Two-armed Spiral Structure Research Articles

Exciting spiral arms in protoplanetary discs from flybys

ABSTRACT Spiral arms are observed in numerous protoplanetary discs. These spiral arms can be excited by companions, either on bound or unbound orbits. We simulate a scenario where an unbound perturber, i.e. a flyby, excites spiral arms during a periastron passage. We run three-dimensional hydrodynamical simulations of a parabolic flyby encountering a gaseous protoplanetary disc. The perturber mass ranges from $10\, \rm M_J$ to $1\, \rm {\rm M}_{\odot }$. The perturber excites a two-armed spiral structure, with a more prominent spiral feature for higher mass perturbers. The two arms evolve over time, eventually winding up, consistent with previous works. We focus on analysing the pattern speed and pitch angle of these spirals during the whole process. The initial pattern speed of the two arms are close to the angular velocity of the perturber at periastron, and then it decreases over time. The pitch angle also decreases over time as the spiral winds up. The spirals disappear after several local orbital times. An inclined prograde orbit flyby induces similar disc substructures as a coplanar flyby. A solar-mass flyby event causes increased eccentricity growth in the protoplanetary disc, leading to an eccentric disc structure which dampens over time. The spirals’ morphology and the disc eccentricity can be used to search for potential unbound stars or planets around discs where a flyby is suspected. Future disc observations at high resolution and dedicated surveys will help to constrain the frequency of such stellar encounters in nearby star-forming regions.

Tidally induced spiral arm wraps encoded in phase space

Context. The impact of Sagittarius and other satellite galaxies such as the Large Magellanic Cloud on our Galaxy and in particular its disc is gradually being disclosed. Simulations tailored to the interaction of the Milky Way (MW) and Sagittarius show rings and spiral arms appearing in the Galaxy disc. However, spiral arms can also be induced by the bar or by disc instabilities. Aims. We aim to study the dynamics of tidally induced spiral arms in the context of the different encounters with Sagittarius and determine their kinematic signatures in the shape of ridges and waves in angular momentum, similar to those detected with Gaia DR2. Methods. We built toy models of the interaction between a host and a satellite galaxy using orbital integrations after a tidal distant encounter. We derived analytically the shape of the structures seen in phase space as a function of time for simple power-law potential models. We compared these models to a more realistic N-body simulation of the MW Sagittarius-like interaction and also to real data from Gaia DR3. Results. As previously found, an impulsive distant tidal approach of a galactic satellite generates a kick in velocities that leads to a two-armed spiral structure. The arms are made of orbits in between their apocentres and pericentres, thus, they correspond to regions with average negative galactocentric radial velocity. The two-arm pattern rotates at an angular speed of ω − 1/2κ which depends on Galactocentric radius, thus causing winding with time range of 0.8–2.1 Gyr, respectively. This winding produces ridges in the R − Vϕ projection with alternating signs of VR and oscillations of VR in the LZ − ϕ space, similar to those observed in the Gaia data. The frequency of these kinematic features increases with time, offering a powerful means to infer the potential and the perturbation’s onset time and azimuthal phase. Fourier analysis allows us to date the impact times of simple models and even to date perturbations induced from subsequent pericentric passages that appear as simultaneous waves. For the MW, the Fourier analysis indicates a superposition of two different frequencies, confirming previous studies. Assuming that both are due to impulsive and distant pericentre passages, we find perturbation times < 0.6 Gyr and in the range of 0.8–2.1 Gyr. The latter is compatible with a previous pericentre of Sagittarius and would be associated to about four wraps of the spiral arms in the observed radial range. Conclusions. Further work on the self-gravitating response of galactic discs and possible degeneracies with secular processes induced by the bar is necessary. Our study is a first step towards shedding more light on the elusive structure and dynamics of the spiral arms of the Galaxy.

A Dynamical Measurement of the Disk Mass in Elias 2–27

Abstract Recent multi-wavelength Atacama Large Millimeter/submillimeter Array (ALMA) observations of the protoplanetary disk orbiting around Elias 2–27 revealed a two-armed spiral structure. The observed morphology, together with the young age of the star and the disk-to-star mass ratio estimated from dust-continuum emission, make this system a perfect laboratory to investigate the role of self-gravity in the early phases of star formation. This is particularly interesting if we consider that gravitational instabilities could be a fundamental first step for the formation of planetesimals and planets. In this Letter, we model the rotation curve obtained by CO data of Elias 2–27 with a theoretical rotation curve, including both the disk self-gravity and the star contribution to the gravitational potential. We compare this model with a purely Keplerian one and with a simple power-law function. We find that (especially for the 13CO isotopologue) the rotation curve is better described by considering not only the star, but also the disk self-gravity. We are thus able to obtain for the first time a dynamical estimate of the disk mass of 0.08 ± 0.04 M ⊙ and the star mass of 0.46 ± 0.03 M ⊙ (in the more general case), the latter being comparable with previous estimates. From these values, we derive that the disk is 17% of the star mass, meaning that it could be prone to gravitational instabilities. This result would strongly support the hypothesis that the two spiral arms are generated by gravitational instabilities.

Modelling the Galactic very-high-energyγ-ray source population

Context.The High Energy Stereoscopic System Galactic plane survey (HGPS) is to date the most comprehensive census of Galacticγ-ray sources at very high energies (VHE; 100 GeV ≤ E ≤ 100 TeV). As a consequence of the limited sensitivity of this survey, the 78 detectedγ-ray sources comprise only a small and biased subsample of the overall population. The larger part consists of currently unresolved sources, which contribute to large-scale diffuse emission to a still uncertain amount.Aims.We study the VHEγ-ray source population in the Milky Way. For this purpose population-synthesis models are derived based on the distributions of source positions, extents, and luminosities.Methods.Several azimuth-symmetric and spiral-arm models are compared for spatial source distribution. The luminosity and radius function of the population are derived from the source properties of the HGPS data set and are corrected for the sensitivity bias of the HGPS. Based on these models, VHE source populations are simulated and the subsets of sources detectable according to the HGPS are compared with HGPS sources.Results.The power-law indices of luminosity and radius functions are determined to range between −1.6 and −1.9 for luminosity and −1.1 and −1.6 for radius. A two-arm spiral structure with central bar is discarded as spatial distribution of VHE sources, while azimuth-symmetric distributions and a distribution following a four-arm spiral structure without bar describe the HGPS data reasonably well. The total number of Galactic VHE sources is predicted to be in the range from 800 to 7000 with a total luminosity and flux of (1.6 − 6.3) × 1036ph s−1and (3 − 15) × 10−10ph cm−2s−1, respectively.Conclusions.Depending on the model, the HGPS sample accounts for (68 − 87)% of the emission of the population in the scanned region. This suggests that unresolved sources represent a critical component of the diffuse emission measurable in the HGPS. With the foreseen jump in sensitivity of the Cherenkov Telescope Array, the number of detectable sources is predicted to increase by a factor between 5 and 9.

Spiral structure generated by major planets in protoplanetary disks

In this paper I describe numerical calculations of the motion of particles in a disk about a solar-mass object perturbed by a planet on a circular orbit with mass greater than 0.001 of the stellar mass. A simple algorithm for simulating bulk viscosity is added to the ensemble of particles, and the response of the disk is followed for several planet orbital periods. A two-arm spiral structure forms near the inner resonance (2–1) and extends to the planetary orbit radius (corotation). In the same way for gaseous disks on a galactic scale perturbed by a weak rotating bar-like distortion, this is shown to be related to the appearance of two perpendicular families of periodic orbits near the resonance combined with dissipation which inhibits the crossing of streamlines. Spiral density enhancements result from the crowding of streamlines due to the gradual shift between families. The results, such as the dependence of pitch-angle on radius and the asymmetry of the spiral features, resemble those of sophisticated calculations that include more physical effects. The morphology of structure generated in this way clearly resembles that observed in objects with well-defined two-arm spirals, such as SAO 206462. This illustrates that the process of spiral formation via interaction with planets in such disks can be due to orbital motion in a perturbed Keplerian field combined with kinematic viscosity.

A possible origin of kilohertz quasi-periodic oscillations in low-mass X-ray binaries

Abstract A possible origin of kilohertz quasi-periodic oscillations (QPOs) in low-mass X-ray binaries is proposed. Recent numerical magnetohydrodynamic simulations of accretion disks with turbulent magnetic fields of magneto-rotational instability definitely show the presence of two-armed spiral structure in the quasi-steady state of accretion disks. In such deformed disks, two-armed ($m=2$) c-mode ($n=1$) oscillations are excited by wave–wave resonant instability. Among these excited oscillations, the fundamental in the radial direction ($n_{\rm r}=0$) will be the higher kHz QPO of twin QPOs, and the first overtone ($n_{\rm r}=1$) in the radial direction will be the lower kHz QPO of the twin. A possible cause of the twin high-frequency QPOs in black hole X-ray binaries is also discussed in the final section.

3D MHD simulations and synthetic radio emission from an oblique rotating magnetic massive star

ABSTRACT We have performed 3D isothermal MHD simulation of a magnetic rotating massive star with a non-zero dipole obliquity and predicted the radio/sub-mm observable light curves and continuum spectra for a frequency range compatible with ALMA. From these results we also compare the model input mass-loss to that calculated from the synthetic thermal emission. Spherical and cylindrical symmetry is broken due to the obliquity of the stellar magnetic dipole resulting in an inclination and phase dependence of both the spectral flux and inferred mass-loss rate, providing testable predictions of variability for oblique rotator. Both quantities vary by factors between 2 and 3 over a full rotational period of the star, demonstrating that the role of rotation as critical in understanding the emission. This illustrates the divergence from a symmetric wind, resulting in a two-armed spiral structure indicative of an oblique magnetic rotator. We show that a constant spectral index, α, model agrees well with our numerical prediction for a spherical wind for ν < 103 GHz; however it is unable to capture the behaviour of emission at ν > 103 GHz. As such we caution the use of such constant α models for predicting emission from non-spherical winds such as those which form around magnetic massive stars.

The Disk Substructures at High Angular Resolution Project (DSHARP). IV. Characterizing Substructures and Interactions in Disks around Multiple Star Systems

Abstract To characterize the substructures induced in protoplanetary disks by the interaction between stars in multiple systems, we study the 1.25 mm continuum and the 12CO(J = 2–1) spectral line emission of the triple systems HT Lup and AS 205, at scales of ≈5 au, as part of the “Disk Substructures at High Angular Resolution Project” (DSHARP). In the continuum emission, we find two symmetric spiral arms in the disk around AS 205 N, with a pitch angle of 14°, while the southern component AS 205 S, itself a spectroscopic binary, is surrounded by a compact inner disk and a bright ring at a radius of 34 au. The 12CO line exhibits clear signatures of tidal interactions, with spiral arms, extended arc-like emission, and high velocity gas, possible evidence of a recent close encounter between the disks in the AS 205 system, as these features are predicted by hydrodynamic simulations of flyby encounters. In the HT Lup system, we detect continuum emission from all three components. The primary disk, HT Lup A, also shows a two-armed symmetric spiral structure with a pitch angle of 4°, while HT Lup B and C, located at 25 and 434 au in projected separation from HT Lup A, are barely resolved with ∼5 and ∼10 au in diameter, respectively. The gas kinematics for the closest pair indicates a different sense of rotation for each disk, which could be explained by either a counter rotation of the two disks in different, close to parallel, planes, or by a projection effect of these disks with a close to 90° misalignment between them.

Tidally Induced Morphology of M33 in Hydrodynamical Simulations of Its Recent Interaction with M31

Abstract We present a hydrodynamical model of M33 and its recent interaction with M31. This scenario was previously proposed in the literature in order to explain the distorted gaseous and stellar disks of M33, as well as the increased star formation rate in both objects around 2 Gyr ago. We used an orbit integration scheme to find which estimate of the transverse velocity of M31 favors the interaction scenario more and then tried to reproduce it in our simulations. M33 was modeled as a stellar and gaseous disk embedded in a live dark matter halo, while M31 was approximated only with a live dark halo. In the simulations, the two galaxies passed each other with a pericenter distance of 37 kpc. Tides excited a two-armed spiral structure in the M33 disk, which is found to be the predominant spiral signal in the observed galaxy and has long been known as a feature easily induced by tidal interactions. We found that the gaseous warp produced by the interaction did not resemble enough the observed one, and we performed an additional simulation including the hot gas halo of M31 to show that this feature can be properly reproduced by tidal forces and ram pressure stripping acting simultaneously on the gaseous disk. In addition to the spiral arms, tidal forces produced a stellar stream similar to the one observed and triggered a star formation burst at radii similar to where it is observed.

Impact of cosmological satellites on stellar discs: dissecting one satelliteat a time

Within the standard hierarchical structure formation scenario, Milky Way-mass dark matter haloes have hundreds of dark matter subhaloes with mass $\gtrsim 10^8 \, {\rm M_{\odot}}$. Over the lifetime of a galactic disc a fraction of these may pass close to the central region and interact with the disc. We extract the properties of subhaloes, such as their mass and trajectories, from a realistic cosmological simulation to study their potential effect on stellar discs. We find that massive subhalo impacts can generate disc heating, rings, bars, warps, lopsidedness as wells as spiral structures in the disc. Specifically, strong counter-rotating single-armed spiral structures form each time a massive subhalo passes through the disc. Such single-armed spirals wind up relatively quickly (over $1-2$ Gyrs) and are generally followed by co-rotating two-armed spiral structures that both develop and wind up more slowly. In our simulations self-gravity in the disc is not very strong and these spiral structures are found to be kinematic density waves. We demonstrate that there is a clear link between each spiral mode in the disc and a given subhalo that caused it, and by changing the mass of the subhalo we can modulate the strength of the spirals. Furthermore, we find that the majority of subhaloes interact with the disc impulsively, such that the strength of spirals generated by subhaloes is proportional to the total torque they exert. We conclude that only a handful of encounters with massive subhaloes is sufficient for re-generating and sustaining spiral structures in discs over their entire lifetime.

The Orbit of the Companion to HD 100453A: Binary-driven Spiral Arms in a Protoplanetary Disk

Abstract HD 100453AB is a 10 ± 2 Myr old binary whose protoplanetary disk was recently revealed to host a global two-armed spiral structure. Given the relatively small projected separation of the binary (1.″05, or ∼108 au), gravitational perturbations by the binary seemed to be a likely driving force behind the formation of the spiral arms. However, the orbit of these stars remained poorly understood, which prevented a proper treatment of the dynamical influence of the companion on the disk. We observed HD 100453AB between 2015 and 2017, utilizing extreme adaptive optics systems on the Very Large Telescope and the Magellan Clay Telescope. We combined the astrometry from these observations with published data to constrain the parameters of the binary’s orbit to a = 1.″06 ± 0.″09, e = 0.17±0.07, and i = 32.°5 ± 6.°5. We utilized publicly available ALMA 12CO data to constrain the inclination of the disk, , which is relatively coplanar with the orbit of the companion and consistent with previous estimates from scattered light images. Finally, we input these constraints into hydrodynamic and radiative transfer simulations to model the structural evolution of the disk. We find that the spiral structure and truncation of the circumprimary disk in HD 100453 are consistent with a companion-driven origin. Furthermore, we find that the primary star’s rotation, its outer disk, and the companion exhibit roughly the same direction of angular momentum, and thus the system likely formed from the same parent body of material.

Stellar spiral structures in triaxial dark matter haloes

We employ very high resolution simulations of isolated Milky Way-like galaxies to study the effect of triaxial dark matter haloes on exponential stellar discs. Non-adiabatic halo shape changes can trigger two-armed grand-design spiral structures which extend all the way to the edge of the disc. Their pattern speed coincides with the inner Lindblad resonance indicating that they are kinematic density waves which can persist up to several Gyr. In dynamically cold discs, grand-design spirals are swing amplified and after a few Gyr can lead to the formation of (multi-armed) transient recurrent spirals. Stellar discs misaligned to the principal planes of the host triaxial halo develop characteristic integral shaped warps, but otherwise exhibit very similar spiral structures as aligned discs. For the grand-design spirals in our simulations, their strength dependence with radius is determined by the torque on the disc, suggesting that by studying grand-design spirals without bars it may be possible to set constraints on the tidal field and host dark matter halo shape.

DISCOVERY OF A TWO-ARMED SPIRAL STRUCTURE IN THE GAPPED DISK AROUND HERBIG Ae STAR HD 100453

We present VLT/SPHERE adaptive optics imaging in Y$-$, J$-$, H$-$, and K-bands of the HD 100453 system and the discovery of a two-armed spiral structure in a disk extending to 0.37" ($\sim$42 AU) from the star, with highly symmetric arms to the Northeast and Southwest. Inside of the spiral arms, we resolve a ring of emission from 0.18"-0.25" ($\sim$21-29 AU). By assuming that the ring is intrinsically circular we estimate an inclination of $\sim$34$^{o}$ from face-on. We detect dark crescents on opposite sides (NW and SE) which begin at 0.18" and continue to radii smaller than our inner working angle of 0.15", which we interpret as the signature of a gap at $\lesssim$21 AU that has likely been cleared by forming planets. We also detect the $\sim$120 AU companion HD 100453 B, and by comparing our data to 2003 HST/ACS and VLT/NACO images we estimate an orbital period of $\sim$850 yr. We discuss what implications the discovery of the spiral arms and finer structures of the disk may have on our understanding of the possible planetary system in HD 100453, and how the morphology of this disk compares to other related objects.

Long-slit spectral observations and stellar mass-to-light ratio of spiral galaxy UGC11919

We performed the long-slit observations of spiral galaxy UGC11919 with the Russian 6-m telescope to study its kinematics and stellar population. The previous studies gave basis to suspect that this galaxy possesses a peculiarly low mass-to-light ratio $M/L_B$ of stellar population which could indicate the presence of bottom-light stellar initial mass function (IMF). The ratio $M/L_B$ estimated for different evolutionary models of stellar population using both the broad-band magnitudes and the detailed spectral data confirms this peculiarity if the disc inclination angle $i>30^o$, as it was obtained earlier from the optical photometry, in a good agreement with the HI data cube modelling. However the re-processing of HI data cube we carried out showed that it is compatible with much lower value $i=13^o$ corresponding to the "normal" ratio $M/L_B$, which does not need any peculiar stellar IMF. Stellar velocity dispersion measured at one disc radial scalelength from the center also better agrees with the low disc inclination. However in this case we should admit that the disc possesses a non-axisymmetric shape even after taking into account a two-armed spiral structure. The derived stellar kinematic profiles reveal a signature of kinematically decoupled nuclear disc in the galaxy. Using different evolution models of stellar population we estimated the stellar metallicity [Z/H] (-0.4, -0.5 and -0.3 dex) and the mean luminosity-weighted (for the luminosity in the spectral range $4800-5570$ \AA) stellar age (4.2, 2.6 and 2.3 Gyr) for the bulge, disc and nuclear disc of this galaxy respectively.

Spiral structure in nearby galaxies – II. Comparative analysis and conclusions

This paper presents a detailed analysis of two-armed spiral structure in a sample of galax- ies from the Spitzer Infrared Nearby Galaxies Survey (SINGS), with particular focus on the relationships between the properties of the spiral pattern in the stellar disc and the global struc- ture and environment of the parent galaxies. Following Paper I we have used a combination of Spitzer Space Telescope mid-infrared imaging and visible multi-colour imaging to isolate the spiral pattern in the underlying stellar discs, and we examine the systematic behaviours of the observed amplitudes and shapes (pitch angles) of these spirals. In general, spiral morphology is found to correlate only weakly at best with morphological parameters such as stellar mass, gas fraction, disc/bulge ratio, and vflat. In contrast to weak correlations with galaxy structure a strong link is found between the strength of the spiral arms and tidal forcing from nearby companion galaxies. This appears to support the longstanding suggestion that either a tidal interaction or strong bar is a necessary condition for driving grand-design spiral structure. The pitch angles of the stellar arms are only loosely correlated with the pitch angles of the corresponding arms traced in gas and young stars. We find that the strength of the shock in the gas and the contrast in the star formation rate are strongly correlated with the stellar spiral amplitude.

FEATHERING INSTABILITY OF SPIRAL ARMS. II. PARAMETER STUDY

We report the results of a parameter study of the feathering stability in the galactic spiral arms. A two-dimensional razor-thin magnetized self-gravitating gas disk with an imposed two-armed stellar spiral structure is considered. Using the formulation developed previously by Lee and Shu, a linear stability analysis of the spiral shock is performed in a localized Cartesian geometry. Results of the parameter study of the base state with a spiral shock are also presented. The single-mode feathering instability that leads to growing perturbations may explain the feathering phenomenon found in nearby spiral galaxies. The self-gravity of the gas, characterized by its average surface density, is an important parameter which 1) shifts the spiral shock further downstream; 2) increases the growth rate and decreases the characteristic spacing of the feathering structure due to the instability. On the other hand, while the magnetic field suppresses the velocity fluctuation associated with the feathers, it does not strongly affect their growth rate. Using a set of typical parameters of the grand-design spiral galaxy M51 at 2 kpc from the center, the spacing of the feathers with the maximum growth rate is found to be 530 pc, which agrees with the previous observational studies.

Disk thickness and spiral arm structure of M31

The objective of this work is to obtain the disk thickness and spiral arm structure of M31. The method to estimate the thickness of galactic disk is based on using Poisson’s equation for the logarithmical matter density in three-dimensional disk galaxies. We use new infrared image of M31 at 24μm taken with the Multiband Imaging Photometer for Spitzer (MIPS). Far infrared images would be free of blurring by the bright bulge, so that the spiral arms can be traced inwards to smaller radii (the forbidden radii). We investigate the fitting of a two-armed (a- and b-arm) spiral structure of M31 by logarithmical spirals. From a-arm structure, we acquire the disk inclination γa=74.5±1.5°, and from b-arm structure, γb=70.3±2.0°. The mean values for the forbidden radius r0¯, the winding parameter Λ¯, and the disk thickness H¯ are 2.82±0.23kpc, 12.49±1.50, and 0.45±0.09kpc, respectively.

Radial migration of the Sun in the Galactic disc

In this paper, we present the physics of the gravitational effect of the Galactic bar. Our equations differ from those recently used in the literature. We apply the simultaneous action of the Galactic bar and spiral structure, together with the action of the Galactic halo and disc, to the motion of the Sun. The true multipolar model of the Galactic bar does not generate any relevant radial migration of the Sun. The Galactocentric distance of the Sun R changes by 1 per cent from the current value R0, for the bar model based on Cosmic Background Explorer (COBE) observations, if the gravity of the spiral arms is not considered. Spiral arms can generate more relevant radial migration. If we take into account the values of 220 km s−1, recommended by the International Astronomical Union, and R0 = 8.0 kpc, together with the bar model based on the COBE observations, then we find the following. R ∈ 〈7.85, 8.10〉 kpc, and the results are practically independent of the spiral structure strength εs, for the four-armed spiral structure. R ∈ 〈7.55, 8.90〉 kpc, and the results strongly depend on the spiral structure strength, for the two-armed spiral structure. If we admit that the absolute value of the radial velocity component is smaller than about 10 per cent of the speed of the Sun during its motion, then εs ≤ 0.03 and R ∈ 〈7.75, 8.45〉 kpc. There is significant radial migration of the Sun for the corotation resonance of the spiral arms with the Sun and εs = 0.06: R ∈ 〈4.18, 15.15〉 kpc. The radial migration of the Sun is not a consequence of the existence of the Galactic bar. The same result would yield a spherically symmetric Galactic bulge, except for the special case of the corotation resonance of the spiral arms with the Sun and εs = 0.06.

A Wideband Circularly Polarized Conical Beam From a Two-Arm Spiral Antenna Excited in Phase

Analysis is performed for a two-arm round spiral structure excited in phase, where the arms near the antenna center are backed by a small round disc. This spiral antenna has a frequency range where the radiation is bidirectional, having patterns symmetric with respect to the antenna plane. Subsequently, the bidirectional radiation is transformed into a unidirectional conical beam by backing the spiral with a cavity, where the cavity height is chosen to be extremely small (0.077 wavelength at the lowest design frequency 3.3 GHz). It is found that, as the distance between the spiral plane and the small disc inside the cavity is decreased, variations in the axial ratio and VSWR become smaller, as desired. Other antenna characteristics over a wide frequency range of 3.3 GHz to 9.6 GHz, including the gain and radiation efficiency, are also discussed.

Spiral structure in nearby galaxies - I. Sample, data analysis and overview of results

This paper, the first of two, introduces an observational study of spiral structure in galaxies chosen from the SINGS survey. Near infrared (NIR) and optical data are used to produce mass surface density maps, and from these the morphology of the disc is examined. The aim of this work is to characterise the prevalence of spiral structure in this sample and, in the cases where a clear spiral pattern is found, include the findings in a comparative study (reported in paper II). A two-armed (`grand design') spiral pattern is found in approximately half the galaxies studied, including all those that are designated as grand design in the optical, but also including some, but not all, optically flocculent galaxies. It is found that the level of non-axisymmetric structure in the galaxies' mass distributions is only modestly higher in those galaxies that are classified as `grand design' compared with those that are not, implying that non-grand design galaxies possess significant power in higher order modes. There is no evidence that bars preferentially trigger the spirals, but they do appear to stir up non-axisymmetric structure in the disc. In contrast, there is evidence that strong/close tidal interactions with companion galaxies are associated with strong two-armed spiral structure in the infrared, though there are a number of galaxies with relatively weak infrared spiral structure that do not possess such companions.