Buildup Of Angular Momentum Research Articles

Spin current driven by ultrafast magnetization of FeRh

Laser-induced ultrafast demagnetization is an important phenomenon that probes arguably the ultimate limits of the angular momentum dynamics in solid. Unfortunately, many aspects of the dynamics remain unclear except that the demagnetization transfers the angular momentum eventually to the lattice. In particular, the role and origin of electron-carried spin currents in the demagnetization process are debated. Here we experimentally probe the spin current in the opposite phenomenon, i.e., laser-induced ultrafast magnetization of FeRh, where the laser pump pulse initiates the angular momentum build-up rather than its dissipation. Using the time-resolved magneto-optical Kerr effect, we directly measure the ultrafast-magnetization-driven spin current in a FeRh/Cu heterostructure. A strong correlation between the spin current and the magnetization dynamics of FeRh is found even though the spin filter effect is negligible in this opposite process. This result implies that the angular momentum build-up is achieved by an angular momentum transfer from the electron bath (supplier) to the magnon bath (receiver) and followed by the spatial transport of angular momentum (spin current) and dissipation of angular momentum to the phonon bath (spin relaxation).

Multiple gas acquisition events in galaxies with dual misaligned gas disks

Frequent accretion of external cold gas is thought to play an important role in galaxy assembly. However, almost all known kinematically misaligned galaxies identify only one gas disk that is misaligned with the stellar disk, implying a single gas acquisition event. Here we report a new configuration in two galaxies where both contain two gas disks misaligned with each other and also with the stellar disk. Such systems are not expected to be stable or long-lasting, challenging the traditional picture of gas accretion of galaxies and their angular momentum build-up. The differences in kinematic position angles are larger than 120{\deg} between the two gas disks, and 40{\deg} between each gas disk and the stellar component. The star formation activity is enhanced at the interface of the two gas disks compared with the other regions within the same galaxy. Such systems illustrate that low-redshift galaxies can still experience multiple gas acquisition events, and provide a new view into the origins of galactic gas.

Unveiling the faint ultraviolet Universe

With this paper we participate to the call for ideas issued by the European Space Agency to define the Science Program and plan for space missions from 2035 to 2050. In particular we present five science cases where major advancements can be achieved thanks to space-based spectroscopic observations at ultraviolet (UV) wavelengths. We discuss the possibility to (1) unveil the large-scale structures and cosmic web in emission at redshift lesssim 1.7; (2) study the exchange of baryons between galaxies and their surroundings to understand the contribution of the circumgalactic gas to the evolution and angular-momentum build-up of galaxies; (3) constrain the efficiency of ram-pressure stripping in removing gas from galaxies and its role in quenching star formation; (4) characterize the progenitor population of core-collapse supernovae to reveal the explosion mechanisms of stars; (5) target accreting white dwarfs in globular clusters to determine their evolution and fate. These science themes can be addressed thanks to UV (wavelength range lambda sim 90 - 350 nm) observations carried out with a panoramic integral field spectrograph (field of view sim !1 times 1 arcmin2), and medium spectral (R = 4000) and spatial (sim !1^{prime prime } - 3^{prime prime }) resolution. Such a UV-optimized instrument will be unique in the coming years, when most of the new large facilities such as the Extremely Large Telescope and the James Webb Space Telescope are optimized for infrared wavelengths.

The SAMI Galaxy Survey: first detection of a transition in spin orientation with respect to cosmic filaments in the stellar kinematics of galaxies

ABSTRACT We present the first detection of mass-dependent galactic spin alignments with local cosmic filaments with >2σ confidence using IFS kinematics. The 3D network of cosmic filaments is reconstructed on Mpc scales across GAlaxy and Mass Assembly fields using the cosmic web extractor DisPerSe. We assign field galaxies from the SAMI survey to their nearest filament segment in 3D and estimate the degree of alignment between SAMI galaxies’ kinematic spin axis and their nearest filament in projection. Low-mass galaxies align their spin with their nearest filament while higher mass counterparts are more likely to display an orthogonal orientation. The stellar transition mass from the first trend to the second is bracketed between $10^{10.4}$ and $10^{10.9}\, \mathrm{ M}_{\odot }$, with hints of an increase with filament scale. Consistent signals are found in the Horizon-AGN cosmological hydrodynamic simulation. This supports a scenario of early angular momentum build-up in vorticity rich quadrants around filaments at low stellar mass followed by progressive flip of spins orthogonal to the cosmic filaments through mergers at high stellar mass. Conversely, we show that dark matter only simulations post-processed with a semi-analytical model treatment of galaxy formation struggles to reproduce this alignment signal. This suggests that gas physics is key in enhancing the galaxy-filament alignment.

On the Terminal Rotation Rates of Giant Planets

Abstract Within the general framework of the core-nucleated accretion theory of giant planet formation, the conglomeration of massive gaseous envelopes is facilitated by a transient period of rapid accumulation of nebular material. While the concurrent build-up of angular momentum is expected to leave newly formed planets spinning at near-breakup velocities, Jupiter and Saturn, as well as super-Jovian long-period extrasolar planets, are observed to rotate well below criticality. In this work, we demonstrate that the large luminosity of a young giant planet simultaneously leads to the generation of a strong planetary magnetic field, as well as thermal ionization of the circumplanetary disk. The ensuing magnetic coupling between the planetary interior and the quasi-Keplerian motion of the disk results in efficient braking of planetary rotation, with hydrodynamic circulation of gas within the Hill sphere playing the key role of expelling spin angular momentum to the circumstellar nebula. Our results place early-stage giant planet and stellar rotation within the same evolutionary framework, and motivate further exploration of magnetohydrodynamic phenomena in the context of the final stages of giant planet formation.

Angular-Momentum Management of Electromagnetic Formation Flight Using Alternating Magnetic Fields

An electromagnetic formation flight uses electromagnetic force to control the relative position of satellites and enables formation flying without consuming fuel. When an electromagnetic field is used, an electromagnetic torque is created that acts on the satellites, resulting in the buildup of angular momentum via reaction wheels or control moment gyroscopes. This paper presents a new angular-momentum management algorithm to solve the angular-momentum buildup problem using the Earth’s magnetic field. The proposed method is characterized by combining an alternating electromagnetic field with a constant electromagnetic field. The alternating component is applied to regulate the relative positions of the satellites in the formation, whereas the constant component interacts with the geomagnetic field to reduce excessive angular momentum. With this method, angular-momentum management is decoupled from position control, and several restrictions are removed. A feedback-filter method is proposed to address the high uncertainty caused by the alternating electromagnetic field, and a sliding-mode controller is used to track the trajectory of both translational and attitude movements. The simulation results presented in this study demonstrate the validity and performance of the proposed strategy.

Dipole solution and angular-momentum minimization for two-satellite electromagnetic formation flight

Electromagnetic formation flight is very attractive for no fuel expenditure, and may become an alternative to traditional propellant-based spacecraft formation flying. When electromagnetic forces are used to determine relative satellite positions, electromagnetic torques are created simultaneously and result in angular-momentum buildup. This paper investigates dipole computation and angular-momentum minimization for a two-satellite formation. Instead of using dipole vectors, a group of parameters are introduced to represent the relation between the electromagnetic force and torque. These parameters enable a certain freedom of allocating electromagnetic torques, and help find an analytical dipole solution to minimize total electromagnetic torque action. It is shown that an electromagnetic force is unconstrained only if associated torque ratios ranges from 0.5 to 2. The impact of formation configuration and control is also investigated. Torque-free formations are obtained where electromagnetic torques can be simply removed. A parameter optimization model is derived under the framework of sliding mode control to minimize angular-momentum buildup. Simulation results demonstrate the effect of the proposed angular-momentum minimization method.

Angular momentum transfer to a Milky Way disc at high redshift

An Adaptive Mesh Refinement cosmological resimulation is analyzed in order to test whether filamentary flows of cold gas are responsible for the build-up of angular momentum within a Milky Way like disk at z>=3. A set of algorithms is presented that takes advantage of the high spatial resolution of the simulation (12 pc) to identify: (i) the central gas disk and its plane of orientation; (ii) the complex individual filament trajectories that connect to the disk, and; (iii) the infalling satellites. The results show that two filaments at z>5.5, which later merge to form a single filament at z<4, drive the angular momentum and mass budget of the disk throughout its evolution, whereas luminous satellite mergers make negligible fractional contributions. Combined with the ubiquitous presence of such filaments in all large-scale cosmological simulations that include hydrodynamics, these findings provide strong quantitative evidence that the growth of thin disks in haloes with masses below 10^{12} M_{sun}, which host the vast majority of galaxies, is supported via inflowing streams of cold gas at intermediate and high redshifts.

Four phases of angular-momentum buildup in high-z galaxies: from cosmic-web streams through an extended ring to disc and bulge

We study the angular-momentum (AM) buildup in high-$z$ massive galaxies using high-resolution cosmological simulations. The AM originates in co-planar streams of cold gas and merging galaxies tracing cosmic-web filaments, and it undergoes four phases of evolution. (I) Outside the halo virial radius ($R_{\rm v}\!\sim\!100\,{\rm kpc}$), the elongated streams gain AM by tidal torques with a specific AM (sAM) $\sim\!1.7$ times the dark-matter (DM) spin due to the gas' higher quadrupole moment. This AM is expressed as stream impact parameters, from $\sim\!0.3R_{\rm v}$ to counter rotation. (II) In the outer halo, while the incoming DM mixes with the existing halo of lower sAM to a spin $\lambda_{\rm dm}\!\sim\!0.04$, the cold streams transport the AM to the inner halo such that their spin in the halo is $\sim\!3\lambda_{\rm dm}$. (III) Near pericenter, the streams dissipate into an irregular rotating ring extending to $\sim\!0.3R_{\rm v}$ and tilted relative to the inner disc. Torques exerted partly by the disc make the ring gas lose AM, spiral in, and settle into the disc within one orbit. The ring is observable with 30\% probability as a damped Lyman-$\alpha$ absorber. (IV) Within the disc, $<\!0.1R_{\rm v}$, torques associated with violent disc instability drive AM out and baryons into a central bulge, while outflows remove low-spin gas, introducing certain sensitivity to feedback strength. Despite the different AM histories of gas and DM, the disc spin is comparable to the DM-halo spin. Counter rotation can strongly affect disc evolution.

Adaptive Terminal Sliding Mode Control of Electromagnetic Spacecraft Formation Flying in Near-Earth Orbits

An adaptive terminal sliding mode control for six-degree-of-freedom electromagnetic spacecraft formation flying (EMFF) in near-Earth orbits is presented. By using terminal sliding mode (TSM) technique, the output tracking error can converge to zero in finite time, and strong robustness with respect to disturbance forces can be guaranteed. Based on a rotated frame F r and the adaptive TSM controller, the special magnetic moment of the steerable magnetic dipole is computed. The angular momentum management strategy (AMM) is implemented in a periodically switching fashion, by which the angular momentum buildup was limited. Illustrative simulations of EMFF are conducted to verify the effectiveness of the proposed controller.

The angular momentum of disc galaxies: implications for gas accretion, outflows, and dynamical friction

We combine constraints on the galaxy-dark matter connection with structural and dynamical scaling relations to investigate the angular momentum content of disc galaxies. For haloes with masses in the interval 10^{11.3} < M_vir/M_sun < 10^{12.7} we find that the galaxy spin parameters are independent of halo mass with <\lambda'_gal> = (J_gal/M_gal) / (\sqrt{2} R_vir V_vir) = 0.019^{+0.004}_{-0.003} (1sigma). This is significantly lower than for relaxed LCDM haloes, which have an average spin parameter <\lambda'_halo> = 0.031. The average ratio between the specific angular momentum of disk galaxies and their host dark matter haloes is therefore R_j = \lambda'_gal/\lambda'_halo = 0.61^{+0.13}_{-0.10}. This calls into question a standard assumption made in the majority of all (semi-analytical) models for (disc) galaxy formation, namely that R_j=1. Using simple disc formation models we show that it is particularly challenging to understand why R_j is independent of halo mass, while the galaxy formation efficiency (\epsilon_GF, proportional to the ratio of galaxy mass to halo mass) reveals a strong halo mass dependence. We argue that the empirical scaling relations between \epsilon_GF, R_j and halo mass require both feedback (i.e., galactic outflows) and angular momentum transfer from the baryons to the dark matter (i.e., dynamical friction). The efficiency of angular momentum loss need to decrease with increasing halo mass. Such a mass dependence may reflect a bias against forming stable discs in high mass, low spin haloes or a transition from cold-mode accretion in low mass haloes to hot-mode accretion at the massive end. However, current hydrodynamical simulations of galaxy formation, which should include these processes, seem unable to reproduce the empirical relation between \epsilon_GF and R_j. We conclude that the angular momentum build-up of galactic discs remains poorly understood.

The Rotational Excitation Temperature of the λ6614 Diffuse Interstellar Band Carrier

Analysis of high spectral resolution observations of the λ6614 diffuse interstellar band (DIB) line profile shows systematic variations in the positions of the peaks in the substructure of the profile. These variations—shown here for the first time—can be understood most naturally in the framework of rotational contours of large molecules, where the variations are caused by changes in the rotational excitation temperature. We show that the rotational excitation temperature for the DIB carrier is likely significantly lower than the gas kinetic temperature—indicating that for this particular DIB carrier angular momentum buildup is not very efficient.

The Origin of Angular Momentum in Dark Matter Halos

We propose a new explanation for the origin of angular momentum in galaxies and their dark halos, in which the halos obtain their spin through the cumulative acquisition of angular momentum from satellite accretion. In our model, the buildup of angular momentum is a random walk process associated with the mass assembly history of the halo's major progenitor. We assume no correlation between the angular momenta of accreted objects. The main role of tidal torques in this approach is to produce the random tangential velocities of merging satellites. Using the extended Press-Schechter approximation, we calculate the growth of mass, angular momentum, and spin parameter λ for many halos. Our random walk model reproduces the key features of the angular momentum of halos found in ΛCDM N-body simulations: a lognormal distribution in λ with an average of ≈ 0.045 and dispersion σλ = 0.56, independent of mass and redshift. The evolution of the spin parameter in individual halos in this model is quite different from the steady increase with time of angular momentum in the tidal torque picture. We find both in N-body simulations and in our random walk model that the value of λ changes significantly with time for a halo's major progenitor. It typically has a sharp increase due to major mergers and a steady decline during periods of gradual accretion of small satellites. The model predicts that, on average, the λ of ~1012 M☉ halos that had major mergers after redshift z = 3 should be substantially larger than the λ of those that did not. Perhaps surprisingly, this suggests that halos that host later forming elliptical galaxies should rotate faster than halos of spiral galaxies.

Towards a resolution of the galactic spin crisis: mergers, feedback and spin segregation

We model in simple terms the angular momentum problems of galaxy formation in cold dark matter cosmologies, and identify the key elements of a scenario that may solve them. The buildup of angular momentum is modelled viadynamical friction and tidal stripping in a sequence of mergers. We demonstrate how overcooling in incoming haloes leads to a transfer of angular momentum from the baryons to the dark matter, in conflict with observations. By incorporating a simple recipe of supernova feedback, we are able to solve the problems of angular momentum in disc formation. Gas removal from the numerous small incoming haloes, which merge to become the low specific angular momentum (low-j) component of the product, eliminates the low-j baryons. Heating and puffing-up of the gas in larger incoming haloes, combined with efficient tidal stripping, reduces the angular momentum loss of baryons due to dynamical friction. Dependence of the feedback effects on the progenitor halo mass implies that the spin of baryons is typically higher for lower-mass haloes. The observed low baryonic fraction in dwarf galaxies is used to calibrate the characteristic velocity associated with supernova feedback, yielding V f b ∼100 km s - 1 , within the range of theoretical expectations. We then find that the model naturally produces the observed distribution of the spin parameter among dwarf and bright disc galaxies, as well as the j profile inside these galaxies. This suggests that the model indeed captures the main features of a full scenario for resolving the spin crisis.

N-body simulation of large-scale structures in Lambda not = 0 Friedmann models

view Abstract Citations (31) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS N-Body Simulation of Large-Scale Structures in Lambda not = 0 Friedmann Models Martel, Hugo Abstract This paper presents a study of the formation and evolution of large- scale structures in Friedmann models with a positive cosmological constant λ. The goal is to find out if current observations either support or rule out the existence of λ and to suggest observational tests that could answer the question. A series of 18 N-body simulations were carried out, for four different Friedmann models: {OMEGA}_0_ = 1, λ_0_ = 0 (four calculations), {OMEGA} = 0.2, λ_0_ = 0 (five calculations), {OMEGA}_0_ = 0.2, λ_0_ = 0.8 (five calculations), and {OMEGA}_0_ = 0.2, H_0_t_0_ = 2 (four calculations). All simulations were done using a P^3^M algorithm with 32,768 particles. The initial conditions were set up in order to reproduce the linear growing mode of a standard cold dark matter fluctuation spectrum. Biased galaxy formation is implemented into the algorithm using an original technique based on physical considerations on the density and density contrast around particles. In all calculations, galaxy clustering proceeds rapidly, leading to the formation of massive clusters connected by bridges and surrounding vast underdense regions. Several aspects of the final states are compared among the various models, with observations, and with results of simulations carried out by previous authors. The final positions of the particles are very similar in all models, and consequently all results based on these positions only [morphology and topology of the final configurations, two-point correlation function ξ(r) and angular two-point correlation function w(θ)] are essentially independent of the cosmological constant. Since the peculiar velocity field varies among the various models, results involving the final velocities of the particles might show a dependence on {OMEGA}. However, the moments of the relative peculiar velocity between pairs turn out to depend upon {OMEGA}_0_, but not λ_0_. Similarly, the galaxy distribution in "slices," and the number counts in redshift space, which both depend upon the Hubble distance of galaxies to the observer, show very little dependence on {OMEGA}. The only results which are sensitive to the presence of the cosmological constant are the rate of galaxy formation, and the buildup of angular momentum in clusters by merging events. The former result imposes a strong constraint on any phenomenological biasing scheme: the presence of a large cosmological constant inhibits galaxy formation by increasing significantly the minimum density required for a clump to collapse. The later result suggests an observational test: the search for clusters of galaxies with low net angular momentum. Publication: The Astrophysical Journal Pub Date: January 1991 DOI: 10.1086/169570 Bibcode: 1991ApJ...366..353M Keywords: Astronomical Models; Computational Astrophysics; Cosmology; Galactic Evolution; Many Body Problem; Cluster Analysis; Dark Matter; Galactic Clusters; Mathematical Models; Universe; Astrophysics; COSMOLOGY; DARK MATTER; EARLY UNIVERSE; GALAXIES: CLUSTERING; NUMERICAL METHODS full text sources ADS |

MAGNETIC TORQUING SCHEME

T note presents a simple scheme to use the earth's magnetic field as an angular momentum reversal device in the attitude control of satellites. The basic control concept involved is the use of momentum storage devices supplemented on a discontinuous basis by magnetic torques for desaturation purposes. Reaction wheels are used to attitude control the vehicle by storing angular momentum, and the magnetic torques are used to remove angular momentum from the system. The system presented here performs only logical (not arithmetic) computations based upon angular momentum and magnetic field measurements and, therefore, is a simplification over the control techniques discussed in Refs. 2 and 3. This magnetic momentum removal system requires only a simple threshold measurement of both the reaction wheel speeds and the body axis components of the earth's magnetic field. These signals are combined using some simple logical operations to produce constant control moments. The concept discussed below is for three-axis attitude stabilization as opposed to those which control a spin stabilized vehicle. A block diagram of the attitude control system under consideration is shown in Fig. 1. The block diagram shows the primary control loop, consisting of attitude sensors, compensation, and reaction wheels to control the body rotational dynamics, and the magnetic torquing system, consisting of magnetometers, logic, and magnetic moment generators. Because of disturbance torques acting on the orbiting satellite, the reaction wheel angular momentum storage will be fluctuating periodically with a component of secular angular momentum buildup. The function of the magnetic system is to remove this secular component of angular momentum. A magnetic dipole moment in a magnetic field experiences a torque given by the cross product of the magnetic moment and the magnetic field vector. Written out in body coordinates, this relationship becomes