates that of the CMB. We find that the velocities imprint Baryon Acoustic Oscillations (BAO) in the power spectrum of the 21-cm signal and enhance the fluctuations, leading to a detectible signature. We also show that the timing of the L-W feedback is crucial: a feedback that saturates early erases BAO, increase clustering of the first stars and improves observational prospects. More info about this paper can be found here.
In Fialkov, Barkana, Visbal, Tseliakhovih & Hirata (submitted to MNRAS) we probe the parameter space of possible L-W feedbacks and quantify the impact on the 21-cm signal. We use results of small-scale simulations at hand (see citations in the paper) to find the dependence of the minimal cooling mass (i.e. the mass of a lightest halo in which stars can form) on the L-W feedback and the value of the relative velocities. The feedback, as well as the relative velocities, delays the heating of the Universe. In addition it lessens the impact of the relative velocities at large scales and erases the BAO. The good news is that it increases clustering and enhances the power. Though our results span over the parameter space of possible L-W feedbacks, precise small-scale numerical simulations which include both the velocities and the feedback are needed to study this effect in more details.
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