Proper motions and variability of the H2 knots in the HH 111 and 121 protostellar outflows

We discuss the formation of spectral features in the decelerating ejecta of gamma-ray bursts, including the possible effect of inhomogeneities. These should lead to blueshifted and broadened absorption edges and resonant features, especially from H and He. An external neutral ISM could produce detectable H and He, as well as Fe X-ray absorption edges and lines. Hypernova scenarios may be diagnosed by Fe Kα and H Lyα emission lines.     

Mattig's relation and dynamical distance indicators

We discuss how the redshift (Mattig) method in the Friedmann cosmology relates to dynamical distance indicators based on Newton's gravity (Teerikorpi 2011). It belongs to the class of indicators where the relevant length inside the system is the distance itself (in this case the proper metric distance). As the Friedmann model has a Newtonian analogy, its use to infer distances has instructive similarities to classical dynamical distance indicators. In view of the theoretical exact linear distance‐velocity law, we emphasize that it is conceptually correct to derive the cosmological distance via the route: redshift (primarily observed) → space expansion velocity (not directly observed) → metric distance (physical length in “cm”). Important properties of the proper metric distance are summarized. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Cosmological perturbation theory and the spherical collapse model — III. The velocity divergence field and the Ω dependence

Cosmological perturbation theory (PT) is a useful tool to study the cumulants of the density and velocity fields in the large-scale structure of the Universe. In Papers I and II of this series we saw that the spherical collapse (SC) model provides the exact solution to PT at tree-level and gives a good approximation to the loop corrections (next-to-leading orders), indicating negligible tidal effects. Here, we derive predictions for the (smoothed) cumulants of the velocity divergence field θ ≡ ▽ ⊙  v for an irrotational fluid in the SC model. By comparing these with the exact analytic results of Scoccimarro &38; Frieman, it is shown that, at least for the unsmoothed case, the loop corrections to the cumulants of θ are dominated by tidal effects. However, most of the tidal contribution seems to cancel out when computing the hierarchical ratios, T _J  = 〈θ ^J 〉 / 〈θ²〉  ^{J −1}. We also extend the work presented in Papers I and II to give predictions for the cumulants of the density and velocity divergence fields in non-flat spaces. In particular, we show the equivalence between the spherically symmetric solution to the equations of motion in the SC model (given in terms of the density) and that of the Lagrangian PT approach (given in terms of the displacement field). It is shown that the Ω dependence is very weak for both cosmic fields even at one loop (a 10 per cent effect at most), except for the overall factor f (Ω) that couples to the velocity divergence.     

Poincarè Chaos and the Dynamical Evolution of Systems of Gravitating Bodies

Some general laws of evolution of a system of a large number of gravitating bodies are discussed. If in the initial stage the dynamics of the system is determined by large-scale perturbations of the gravitational potential associated with excitations of a few collective degrees of freedom, then one can assume, by analogy with chaos in the several-body problem (Poincarè chaos), that randomization will occur in the system over several average crossing times. In the next stage of evolution, the energy of collective modes should be transferred by the cascade mechanism to ever smaller scales, down to invididual particles. Numerical experiments and gross-dynamical considerations that could verify this picture and bring out details are discussed.     

Merger histories in warm dark matter structure formation scenarios

Observations on galactic scales seem to be in contradiction with recent high-resolution N -body simulations. This so-called cold dark matter (CDM) crisis has been addressed in several ways, ranging from a change in fundamental physics by introducing self-interacting cold dark matter particles to a tuning of complex astrophysical processes such as global and/or local feedback. All these efforts attempt to soften density profiles and reduce the abundance of satellites in simulated galaxy haloes. In this paper, we explore a different approach that consists of filtering the dark matter power spectrum on small scales, thereby altering the formation history of low-mass objects. The physical motivation for damping these fluctuations lies in the possibility that the dark matter particles have a different nature, i.e. are warm (WDM) rather than cold. We show that this leads to some interesting new results in terms of the merger history and large-scale distribution of low-mass haloes, compared with the standard CDM scenario. However, WDM does not appear to be the ultimate solution, in the sense that it is not able to fully solve the CDM crisis, even though one of the main drawbacks, namely the abundance of satellites, can be remedied. Indeed, the cuspiness of the halo profiles still persists, at all redshifts, and for all haloes and sub-haloes that we investigated. Despite the persistence of the cuspiness problem of DM haloes, WDM seems to be still worth taking seriously, as it alleviates the problems of over-abundant sub-structures in galactic haloes and possibly the lack of angular momentum of simulated disc galaxies. WDM also lessens the need to invoke strong feedback to solve these problems, and may provide a natural explanation of the clustering properties and ages of dwarfs.     

Testing tidal-torque theory – I. Spin amplitude and direction

We evaluate the success of linear tidal-torque theory (TTT) in predicting galactic-halo spin using a cosmological N -body simulation with thousands of well-resolved haloes. The protohaloes are identified by tracing today's haloes back to the initial conditions. The TTT predictions for the protohaloes match, on average, the spin amplitudes of the virialized haloes of today, if linear growth is assumed until ∼ t ₀/3, or  55–70  per cent of the halo effective turn-around time. This makes it a useful qualitative tool for understanding certain average properties of galaxies, such as total spin and angular momentum distribution within haloes, but with a random scatter of the order of the signal itself. Non-linear changes in spin direction cause a mean error of ∼50° in the TTT prediction at t ₀, such that the linear spatial correlations of spins on scales ≥1  h ⁻¹ Mpc are significantly weakened by non-linear effects. This questions the usefulness of TTT for predicting intrinsic alignments in the context of gravitational lensing. We find that the standard approximations made in TTT, including a second-order expansion of the Zel'dovich potential and a smoothing of the tidal field, provide close-to-optimal results.     

Mapping on the HEALPix grid

The natural spherical projection associated with the Hierarchical Equal-Area and Isolatitude Pixelization (HEALPix) is described and shown to be one of a hybrid class that combines the cylindrical equal-area and Collignon projections, not previously documented in the cartographic literature. Projection equations are derived for the class in general and are used to investigate its properties. It is shown that the HEALPix projection suggests a simple method of (i) storing and (ii) visualizing data sampled on the grid of the HEALPix pixelization, and also suggests an extension of the pixelization that is better suited for these purposes. Potentially useful properties of other members of the class are described, and new triangular and hexagonal pixelizations are constructed from them. Finally, the standard formalism is defined for representing the celestial coordinate system for any member of the class in the FITS data format.