Scaled Brownian motion: a paradoxical process with a time dependent diffusivity for the description of anomalous diffusion

Anomalous diffusion is frequently described by scaled Brownian motion (SBM), a Gaussian process with a power-law time dependent diffusion coefficient. Its mean squared displacement is $\langle x^2(t)\rangle\simeq\mathscr{K}(t)t$ with $\mathscr{K}(t)\simeq t^{\alpha-1}$ for $0<\alpha<2$. SBM may provide a seemingly adequate description in the case of unbounded diffusion, for which its probability density function coincides with that of fractional Brownian motion. Here we show that free SBM is weakly non-ergodic but does not exhibit a significant amplitude scatter of the time averaged mean squared displacement. More severely, we demonstrate that under confinement, the dynamics encoded by SBM is fundamentally different from both fractional Brownian motion and continuous time random walks. SBM is highly non-stationary and cannot provide a physical description for particles in a thermalised stationary system. Our findings have direct impact on the modelling of single particle tracking experiments, in particular, under confinement inside cellular compartments or when optical tweezers tracking methods are used.Comment: 7 pages, 5 figure

The first galaxies: simulating their feedback-regulated assembly

We investigate the formation of a galaxy reaching a virial mass of $~ 10^8$ solar mass at $z=10$ by carrying out a zoomed radiation-hydrodynamical cosmological simulation. This simulation traces Population~III (Pop~III) star formation, characterized by a modestly top-heavy initial mass function (IMF), and considers stellar feedback such as photoionization heating from Pop III and Population~II (Pop~II) stars, mechanical and chemical feedback from supernovae (SNe), and X-ray feedback from accreting black holes (BHs) and high-mass X-ray binaries (HMXBs). We self-consistently impose a transition in star formation mode from top-heavy Pop III to low-mass Pop~II, and find that the star formation rate in the computational box is dominated by Pop~III until $z=13$, and by Pop~II thereafter. The simulated galaxy experiences bursty star formation, with a substantially reduced gas content due to photoionization heating from Pop~III and Pop~II stars, together with SN feedback. All the gas within the simulated galaxy is metal-enriched above $10^{-5}$ solar, such that there are no remaining pockets of primordial gas. The simulated galaxy has an estimated observed flux of $~10^{-3} nJy$, which is too low to be detected by the James Webb Space Telescope (JWST) without strong lensing amplification. We also show that our simulated galaxy is similar in terms of stellar mass to Segue 2, the least luminous dwarf known in the Local Group.Comment: 18 pages 14 figures, Accepted for publication in MNRA

Recovery from population III supernova explosions and the onset of second generation star formation

We use cosmological simulations to assess how the explosion of the first stars in supernovae (SNe) influences early cosmic history. Specifically, we investigate the impact by SNe on the host systems for Population~III (Pop~III) star formation and explore its dependence on halo environment and Pop~III progenitor mass. We then trace the evolution of the enriched gas until conditions are met to trigger second-generation star formation. To this extent, we quantify the recovery timescale, which measures the time delay between a Pop~III SN explosion and the appearance of cold, dense gas, out of which second-generation stars can form. We find that this timescale is highly sensitive to the Pop~III progenitor mass, and less so to the halo environment. For more massive progenitors, including those exploding in pair instability SNe, second-generation star formation is delayed significantly, for up to a Hubble time. The dependence of the recovery time on the mass of the SN progenitor is mainly due to the ionizing impact of the progenitor star. Photoionization heating increases the gas pressure and initiates a hydrodynamical response that reduces the central gas density, an effect that is stronger in more massive. The gas around lower mass Pop~III stars remains denser and hence the SN remnants cool more rapidly, facilitating the subsequent re-condensation of the gas and formation of a second generation of stars. In most cases, the second-generation stars are already metal-enriched to ~2-5 X 10^{-4}\zsun, thus belonging to Population~II. The recovery timescale is a key quantity governing the nature of the first galaxies, able to host low-mass, long-lived stellar systems. These in turn are the target of future deep-field campaigns with the James Webb Space Telescope.Comment: 12 pages, 10 figures, Accepted for publication in MNRA

Anomalous Multiplicity Fluctuations from Phase Transitions in Heavy Ion Collisions

Event-by-event fluctuations and correlations between particles produced in relativistic nuclear collisions are studied. The fluctuations in positive, negative, total and net charge are closely related through correlations. In the event of a phase transitions to a quark-gluon plasma, fluctuations in total and net charge can be enhanced and reduced respectively which, however, is very sensitive to the acceptance and centrality. If the colliding system experiences strong density fluctuations due, e.g., to droplet formation in a first-order phase transition, all fluctuations can be enhanced substantially. The importance of fluctuations and correlations is exemplified by event-by-event measurement of the multiplicities of $J/\Psi$'s and charged particles since these observables should anti-correlate in the presence of co-mover or anomalous absorption.Comment: revised version to appear in Phys. Rev. C, 5 page

Radiative Feedback from high mass X-ray binaries on the formation of the first galaxies and early reionization

Recent work suggests that the first generation of stars, the so-called Population III (Pop III), could have formed primarily in binaries or as members of small multiple systems. Here we investigate the impact of X-ray feedback from High-Mass X-ray Binaries (HMXBs) left behind in stellar binary systems after the primary forms a black hole (BH), accreting gas at a high rate from the companion, a process that is thought to be favored at the low metallicities characteristic of high-redshift gas. Thanks to their large mean free path, X-rays are capable of preionizing and preheating the gas in the intergalactic medium (IGM) and in haloes long before the reionization of the Universe is complete, and thus could have strongly affected the formation of subsequent generations of stars as well as reionization. We have carried out zoomed hydrodynamical cosmological simulations of minihaloes, accounting for the formation of Pop III stars and their collapse into BHs and HMXBs, and the associated radiation-hydrodynamic feedback from UV and X-ray photons. We find no strong net feedback from HMXBs on the simulated star formation history. On the other hand, the preheating of the IGM by HMXBs leads to a strong suppression of small-scale structures and significantly lowers the recombination rate in the IGM, thus yielding a net positive feedback on reionization. We further show that X-ray feedback from HMXBs can augment the ionizing feedback from the Pop III progenitor stars to suppress gas accretion onto the first BHs, limiting their growth into supermassive BHs. Finally, we show that X-ray ionization by HMXBs leaves distinct signatures in the properties of the high-redshift hydrogen that may be probed in upcoming observations of the redshifted 21cm spin-flip line.Comment: 20 pages, 12 figures. Accepted for publication in MNRA

How dsDNA breathing enhances its flexibility and instability on short length scales

We study the unexpected high flexibility of short dsDNA which recently has been reported by a number of experiments. Via the Langevin dynamics simulation of our Breathing DNA model, first we observe the formation of bubbles within the duplex and also forks at the ends, with the size distributions independent of the contour length. We find that these local denaturations at a physiological temperature, despite their rare and transient presence, can lower the persistence length drastically for a short DNA segment in agreement with experiment