Dynamical Friction in Nonlocal Gravity

We study dynamical friction in the Newtonian regime of nonlocal gravity (NLG), which is a classical nonlocal generalization of Einstein’s theory of gravitation. The nonlocal aspect of NLG simulates dark matter. The attributes of the resulting effective dark matter are described and the main physical predictions of NLG, which has a characteristic length scale of order 1 kpc, for galactic dynamics are presented. Within the framework of NLG, we derive the analog of Chandrasekhar’s formula for dynamical friction. The astrophysical implications of the results for the apparent rotation of a central bar subject to dynamical friction in a barred spiral galaxy are briefly discussed.

Interrelations Between Astrochemistry and Galactic Dynamics

This paper presents a review of ideas that interconnect astrochemistry and galactic dynamics. Since these two areas are vast and not recent, each one has already been covered separately by several reviews. After a general historical introduction, and a needed quick review of processes such as stellar nucleosynthesis that gives the base to understand the interstellar formation of simple chemical compounds (e.g., H2, CO, NH3, and H2O), we focus on a number of topics that are at the crossing of the two big areas, dynamics and astrochemistry. Astrochemistry is a flourishing field that intends to study the presence and formation of molecules as well as the influence of them on the structure, evolution, and dynamics of astronomical objects. The progress in the knowledge on the existence of new complex molecules and of their process of formation originates from the observational, experimental, and theoretical areas that compose the field. The interfacing areas include star formation, protoplanetary disks, the role of the spiral arms, and the chemical abundance gradients in the galactic disk. It often happens that the physical conditions in some regions of the interstellar medium are only revealed by means of molecular observations. To organize a rough classification of chemical evolution processes, we discuss about how astrochemistry can act in three different contexts, namely, the chemistry of the early universe, including external galaxies, star-forming regions, and asymptotic giant branch (AGB) stars and circumstellar envelopes. We mention that our research is stimulated by plans for instruments and projects, such as the ongoing Large Latin American Millimeter Array (LLAMA), which consists in the construction of a 12 m sub-mm radio telescope in the Andes. Thus, modern and new facilities can play a key role in new discoveries not only in astrochemistry but also in radio astronomy and related areas. Furthermore, the research on the origin of life is also a stimulating perspective.

A second look at the Kurth solution in galactic dynamics

<p style='text-indent:20px;'>The Kurth solution is a particular non-isotropic steady state solution to the gravitational Vlasov-Poisson system. It has the property that by means of a suitable time-dependent transformation it can be turned into a family of time-dependent solutions. Therefore, for a general steady state <inline-formula><tex-math id="M1">\begin{document}$ Q(x, v) = \tilde{Q}(e_Q, \beta) $\end{document}</tex-math></inline-formula>, depending upon the particle energy <inline-formula><tex-math id="M2">\begin{document}$ e_Q $\end{document}</tex-math></inline-formula> and <inline-formula><tex-math id="M3">\begin{document}$ \beta = \ell^2 = |x\wedge v|^2 $\end{document}</tex-math></inline-formula>, the question arises if solutions <inline-formula><tex-math id="M4">\begin{document}$ f $\end{document}</tex-math></inline-formula> could be generated that are of the form</p><p style='text-indent:20px;'><disp-formula> <label/> <tex-math id="FE1"> \begin{document}$ f(t) = \tilde{Q}\Big(e_Q(R(t), P(t), B(t)), B(t)\Big) $\end{document} </tex-math></disp-formula></p><p style='text-indent:20px;'>for suitable functions <inline-formula><tex-math id="M5">\begin{document}$ R $\end{document}</tex-math></inline-formula>, <inline-formula><tex-math id="M6">\begin{document}$ P $\end{document}</tex-math></inline-formula> and <inline-formula><tex-math id="M7">\begin{document}$ B $\end{document}</tex-math></inline-formula>, all depending on <inline-formula><tex-math id="M8">\begin{document}$ (t, r, p_r, \beta) $\end{document}</tex-math></inline-formula> for <inline-formula><tex-math id="M9">\begin{document}$ r = |x| $\end{document}</tex-math></inline-formula> and <inline-formula><tex-math id="M10">\begin{document}$ p_r = \frac{x\cdot v}{|x|} $\end{document}</tex-math></inline-formula>. We are going to show that, under some mild assumptions, basically if <inline-formula><tex-math id="M11">\begin{document}$ R $\end{document}</tex-math></inline-formula> and <inline-formula><tex-math id="M12">\begin{document}$ P $\end{document}</tex-math></inline-formula> are independent of <inline-formula><tex-math id="M13">\begin{document}$ \beta $\end{document}</tex-math></inline-formula>, and if <inline-formula><tex-math id="M14">\begin{document}$ B = \beta $\end{document}</tex-math></inline-formula> is constant, then <inline-formula><tex-math id="M15">\begin{document}$ Q $\end{document}</tex-math></inline-formula> already has to be the Kurth solution.</p><p style='text-indent:20px;'>This paper is dedicated to the memory of Professor Robert Glassey.</p>

The history of dynamics and stellar feedback revealed by the H I filamentary structure in the disk of the Milky Way

We present a study of the filamentary structure in the emission from the neutral atomic hydrogen (HI) at 21 cm across velocity channels in the 40′′ and 1.5-km s−1 resolution position-position-velocity cube, resulting from the combination of the single-dish and interferometric observations in The HI/OH/recombination-line survey of the inner Milky Way. Using the Hessian matrix method in combination with tools from circular statistics, we find that the majority of the filamentary structures in the HI emission are aligned with the Galactic plane. Part of this trend can be assigned to long filamentary structures that are coherent across several velocity channels. However, we also find ranges of Galactic longitude and radial velocity where the HI filamentary structures are preferentially oriented perpendicular to the Galactic plane. These are located (i) around the tangent point of the Scutum spiral arm and the terminal velocities of the Molecular Ring, around l ≈ 28° and vLSR ≈ 100 km s−1, (ii) toward l ≈ 45° and vLSR ≈ 50 km s−1, (iii) around the Riegel-Crutcher cloud, and (iv) toward the positive and negative terminal velocities. A comparison with numerical simulations indicates that the prevalence of horizontal filamentary structures is most likely the result of large-scale Galactic dynamics and that vertical structures identified in (i) and (ii) may arise from the combined effect of supernova (SN) feedback and strong magnetic fields. The vertical filamentary structures in (iv) can be related to the presence of clouds from extra-planar HI gas falling back into the Galactic plane after being expelled by SNe. Our results indicate that a systematic characterization of the emission morphology toward the Galactic plane provides an unexplored link between the observations and the dynamical behavior of the interstellar medium, from the effect of large-scale Galactic dynamics to the Galactic fountains driven by SNe.

Debate on the Physics of Galactic Rotation and the Existence of Dark Matter

This Special Issue was motivated by the disparate explanations of galactic dynamics promulgated by different philosophical camps [...]

Simplification of Galactic Dynamic equations

Most fully developed galaxies have vivid spiral structure, but the formation and evolution of spiral structure is still a mystery that is not fully understood in astrophysics. We find that the currently used equations of galactic dynamics contain some unreasonable components. In this paper, the following three working assumptions are introduced to simplify the galactic structural equations. 1. In the research of large-scale structure, the retarded potential of the gravitational field should be taken into account. The propagating time of the gravitational field from center to border is longer than the revolution periods of the stars near the center of galaxy. Newton's gravitational potential is unreasonable for such case, and the weak field and low velocity approximation of Einstein's field equation should be adopted. 2. The stars in a fully developed galaxy should be zero-pressure and inviscid fluid, and the equation of motion is different from that of ordinary continuum mechanics. Stars move along geodesics. 3. The structure of the galaxy is only related to the total mass density distribution. The equation of state of dark halo is different from that of ordinary luminous interstellar matter, so their trajectories are also very different. Dark halo and ordinary matter in galaxy are automatically separated. The total mass density distribution can be presupposed according to the observation data, and then it can be determined by comparing the solution of the equations with the observed data. These assumptions and treatments are supported by theory and observation. The variables of the equations of simplified galactic dynamics are separated from each other, and the equations are well-posed and can be solved according to a definite procedure. Therefore, this simplified dynamic equation system provides a more reasonable and practical framework for the further study of galactic structure, and can solve many practical problems. Besides, it is closely related to the study of dark energy and dark matter.