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Register a new userOn the formation of spiral arms in dwarf galaxies
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Added by
Alexander Khoperskov V.
Publication date
2021
fields
Physics
and research's language is
English
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Spiral structure (both flocculent and Grand Design types) is very rarely observed in dwarf galaxies because the formation of spiral arms requires special conditions. In this work we analyze the sample of about 40 dS-galaxies found by scanning by eye the images of late-type galaxies with $m_B<15^m$ and $M_B>-18^m$ and photometric diameter $D_{25}<12$~kpc. We found that apart from the lower average gas (HI) fraction the other properties of dS-galaxies including the presence of a bar and the isolation index do not differ much from those for dwarf Irr or Sm-types of similar luminosity and rotation velocity (or specific angular momentum).There are practically no dS-galaxies with rotation velocity below 50,--,60~km,sec$^{-1}$. To check the conditions of formation of spiral structure in dwarf galaxies we carried out a series of N-body/hydrodynamic simulations of low-mass stellar-gaseous discy galaxies by varying the model kinematic parameters of discs, their initial thickness, relative masses and scale lengths of stellar and gaseous disc components, and stellar-to-dark halo masses. We came to conclusion that the gravitational mechanism of spiral structure formation is effective only for thin stellar discs, which are non-typical for dwarf galaxies, and for not too slowly rotating galaxies. Therefore, only a small fraction of dwarf galaxies with stellar/gaseous discs have spiral or ring structures. The thicker stellar disc, the more gas is required for the spiral structure to form. The reduced gas content in many dS-galaxies compared to non-spiral ones may be a result of more efficient star formation due to a higher volume gas density thank to the thinner stellar/gaseous discs.
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A model based on disk-stability criteria to determine the number of spiral arms of a general disk galaxy with an exponential disk, a bulge and a dark halo described by a Hernquist model is presented. The multifold rotational symmetry of the spiral structure can be evaluated analytically once the structural properties of a galaxy, such as the circular speed curve, and the disk surface brightness, are known. By changing the disk mass, these models are aimed at varying the critical length scale parameter of the disk and lead to a different spiral morphology in agreement with prior models. Previous studies based on the swing amplification and disk stability have been applied to constrain the mass-to-light ratio in disk galaxies. This formalism provides an analytic expression to estimate the number of arms expected by swing amplification making its application straight-forward to large surveys. It can be applied to predict the number of arms in the Milky Way as a function of radius and to constrain the mass-to-light ratio in disk galaxies for which photometric and kinematic measurements are available, like in the DiskMass survey. Hence, the halo contribution to the total mass in the inner parts of disk galaxies can be inferred in light of the ongoing and forthcoming surveys.
We propose a new theory to explain the formation of spiral arms and of all types of outer rings in barred galaxies. We have extended and applied the technique used in celestial mechanics to compute transfer orbits. Thus, our theory is based on the chaotic orbital motion driven by the invariant manifolds associated to the periodic orbits around the hyperbolic equilibrium points. In particular, spiral arms and outer rings are related to the presence of heteroclinic or homoclinic orbits. Thus, R1 rings are associated to the presence of heteroclinic orbits, while R1R2 rings are associated to the presence of homoclinic orbits. Spiral arms and R2 rings, however, appear when there exist neither heteroclinic nor homoclinic orbits. We examine the parameter space of three realistic, yet simple, barred galaxy models and discuss the formation of the different morphologies according to the properties of the galaxy model. The different morphologies arise from differences in the dynamical parameters of the galaxy.
One of the scenarios for the formation of grand-design spiral arms in disky galaxies involves their interactions with a satellite or another galaxy. Here we consider another possibility, where the perturbation is instead due to the potential of a galaxy cluster. Using $N$-body simulations we investigate the formation and evolution of spiral arms in a Milky Way-like galaxy orbiting a Virgo-like cluster. The galaxy is placed on a few orbits of different size but similar eccentricity and its evolution is followed for 10 Gyr. The tidally induced, two-armed, approximately logarithmic spiral structure forms on each of them during the pericenter passages. The spiral arms dissipate and wind up with time, to be triggered again at the next pericenter passage. We confirm this transient and recurrent nature of the arms by analyzing the time evolution of the pitch angle and the arm strength. We find that the strongest arms are formed on the tightest orbit, however they wind up rather quickly and are disturbed by another pericenter passage. The arms on the most extended orbit, which we analyze in more detail, wind up slowly and survive for the longest time. Measurements of the pattern speed of the arms indicate that they are kinematic density waves. We attempt a comparison with observations by selecting grand-design spiral galaxies in the Virgo cluster. Among those, we find nine examples bearing no signs of recent interactions or the presence of companions. For three of them we present close structural analogues among our simulated spiral galaxies.
Spitzer Space Telescope observations of 15 spiral galaxies show numerous dense cores at 8 microns that are revealed primarily in unsharp mask images. The cores are generally invisible in optical bands because of extinction, and they are also indistinct at 8 microns alone because of contamination by more widespread diffuse emission. Several hundred core positions, magnitudes, and colors from the four IRAC bands are measured and tabulated for each galaxy. The larger galaxies, which tend to have longer and more regular spiral arms, often have their infrared cores aligned along these arms, with additional cores in spiral arm spurs. Galaxies without regular spirals have their cores in more irregular spiral-like filaments, with typically only one or two cores per filament. Nearly every elongated emission feature has 8 micron cores strung out along its length. The occurrence of dense cores in long and thin filaments is reminiscent of filamentary star formation in the solar neighborhood, although on a scale 100 times larger in galaxies. The cores most likely form by gravitational instabilities and cloud agglomeration in the filaments. The simultaneous occurrence of several cores with regular spacings in some spiral arms suggests that in these cases, all of the cores formed at about the same time and the corresponding filaments are young. Total star formation rates for the galaxies correlate with the total embedded stellar masses in the cores with an average ratio corresponding to a possible age between 0.2 Myr and 2 Myr. This suggests that the identified cores are the earliest phase for most star formation.
Context. Observations of polarized radio emission show that large-scale (regular) magnetic fields in spiral galaxies are not axisymmetric, but generally stronger in interarm regions. In some nearby galaxies such as NGC 6946 they are organized in narrow magnetic arms situated between the material spiral arms. Aims. The phenomenon of magnetic arms and their relation to the optical spiral arms (the material arms) call for an explanation in the framework of galactic dynamo theory. Several possibilities have been suggested but are not completely satisfactory; here we attempt a consistent investigation. Methods. We use a 2D mean-field dynamo model in the no-z approximation and add injections of small-scale magnetic field, taken to result from supernova explosions, to represent the effects of dynamo action on smaller scales. This injection of small scale field is situated along the spiral arms, where star-formation mostly occurs. Results. A straightforward explanation of magnetic arms as a result of modulation of the dynamo mechanism by material arms struggles to produce pronounced magnetic arms, at least with realistic parameters, without introducing new effects such as a time lag between Coriolis force and {alpha}-effect. In contrast, by taking into account explicitly the small-scale magnetic field that is injected into the arms by the action of the star forming regions that are concentrated there, we can obtain dynamo models with magnetic structures of various forms that can be compared with magnetic arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered as coherent magnetic structures generated by large-scale dynamo action, and associated with spatially modulated small-scale magnetic fluctuations, caused by enhanced star formation rates within the material arms.
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