Galaxies in Milgromian Dynamics

Since its first formulation in 1983, Milgromian dynamics (also known as modified Newtonian dynamics or MOND) has been very successful in predicting the gravitational potential of galaxies from the distribution of baryons alone, including general scaling relations and detailed rotation curves of large statistical samples of individual galaxies covering a large range of masses and sizes. To date, with only very few Milgromian N-body codes and simulations available, only little is actually known about the time-evolution of dynamical objects within that framework, and generic, fully dynamical tests using N-body codes with live particles, high spatial resolution, and full treatment of hydrodynamics are still completely missing. In the course of this thesis, I developed such a code, allowing to simulate generically the formation and evolution of galaxies in the Milgromian framework for the first time. The code is based on the hydrodynamics and N-body code RAMSES, and achieves exceptionally high spatial and temporal resolutions thanks to the adaptive mesh refinement technique. Three research projects emerged during this development: (i) Polar ring galaxies are modelled and compared to observational data. The qualitative and quantitative analysis of those models shows that the derived Milgromian potential accounts well for their dynamical properties, whereby this is not possible in the dark matter framework unless extreme fine tuning is assumed. (ii) The dynamical properties of the five most massive Milky Way dwarf spheroidal satellites expected in Milgrom's modified gravity framework are investigated and compared to available observational data, taking into account self-consistently the external field of the Milky Way, which strongly affects the satellites' internal dynamics. This external field effect provides further, very stringent tests on this theoretical framework, and it turns out to resolve the "dark matter `core vs. cusp' problem" naturally in the tested objects. (iii) The final N-body code is presented and explained. Test results are shown, and sample applications are provided (spherical equilibrium models, exponential disk galaxies, with and without initial bulge), which constitute the basis of future projects with the new Milgromian dynamics simulation code, as is evident from the significant interest this code has been receiving

Polar ring galaxies as tests of gravity

Polar ring galaxies are ideal objects with which to study the three-dimensional shapes of galactic gravitational potentials since two rotation curves can be measured in two perpendicular planes. Observational studies have uncovered systematically larger rotation velocities in the extended polar rings than in the associated host galaxies. In the dark matter context, this can only be explained through dark halos that are systematically flattened along the polar rings. Here, we point out that these objects can also be used as very effective tests of gravity theories, such as those based on Milgromian dynamics (MOND). We run a set of polar ring models using both Milgromian and Newtonian dynamics to predict the expected shapes of the rotation curves in both planes, varying the total mass of the system, the mass of the ring with respect to the host, as well as the size of the hole at the center of the ring. We find that Milgromian dynamics not only naturally leads to rotation velocities being typically higher in the extended polar rings than in the hosts, as would be the case in Newtonian dynamics without dark matter, but that it also gets the shape and amplitude of velocities correct. Milgromian dynamics thus adequately explains this particular property of polar ring galaxies.Comment: 9 pages, 8 Figures, 1 Table, Accepted for publication by MNRA

PENGARUH PENAMBAHAN JUMLAH WAJIB PAJAK MELALUI PEMBERI KERJA TERHADAP PENERIMAAN PAJAK DI KANTOR PELAYANAN PAJAK PRATAMA KARANGANYAR

Context: Local Group (LG) timing is one of the first historical probes of the so-called missing mass problem. Whilst modern cosmological probes indicate that pure baryonic dynamics is not sufficient on the largest scales, nearby galaxies and small galaxy groups persistently obey Milgrom’s MOND law, which implies that dynamics at small scales is possibly entirely predicted by the baryons. Aims: Here, we investigate LG timing in this context of Milgromian dynamics. Methods: We used the latest measured proper motions and radial velocities for Andromeda and the Magellanic clouds, and we integrated their orbits backwards by making use of the Milgromian two-body equation of motion. Results: With the currently measured proper motions and radial velocity of M31, we find that MOND would imply that the Milky Way (MW) and M31 first moved apart via Hubble expansion after birth, but then necessarily were attracted again by the Milgromian gravitational attraction, and had a past fly-by encounter before coming to their present positions. This encounter would most probably have happened 7 to 11 Gyr ago (0.8 < z < 3). The absence of a dark matter halo and its associated dynamical friction is necessary for such a close encounter not to have triggered a merger. Observational arguments which could exclude or favour such a past encounter would thus be very important in order to falsify or vindicate Milgromian dynamics on the scale of the LG. Interestingly, the closest approach of the encounter is small enough (<55 kpc) to have had severe consequences on the disc dynamics, including perhaps thick disc formation, and on the satellite systems of both galaxies. The ages of the satellite galaxies and of the young halo globular clusters, all of which form the vast polar structure around the MW, are consistent with these objects having been born during this encounter.Publisher PDFPeer reviewe

The mass function and dynamical mass of young star clusters: Why their initial crossing-time matters crucially

We highlight the impact of cluster-mass-dependent evolutionary rates upon the evolution of the cluster mass function during violent relaxation, that is, while clusters dynamically respond to the expulsion of their residual star-forming gas. Mass-dependent evolutionary rates arise when the mean volume density of cluster-forming regions is mass-dependent. In that case, even if the initial conditions are such that the cluster mass function at the end of violent relaxation has the same shape as the embedded-cluster mass function (i.e. infant weight-loss is mass-independent), the shape of the cluster mass function does change transiently {\it during} violent relaxation. In contrast, for cluster-forming regions of constant mean volume density, the cluster mass function shape is preserved all through violent relaxation since all clusters then evolve at the same mass-independent rate. On the scale of individual clusters, we model the evolution of the ratio between the dynamical mass and luminous mass of a cluster after gas expulsion. Specifically, we map the radial dependence of the time-scale for a star cluster to return to equilibrium. We stress that fields-of-view a few pc in size only, typical of compact clusters with rapid evolutionary rates, are likely to reveal cluster regions which have returned to equilibrium even if the cluster experienced a major gas expulsion episode a few Myr earlier. We provide models with the aperture and time expressed in units of the initial half-mass radius and initial crossing-time, respectively, so that our results can be applied to clusters with initial densities, sizes, and apertures different from ours.Comment: 14 pages, 10 figures, accepted for publication in MNRA

Comment on "Evidence for dark matter in the inner Milky Way"

This is a brief rebuttal to arXiv:1502.03821, which claims to provide the first observational proof of dark matter interior to the solar circle. We point out that this result is not new, and can be traced back at least a quarter century.Comment: 3 pages, 2 figures. In this version we add a figure from a 1998 paper that shows the same result that arXiv:1502.03821 claims to be novel. We also add a short note rebutting arXiv:1503.08784 which was written in response to the first versio

Agiles Arbeiten im strategischen Handlungsfeld Lehrentwicklung an der Hochschule Niederrhein

„Agilität“ ist ein für Unternehmen bekannter, in Hochschulen neuer Ansatz der kollaborativen Zusammenarbeit bei der zeitkritischen Entwicklung von Projekten. Der Werkstattbericht reflektiert das Zusammenfinden verschiedener lateral agierender Arbeitsbereiche zu einem „Team Lehrentwicklung“ und die Integration der jeweiligen Perspektiven für die Gestaltung des strategischen Hochschulentwicklungsplans in einem agilen Sprint. Die für diesen Zweck angewandten agilen Methoden beförderten nachhaltig die Zusammenarbeit der Akteur*innen und die Qualität der entwickelten Gestaltungsprinzipien im disruptiven Szenario des digitalen Semesters, das durch die Covid-19-Pandemie ausgelöst wurde

Beyond ΛCDM:problems, solutions, and the road ahead

97 pages, 8 figuresInternational audienceDespite its continued observational successes, there is a persistent (and growing) interest in extending cosmology beyond the standard model, $\Lambda$CDM. This is motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm. In this paper, we summarize the current status of $\Lambda$CDM as a physical theory, and review investigations into possible alternatives along a number of different lines, with a particular focus on highlighting the most promising directions. While the fundamental problems are proving reluctant to yield, the study of alternative cosmologies has led to considerable progress, with much more to come if hopes about forthcoming high-precision observations and new theoretical ideas are fulfilled