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 Physics , 2010, DOI: 10.1088/0004-637X/730/2/109 Abstract: It has been believed that spirals in pure stellar disks, especially the ones spontaneously formed, decay in several galactic rotations due to the increase of stellar velocity dispersions. Therefore, some cooling mechanism, for example dissipational effects of the interstellar medium, was assumed to be necessary to keep the spiral arms. Here we show that stellar disks can maintain spiral features for several tens of rotations without the help of cooling, using a series of high-resolution three-dimensional $N$-body simulations of pure stellar disks. We found that if the number of particles is sufficiently large, e.g., $3\times 10^6$, multi-arm spirals developed in an isolated disk can survive for more than 10 Gyrs. We confirmed that there is a self-regulating mechanism that maintains the amplitude of the spiral arms. Spiral arms increase Toomre's $Q$ of the disk, and the heating rate correlates with the squared amplitude of the spirals. Since the amplitude itself is limited by the value of $Q$, this makes the dynamical heating less effective in the later phase of evolution. A simple analytical argument suggests that the heating is caused by gravitational scattering of stars by spiral arms, and that the self-regulating mechanism in pure-stellar disks can effectively maintain spiral arms on a cosmological timescale. In the case of a smaller number of particles, e.g., $3\times 10^5$, spiral arms grow faster in the beginning of the simulation (while $Q$ is small) and they cause a rapid increase of $Q$. As a result, the spiral arms become faint in several Gyrs.
 Physics , 2000, Abstract: Two dimensional Fourier spectra of near-infrared images of galaxies provide a powerful diagnostic tool for the detection of spiral arm modulation in stellar disks. Spiral arm modulation may be understood in terms of interference patterns of outgoing and incoming density wave packets or modes. The brightness along a spiral arm will be increased where two wave crests meet and constructively interfere, but will be decreased where a wave crest and a wave trough destructively interfere. Spiral arm modulation has hitherto only been detected in grand design spirals (such as Messier 81). Spiral arm amplitude variations have the potential to become a powerful constraint for the study of galactic dynamics. We illustrate our method in two galaxies: NGC 4062 and NGC 5248. In both cases, we have detected trailing and leading m=2 waves with similar pitch angles. This suggests that the amplification mechanism is the WASER type II. In this mechanism, the bulge region reflects (rather than refracts) incoming waves with no change of pitch angle, but only a change of their sense of winding. The ratio between the amplitudes of the leading and the trailing waves is about 0.5 in both cases, wherein the higher amplitude is consistently assigned to the trailing (as opposed to leading) mode. The results are particularly significant because NGC 5248 is an optically grand design galaxy, whereas NGC 4062 is optically flocculent. NGC 4062 represents the very first detection of spiral arm modulation in the stellar disk of an optically flocculent galaxy.
 Physics , 2014, DOI: 10.1088/0004-637X/788/1/56 Abstract: Low resolution ESO VLT/FORS spectra of blue supergiant stars are analyzed to determine stellar metallicities (based on elements such as Fe, Ti, Mg) in the extended disk of the spiral galaxy NGC3621. Mildly subsolar metallicity (-0.30 dex) is found for the outer objects beyond 7 kpc independent of galactocentric radius and compatible with the absence of a metallicity gradient confirming the results of a recent investigation of interstellar medium HII region gas oxygen abundances. The stellar metallicities are slightly higher than those from the HII regions when based on measurements of the weak forbidden auroral oxygen line at 4363 AE but lower than the ones obtained with the R23 strong line method. It is shown that the present level of metallicity in the extended disk cannot be the result of chemical evolution over the age of the disk with the present rate of in situ star formation. Additional mechanisms must be involved. In addition to metallicity, stellar effective temperatures, gravities, interstellar reddening, and bolometric magnitudes are determined. After application of individual reddening corrections for each target the flux-weighted gravity-luminosity relationship of blue supergiant stars is used to obtain a distance modulus of 29.07+/-0.09 mag (distance D=6.52+/-0.28 Mpc). This new distance is discussed in relation to Cepheid and tip of the red giant branch distances.
 Physics , 2012, DOI: 10.1088/0004-637X/763/1/46 Abstract: In order to understand the physical mechanisms underlying non-steady stellar spiral arms in disk galaxies, we analyzed the growing and damping phases of their spiral arms using three-dimensional $N$-body simulations. We confirmed that the spiral arms are formed due to a swing amplification mechanism that reinforces density enhancement as a seeded wake. In the damping phase, the Coriolis force exerted on a portion of the arm surpasses the gravitational force that acts to shrink the portion. Consequently, the stars in the portion escape from the arm, and subsequently they form a new arm at a different location. The time-dependent nature of the spiral arms are originated in the continual repetition of this non-linear phenomenon. Since a spiral arm does not rigidly rotate, but follows the galactic differential rotation, the stars in the arm rotate at almost the same rate as the arm. In other words, every single position in the arm can be regarded as the co-rotation point. Due to interaction with their host arms, the energy and angular momentum of the stars change, thereby causing the radial migration of the stars. During this process, the kinetic energy of random motion (random energy) of the stars does not significantly increase, and the disk remains dynamically cold. Owing to this low degree of disk heating, the short-lived spiral arms can recurrently develop over many rotational periods. The resultant structure of the spiral arms in the $N$-body simulations is consistent with some observational nature of spiral galaxies. We conclude that the formation and structure of spiral arms in isolated disk galaxies can be reasonably understood by non-linear interactions between a spiral arm and its constituent stars.
 EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20121907009 Abstract: It has been believed that spiral arms in pure stellar disks decay in several galactic rotations due to the heating by the spiral arms. However, it might be caused by a numerical heating. We performed a three-dimensional N-body simulations with a sufficiently large number of particles and found that stellar disks can maintain spiral arms for more than 10 Gyr without the help of cooling. Spiral arms are transient and recurrent and they heat disk with a heating rate, dQ/dt, correlated to the spiral amplitude |Am|. On the other hand, |Am| is suppressed by Toomre’s Q. Therefore, the dynamical heating becomes less effective in the later phase of the evolution. This mechanism maintain the spiral arms for more than 10 Gyr.
 Physics , 2015, DOI: 10.1093/mnras/stv307 Abstract: We measure the stellar mass-star formation rate relation in star-forming disk galaxies at z<0.085, using Galaxy~Zoo morphologies to examine different populations of spirals as classified by their kiloparsec-scale structure. We examine the number of spiral arms, their relative pitch angle, and the presence of a galactic bar in the disk, and show that both the slope and dispersion of the M-SFR relation is constant when varying all the above parameters. We also show that mergers (both major and minor), which represent the strongest conditions for increases in star formation at a constant mass, only boost the SFR above the main relation by ~0.3 dex; this is significantly smaller than the increase seen in merging systems at z>1. Of the galaxies lying significantly above the M-SFR relation in the local Universe, more than 50% are mergers. We interpret this as evidence that the spiral arms, which are imperfect reflections of the galaxy's current gravitational potential, are either fully independent of the various quenching mechanisms or are completely overwhelmed by the combination of outflows and feedback. The arrangement of the star formation can be changed, but the system as a whole regulates itself even in the presence of strong dynamical forcing.
 Physics , 2009, DOI: 10.1088/2041-8205/714/2/L275 Abstract: Spiral galaxies have most of their stellar mass in a large rotating disk, and only a modest fraction in a central spheroidal bulge. This poses a major challenge for cosmological models of galaxy formation. Galaxies form at the centre of dark matter halos through a combination of hierarchical merging and gas accretion along cold streams, and should rapidly grow their bulge through mergers and instabilities. Cosmological simulations predict galaxies to have most of their mass in the central bulge, and therefore an angular momentum much below the observed level, except in dwarf galaxies. We propose that the continuous return of fresh gas by stellar populations over cosmic times could solve this issue. A population of stars formed at a given instant typically returns half of its initial mass in the form of gas over 10 billion years, and the process is not dominated by rapid supernovae explosions but by the long-term mass-loss from low- and intermediate-mass stars. Using simulations of galaxy formation, we show that this recycling of gas can strongly affect the structural evolution of massive galaxies, potentially solving the bulge fraction issue: we find that the bulge-to-disk ratio of a massive galaxy can be divided by a factor of 3. The continuous recycling of baryons through star formation and stellar mass loss helps the growth of disks and their survival to interactions and mergers. Instead of forming only early-type, spheroid-dominated galaxies (S0 and ellipticals), the standard cosmological model can then successfully account for massive late-type, disk-dominated spiral galaxies (Sb-Sc).
 Physics , 2002, Abstract: Over the last decade it has become clear that there is a decoupling between the old stellar disk and young stellar disk in spiral galaxies. This has led to a scheme for classifying galaxies on the basis of their near-infrared morphology. The near-infrared provides a more physical framework for classifying galaxies as it is both relatively free from extinction and it traces the old stellar population, i.e. the dominant stellar mass distribution. The `dust penetrated class' is dependent upon the spiral pitch angle of arms. We have observed 8 galaxies with UFTI on UKIRT in the K-band in order to investigate the theoretical link between disk dynamics and arm morphology, which is suggested both from numerical models and the dust penetrated class. We find that the pitch angle of spiral arms, i, correlates well with the shear rate of rotation curves, $A/\omega$ (where A is the first Oort constant and $\omega$ is the rotational velocity), over the same radial range.
 Physics , 2011, DOI: 10.1111/j.1365-2966.2012.21860.x Abstract: We seek to understand the origin of radial migration in spiral galaxies by analyzing in detail the structure and evolution of an idealized, isolated galactic disk. To understand the redistribution of stars, we characterize the time-evolution of properties of spirals that spontaneously form in the disk. Our models unambiguously show that in such disks, single spirals are unlikely, but that a number of transient patterns may coexist in the disk. However, we also show that while spirals are transient in amplitude, at any given time the disk favors patterns of certain pattern speeds. Using several runs with different numerical parameters we show that the properties of spirals that occur spontaneously in the disk do not sensitively depend on resolution. The existence of multiple transient patterns has large implications for the orbits of stars in the disk, and we therefore examine the resonant scattering mechanisms that profoundly alter angular momenta of individual stars. We confirm that the corotation scattering mechanism described by Sellwood & Binney (2002) is responsible for the largest angular momentum changes in our simulations.
 EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20121907006 Abstract: Spiral density wave theory attempts to describe the spiral pattern in spiral galaxies in terms of a long-lived wave structure with a constant pattern speed in order to avoid the winding dilemma. The pattern is consequently a rigidly rotating, long-lived feature. We run an N-body/SPH simulation of a Milky Way-sized barred disk, and find that the spiral arms are transient features whose pattern speeds decrease with radius, in such a way that the pattern speed is almost equal to the rotation curve of the galaxy. We trace particle motion around the spiral arms. We show that particles from behind and in front of the spiral arm are drawn towards and join the arm. Particles move along the arm in the radial direction and we find a clear trend that they migrate toward the outer (inner) radii on the trailing (leading) side of the arm. Our simulations demonstrate that tat all radii where there is a co-rotating spiral arm the particles continue to be accelerated (decelerated) by the spiral arm for long periods, which leads to strong migration.
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