Lyman-alpha radiative transfer during the Epoch of Reionization: contribution to 21-cm signal fluctuations

During the epoch of reionization, Ly-alpha photons emitted by the first stars can couple the neutral hydrogen spin temperature to the kinetic gas temperature, providing the opportunity to observe the gas in emission or absorption in the 21-cm line. Given the bright foregrounds, it is of prime importance to determine precisely the fluctuations signature of the signal, to be able to extract it by its correlation power. LICORICE is a Monte-Carlo radiative transfer code, coupled to the dynamics via an adaptative Tree-SPH code. We present here the Ly-alpha part of the implementation, and validate it through three classical tests. Contrary to previous works, we do not assume that P_alpha, the number of scatterings of Ly-alpha photons per atom per second, is proportional to the Ly-alpha background flux, but take into account the scatterings in the Ly-alpha line wings. The latter have the effect to steepen the radial profile of P_alpha around each source, and re-inforce the contrast of the fluctuations. In the particular geometry of cosmic filaments of baryonic matter, Ly-alpha photons are scattered out of the filament, and the large scale structure of P_alpha is significantly anisotropic. This could have strong implications for the possible detection of the 21-cm signal.Comment: 13 pages, 9 figures. To be published in A&

Formation and Evolution of Galactic Disks with a Multiphase Numerical Model

The formation and evolution of galactic disks are complex phenomena, where gas and star dynamics are coupled through star formation and the related feedback. The physical processes are so numerous and intricate that numerical models focus, in general, on one or a few of them only. We propose here a numerical model with particular attention to the multiphase nature of the interstellar medium; we consider a warm gas phase (> 10^4 K), treated as a continuous fluid by an SPH algorithm, and a cold gas phase (down to 10K), fragmented in clouds, treated by a low-dissipation sticky particles component. The two gas phases do not have the same dynamics, nor the same spatial distribution. In addition to gravity, they are coupled through mass exchanges due to heating/cooling processes, and supernovae feedback. Stars form out of the cold phase, and re-inject mass to the warm phase through SN explosions and stellar winds. The baryons are embedded in a live cold dark matter component. Baryonic disks, initially composed of pure gas, encounter violent instabilities, and a rapid phase of star formation, that slows down exponentially. Stars form in big clumps, that accumulate in the center to build a bulge. Exponential metallicity gradients are obtained. External infall of gas should be included to maintain a star formation rate in the disk comparable to what is observed in present disk galaxies

Star formation efficiency in galaxy interactions and mergers: a statistical study

We investigate the enhancement of star formation efficiency in galaxy interactions and mergers, by numerical simulations of several hundred galaxy collisions. All morphological types along the Hubble sequence are considered in the initial conditions of the two colliding galaxies, with varying bulge-to-disk ratios and gas mass fractions. Different types of orbits are simulated, direct and retrograde, according to the initial relative energy and impact parameter, and the resulting star formation history is compared to that occuring in the two galaxies when they are isolated. Our principal results are: (1) retrograde encounters have a larger star formation efficiency (SFE) than direct encounters; (2) the amount of gas available in the galaxy is not the main parameter governing the SFE in the burst phase; (3) there is an anticorrelation between the amplitude of the star forming burst and the tidal forces exerted per unit of time, which is due to the large amount of gas dragged outside the galaxy by tidal tails in strong interactions; (4) globally, the Kennicutt-Schmidt law is retrieved statistically for isolated galaxies, interacting pairs and mergers; (5) the enhanced star formation is essentially occurring in nuclear starbursts, triggered by inward gas flows driven by non-axisymmetries in the galaxy disks. Direct encounters develop more pronounced asymmetries than retrograde ones. Based on these statistical results, we derive general laws for the enhancement of star formation in galaxy interactions and mergers, as a function of the main parameters of the encounter.Comment: 22 pages, 37 figures, 4 tables. Accepted on Astronomy & Astrophysic

Signatures of radial migration in barred galaxies: Azimuthal variations in the metallicity distribution of old stars

By means of N-body simulations, we show that radial migration in galaxy disks, induced by bar and spiral arms, leads to significant azimuthal variations in the metallicity distribution of old stars at a given distance from the galaxy center. Metals do not show an axisymmetric distribution during phases of strong migration. Azimuthal variations are visible during the whole phase of strong bar phase, and tend to disappear as the effect of radial migration diminishes, together with a reduction in the bar strength. These results suggest that the presence of inhomogeneities in the metallicity distribution of old stars in a galaxy disk can be a probe of ongoing strong migration. Such signatures may be detected in the Milky Way by Gaia (and complementary spectroscopic data), as well as in external galaxies, by IFU surveys like CALIFA and ATLAS3D. Mixing - defined as the tendency toward a homogeneous, azimuthally symmetric, stellar distribution in the disk - and migration turns out to be two distinct processes, the effects of mixing starting to be visible when strong migration is over.Comment: 8 pages, 10 figures, accepted for publication on Astronomy and Astrophysic

The life-time of galactic bars: central mass concentrations and gravity torques

Bars in gas-rich spiral galaxies are short-lived. They drive gas inflows through their gravity torques, and at the same time self-regulate their strength. Their robustness has been subject of debate, since it was thought that only the resulting central mass concentrations (CMCs) were weakening bars, and only relatively rare massive CMCs were able to completely destroy them. Through numerical simulations including gas dynamics, we find that with the gas parameters of normal spiral galaxies, the CMC is not sufficient to fully dissolve the bar. But another overlooked mechanism, the transfer of angular momentum from the infalling gas to the stellar bar, can also strongly weaken the bar. In addition, we show that gravity torques are correctly reproduced in simulations, and conclude that bars are transient features, with life-time of 1-2 Gyr in typical Sb-Sc galaxies, because of the combined effects of CMCs and gravity torques, while most existing works had focussed on the CMC effects alone.Comment: accepted for publication in MNRAS. The definitive version is available at www.blackwell-synergy.co