We investigate the in‐spiralling time‐scales of globular clusters (GCs) in dwarf spheroidal (dSph) and dwarf elliptical (dE) galaxies, due to dynamical friction (DF). We address the problem of these time‐scales having been variously estimated in the literature as much shorter than a Hubble time. Using self‐consistent two‐component (dark matter and stars) models, we explore mechanisms which may yield extended DF time‐scales in such systems in order to explain why dwarf galaxies often show GC systems. As a general rule, dark matter and stars both give a comparable contribution to the dynamical drag. By exploring various possibilities for their gravitational make‐up, it is shown that these studies help to constrain the parameters of the dark matter haloes in these galaxies, as well as to test alternatives to dark matter. Under the assumption of a dark halo having a central density core with a typical King core radius somewhat larger than the observed stellar core radius, DF time‐scales are naturally extended upwards of a Hubble time. Cuspy dark haloes yield time‐scales ≲4.5 Gyr, for any dark halo parameters in accordance with observations of stellar line‐of‐sight velocity dispersion in dSph galaxies. We confirm, after a detailed formulation of the DF problem under the alternative hypothesis of modified Newtonian dynamics (MOND) and in the lack of any dark matter, that due to the enhanced dynamical drag of the stars, the DF time‐scales in MOND would be extremely short. Taking the well‐measured structural parameters of the Fornax dSph and its GC system as a case study, we conclude that requiring DF time‐scales comparable to the Hubble time strongly favours dark haloes with a central core.

We consider the hypothesis that galactic dark matter is composed of ultralight scalar particles and use internal properties of dwarf spheroidal galaxies to establish a preferred range for the mass m φ of these bosonic particles. We re-investigate the problem of the longevity of the cold clump in Ursa Minor and the problem of the rapid orbital decay of the globular clusters in Fornax and dwarf ellipticals. Treating the scalar field halo as a rigid background gravitational potential and using N -body simulations, we have explored how the dissolution timescale of the cold clump in Ursa Minor depends on m φ . It is demonstrated that for masses in the range 0.3 × 10 −22 eV < m φ < 1 × 10 −22 eV, scalar field dark halos without self-interaction would have cores large enough to explain the longevity of the cold clump in Ursa Minor and the wide distribution of globular clusters in Fornax, but small enough to make the mass of the dark halos compatible with dynamical limits. It is encouraging to see that this interval of m φ is consistent with that needed to suppress the overproduction of substructure in galactic halos and is compatible with the acoustic peaks of cosmic microwave radiation. On the other hand, for self-interacting scalar fields with coupling constant λ, values of m 4 φ /λ 0.55 × 10 3 eV 4 are required to account for the properties of the dark halos of these dwarf spheroidal galaxies.

We discuss the nonlinear development of the isobaric mode of thermal instability (TI) in the context of the atomic interstellar medium (ISM), both in isolation and in the presence of either density or velocity fluctuations, in order to assess the ability of TI to establish a well-segregated multi-phase structure in the turbulent ISM. The key parameter is the ratio of the cooling time to the dynamical crossing time η. First, we discuss the degree to which the condensation process of large-scale perturbations generates large velocities, and the times required for them to subside. Using high-resolution simulations in 1D and fits to recently published cooling rates, we find that density perturbations of sizes 15 pc in media with mean density ∼ 1 cm −3 develop inflow motions with Mach numbers larger than 0.5 and a shock that propagates outwards from the condensation, bringing the surrounding medium out of thermal equilibrium. The time for the dynamical transient state to subside ranges from 4 to 30 Myr for initial density perturbations of 20% and sizes 3 to 75 pc. By the time the condensations have formed, a substantial fraction of the mass is still traversing the unstable range. Smaller (0.3 -3 pc) perturbations may condense less dynamically, and reach nearly static configurations in shorter times (e.g., ∼ 3.5 Myr for perturbations of ∼ 0.3 pc), but they may be stable if they have a turbulent origin (see below). We thus suggest that, even if TI were the sole cloud-forming agent in the ISM, clouds formed by it should be bounded by accreting gas traversing the unstable range, rather than by sharp transitions to the stable warm phase. Second, we discuss the competition between a spectrum of density perturbations of various sizes. We empirically find that, in order for small-scale perturbations not to alter significantly the global evolution, progressively larger values of η are necessary as the initial spectrum becomes shallower. Finally, we discuss the development of the instability in the presence of random velocity forcing, which we argue is the most realistic way to emulate density fluctuation production in the actual ISM. Such fluctuations are quasi-adiabatic rather than quasi-isobaric in the large-η limit, and trigger the wave mode of TI, rather than the condensation mode, being stable to first order. Indeed, we find that the condensation process can be suppressed for arbitrarily long times if the forcing causes a moderate rms Mach number ( 0.3) and extends to small enough scales or occurs in low enough density environments that the turbulent crossing time becomes smaller than the cooling time at those scales. We suggest that this mechanism, and the long times required to evacuate the unstable phase, may be at the origin of the relatively large amounts of gas mass in the unstable regime found in both observations and simulations of the ISM. The gas with unstable temperatures is expected to be out of thermal equilibrium, suggesting that it can be observationally distinguished by simultaneously measuring two of its ...

We discuss the temperature distribution in a two-dimensional, thermally unstable numerical simulation of the warm and cold gas in the Galactic disk, including the magnetic field, self-gravity, the Coriolis force, stellar energy injection and a realistic cooling function. We find that ~50% of the turbulent gas mass has temperatures in what would be the thermally unstable range if thermal instability were to be considered alone. This appears to be a consequence of there being many other forces at play than just thermal pressure. We also point out that a bimodal temperature pdf is a simple consequence of the form of the interstellar cooling function and is not necessarily a signature of discontinuous phase transitions.Comment: video address has been update

We study the formation of structure in the Universe assuming that dark matter can be described by a scalar fieldΦ with a potential V (Φ) = −m 2Φ2 /2 + λΦ 4 /4. We derive the evolution equations of the scalar field in the linear regime of perturbations. We investigate the symmetry breaking and possibly a phase transition of this scalar field in the early Universe. At low temperatures, the scalar perturbations have an oscillating growing mode and therefore, this kind of dark matter could lead to the formation of gravitational structures. In order to study the nonlinear regime, we use the spherical collapse model and show that, in the quadratic potential limit, this kind of dark matter can form virialized structures. The main difference with the traditional Cold Dark Matter paradigm is that the formation of structure in the scalar field model can occur at earlier times. Thus, if the dark matter behaves as a scalar field, large galaxies are expected to be formed already at high redshifts.

In many astrophysical situations, as in the coalescence of supermassive black hole pairs at gas rich galactic nuclei, the dynamical friction experienced by an object is a combination of its own wake as well as the wakes of its companions. Using a semi-analytic approach, we investigate the composite wake due to, and the resulting drag forces on, double perturbers that are placed at the opposite sides of the orbital center and move on a circular orbit in a uniform gaseous medium. The circular orbit makes the wake of each perturber asymmetric, creating an overdense tail at the trailing side. The tail not only drags the perturber backward but it also exerts a positive torque on the companion. For equal-mass perturbers, the positive torque created by the companion wake is, on average, a fraction ~40-50% of the negative torque created by its own wake, but this fraction may be even larger for perturbers moving subsonically. This suggests that the orbital decay of a perturber in a double system, especially in the subsonic regime, can take considerably longer than in isolation. We provide the fitting formulae for the forces due to the companion wake and discuss our results in light of recent numerical simulations for mergers of binary black holes.Comment: 4 pages, 3 figures, accepted for publication in ApJ

The dynamical friction experienced by a body moving in a gaseous medium is different from the friction in the case of a collisionless stellar system. Here we consider the orbital evolution of a gravitational perturber inside a gaseous sphere using three-dimensional simulations, ignoring however self-gravity. The results are analysed in terms of a `local' formula with the associated Coulomb logarithm taken as a free parameter. For forced circular orbits, the asymptotic value of the component of the drag force in the direction of the velocity is a slowly varying function of the Mach number in the range 1.0-1.6. The dynamical friction timescale for free decay orbits is typically only half as long as in the case of a collisionless background, which is in agreement with E.C. Ostriker's recent analytic result. The orbital decay rate is rather insensitive to the past history of the perturber. It is shown that, similar to the case of stellar systems, orbits are not subject to any significant circularization. However, the dynamical friction timescales are found to increase with increasing orbital eccentricity for the Plummer model, whilst no strong dependence on the initial eccentricity is found for the isothermal sphere.Comment: 13 pages, 13 figures, MNRAS accepte

The drag force experienced by a gravitational body moving in a straight-line trajectory through a homogeneous isothermal gaseous medium of given sound speed is investigated numerically. For perturbers with constant velocity, linear theory describes successfully the temporal evolution and magnitude of the force. The result obtained recently by E. Ostriker-that for Mach numbers-2 the force is stronger in a gaseous medium than in a M ϭ 1 collisionless medium, as described by the standard Chandrasekhar formula-is confirmed. The corresponding minimum impact radius r min for a body described with a Plummer model with core radius R soft is r /R ≈ min soft. When , the drag force is strongly suppressed, which is consistent with Ostriker's results but in 2.25 M ! 1 disagreement with the Chandrasekhar formula. However, when the perturber is decelerated by its own wake to , the effective drag force remains initially somewhat larger than the value in the case of constant velocity M ! 1 because it takes some time to get rid of the wake that was generated during its supersonic history.