Grand-design Spiral Structure Research Articles

Cosmic-Ray Diffusion Suppression in Star-forming Regions Inhibits Clump Formation in Gas-rich Galaxies

Abstract Observations of the γ-ray emission around star clusters, isolated supernova remnants, and pulsar wind nebulae indicate that the cosmic-ray (CR) diffusion coefficient near acceleration sites can be suppressed by a large factor compared to the Galaxy average. We explore the effects of such local suppression of CR diffusion on galaxy evolution using simulations of isolated disk galaxies with regular and high gas fractions. Our results show that while CR propagation with constant diffusivity can make gaseous disks more stable by increasing the midplane pressure, large-scale CR pressure gradients cannot prevent local fragmentation when the disk is unstable. In contrast, when CR diffusivity is suppressed in star-forming regions, the accumulation of CRs in these regions results in strong local pressure gradients that prevent the formation of massive gaseous clumps. As a result, the distribution of dense gas and star formation changes qualitatively: a globally unstable gaseous disk does not violently fragment into massive star-forming clumps but maintains a regular grand-design spiral structure. This effect regulates star formation and disk structure and is qualitatively different from and complementary to the global role of CRs in vertical hydrostatic support of the gaseous disk and in driving galactic winds.

How the bar properties affect the induced spiral structure

ABSTRACT Stellar bars and spiral arms coexist and co-evolve in most disc galaxies in the local Universe. However, the physical nature of this interaction remains a matter of debate. In this work, we present a set of numerical simulations based on isolated galactic models aimed to explore how the bar properties affect the induced spiral structure. We cover a large combination of bar properties, including the bar length, axial ratio, mass, and rotation rate. We use three galactic models describing galaxies with rising, flat, and declining rotation curves. We found that the pitch angle best correlates with the bar pattern speed and the spiral amplitude with the bar quadrupole moment. Our results suggest that galaxies with declining rotation curves are the most efficient forming grand design spiral structure, evidenced by spirals with larger amplitude and pitch angle. We also test the effects of the velocity ellipsoid in a subset of simulations. We found that as we increase the radial anisotropy, spirals increase their pitch angle but become less coherent with smaller amplitude.

Numerical integration of gravitational field for general three-dimensional objects and its application to gravitational study of grand design spiral arm structure

Numerical integration of gravitational field for general three-dimensional objects and its application to gravitational study of grand design spiral arm structure

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.

Overlapping abundance gradients and azimuthal gradients related to the spiral structure of the Galaxy

The connection between some features of the metallicity gradient in the Galactic disk, best revealed by Open Clusters and Cepheids, and the spiral structure, is explored. The step-like abrupt decrease in metallicity at 8.5 kpc (with R_0= 7.5 kpc, or at 9.5 kpc if R_0 = 8.5 kpc is adopted) is well explained by the corotation ring-shaped gap in the density of gas, which isolates the internal and external regions of the disk one from the other. This solves a long standing problem of understanding the different chemical characteristics of the inner and outer parts of the disk. The time required to build up the metallicity difference between the two sides of the step is a measure of the minimal life-time of the present grand-design spiral pattern structure, of the order of 3 Gyr. The plateaux observed on each side of the step are interpreted in terms of the large scale radial motion of the stars and of the gas flow induced by the spiral structure. The star-formation rate revealed by the density of open clusters is maximum in the Galactic radial range from 6 to 12 kpc (with an exception of a narrow gap at corotation), coinciding with the region where the 4-arms mode is allowed to exist. We argue that most of the old open clusters situated at large galactocentric radii were born in this inner region where conditions more favorable to star-formation are found. The ratio of $\alpha$-elements to Fe of the sample of Cepheids does not vary appreciably with the Galactic radius, which reveals an homogeneous history of star formation. Different arguments are given showing that usual approximations of chemical evolution models, which assume fast mixing of metallicity in the azimuthal direction and ignore the existence of the spiral arms, are a poor ones.

Prominent spiral arms in the gaseous outer galaxy disks

Context: Several spiral galaxies, as beautifully exhibited by the case of NGC 6946, display a prominent large-scale spiral structure in their gaseous outer disk. Such structure is often thought to pose a dynamical puzzle, because grand-design spiral structure is traditionally interpreted as the result of density waves carried mostly in the stellar disk. Aims. Here we argue that the outer spiral arms in the cold gas outside the bright optical disk actually have a natural interpretation as the manifestation of the mechanism that excites grand-design spiral structure in the main, star-dominated body of the disk: the excitation is driven by angular momentum transport to the outer regions, through trailing density waves outside the corotation circle that can penetrate beyond the Outer Lindblad Resonance in the gaseous component of the disk. Methods: Because of conservation of the density wave action, these outgoing waves are likely to become more prominent in the outer disk and eventually reach non-linear amplitudes. To calculate the desired amplitude profiles, we make use of the theory of dispersive waves. Results: If the conditions beyond the optical radius allow for an approximate treatment in terms of a linear theory, we show that fitting the observed amplitude profiles leads to a quantitative test on the density of the disk material and thus on the dark matter distribution in the outer parts of the galaxy. Conclusions: This study is thus of interest to the general problem of the disk-halo decomposition of rotation curves.

Global Modeling of Spur Formation in Spiral Galaxies

We investigate the formation of substructure in spiral galaxies using global MHD simulations, including gas self-gravity. Our models extend previous local models by Kim and Ostriker (2002) by including the full effects of curvilinear coordinates, a realistic log-spiral perturbation, self-gravitational contribution from 5 radial wavelengths of the spiral shock, and variation of density and epicyclic frequency with radius. We show that with realistic Toomre Q values, self-gravity and galactic differential rotation produce filamentary gaseous structures with kpc-scale separations, regardless of the strength -- or even presence -- of a stellar spiral potential. However, the growth of sheared features distinctly associated with the spiral arms, described as spurs or feathers in optical and IR observations of many spiral galaxies, requires a sufficiently strong spiral potential in self gravitating models. Unlike independently-growing ''background'' filaments, the orientation of arm spurs depends on galactic location. Inside corotation, spurs emanate outward, on the convex side of the arm; outside corotation, spurs grow inward, on the concave side of the arm. Based on spacing, orientation, and the relation to arm clumps, it is possible to distinguish ''true spurs'' that originate as instabilities in the spiral arms from independently growing ''background'' filaments. Our models also suggest that magnetic fields are important in preserving grand design spiral structure when gas in the arms fragments via self-gravity into GMCs.

Near-infrared constraints on the driving mechanisms for spiral structure

We have imaged a sample of 17 inclined spiral galaxies with measured Hα rotation curves in the K band, in order to determine the morphology of the old stellar population that dominates the mass in the disc. The K-band images of the galaxies have been used to determine the radial extent of grand-design spiral structure and compare this with the turnover radius in their rotation curves, where the rotation curve transforms from solid-body rotation to differential rotation. If the arms do not extend past this radius, the winding problem is solved. We find in all 17 cases that the arms extend past this radius, with the radius of grand-design spiral structure being a factor of 1.3–9.6 times larger than the rotation curve turnover radius. Of these galaxies, four have near neighbours and central bars and a further seven have a central bar. These bars or near neighbours may be the cause of the driving of the spiral potential in the discs of these galaxies. Of the remaining six galaxies, five show some evidence for a bar or oval distortion in their K-band images. The remaining galaxy (UGC 14) shows no evidence for a central bar and has no near neighbours. Finally, we also find that the spiral structure of these galaxies in the near-infrared is extremely regular, although some range in the regularity of spiral structure is found. To quantify this range in spiral structure, we introduce the dust-penetrated arm class, which is analogous to the dust-penetrated classification scheme of Block & Puerari and is based on the optical arm class of Elmegreen & Elmegreen.

DRIVING A GASEOUS NUCLEAR SPIRAL INSIDE THE INNER LINDBLAD RESONANCE

We model grand-design two-arm spiral structure, recently detected in a number of disc galaxies in the near infrared. We explain the observed continuity between the nuclear and kpc-scale spiral structures and the low arm–interarm contrast, using density-wave theory. We find that nuclear spiral arms form within a range of central mass concentrations and sound speeds in the gas and investigate the allowed parameter space.

Density Waves inside the Inner Lindblad Resonance: Nuclear Spirals in Disk Galaxies

We analyze the formation of grand-design two-arm spiral structure in the nuclear regions of disk galaxies. Such morphology has been recently detected in a number of objects using high-resolution near-infrared observations. Motivated by (1) the observed continuity between the nuclear and kiloparsec-scale spiral structures and (2) low arm-interarm contrast, we apply the density wave theory to explain the basic properties of the spiral nuclear morphology. In particular, we address the mechanism for the formation, maintenance, and detailed shape of nuclear spirals. We find that the latter depends mostly on the shape of the underlying gravitational potential and the sound speed in the gas. Detection of nuclear spiral arms provides diagnostics of mass distribution within the central kiloparsec of disk galaxies. Our results are supported by two-dimensional numerical simulations of gas response to the background gravitational potential of a barred stellar disk. We investigate the parameter space allowed for the formation of nuclear spirals using a new method for constructing a gravitational potential in a barred galaxy, where positions of resonances are prescribed.

Extensive Spiral Structure and Corotation Resonance

Spiral density wave theories demand that grand design spiral structure be bounded, at most, between the inner and outer Lindblad resonances of the spiral pattern. The corotation resonance lies between the outer and inner Lindblad resonances. The locations of the resonances are at radii whose ratios to each other are rather independent of the shape of the rotation curve. The measured ratio of outer to inner extent of spiral structure for a given spiral galaxy can be compared to the standard ratio of corotation to inner Lindblad resonance radius. In the case that the measured ratio far exceeds the standard ratio, it is likely that the corotation resonance is within the bright optical disk. Studying such galaxies can teach us how the action of resonances sculpts the appearance of spiral disks. This paper reports observations of 140 disk galaxies, leading to resonance ratio tests for 109 qualified spirals. It lists candidates that have a good chance of having the corotation resonance radius within the bright optical disk.

NGC 4254: A Spiral Galaxy with an M = 1 Mode and Infalling Gas

view Abstract Citations (79) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS NGC 4254: A Spiral Galaxy with an M = 1 Mode and Infalling Gas Phookun, Bikram ; Vogel, Stuart N. ; Mundy, Lee G. Abstract We present high-resolution H I and Hα observations of the spiral galaxy NGC 4254. The observations were obtained with the VLA and the Maryland-Caltech Fabry-Perot camera, respectively. NGC 4254 is unusual in having a grand-design spiral structure with a strong m = 1 component for which there is no obvious cause in optical images. Our observations reveal that, in addition to the usual galactic disk component, there are H I clouds superposed on and beyond the H I disk, at velocities up to 150 km s^-1^ from those established for the disk. The mass in these clouds is ~2.3 x 10^8^ M_sun_, and they may be the remnants of an entity that was tidally disrupted by NGC 4254 and is now merging with it. The direct effects of the interaction between the cloud gas and the galaxy are limited to the region where the gas appears to be merging with the disk, where it may be causing a warp. But the indirect effects of the infalling gas appear profound: it is the most likely cause for the unusual spiral structure of NGC 4254. If so, the m = 1 spiral structure of NGC 4254 is recent, and an internal amplification mechanism such as swing amplification has played a major role in its evolution. Since NGC 4254 does not appear to be exceptionally deficient in dark matter and is apparently a normal Sc galaxy, the nature of the interaction appears important in determining the susceptibility of the disk to various spiral modes (in particular the m = 1,3, and 5 modes of NGC 4254). Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173375 Bibcode: 1993ApJ...418..113P Keywords: GALAXIES: COOLING FLOWS; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4254; GALAXIES: SPIRAL; GALAXIES: STRUCTURE; RADIO LINES: GALAXIES full text sources ADS | data products SIMBAD (4) NED (1)

Observations of Barred Spirals

Observations of barred spiral galaxies are discussed which show that the presence of a bar increases the likelihood for grand design spiral structure only in early Hubble types. This result is contrary to the more common notion that grand design spiral structure generally accompanies bars in galaxies. Enhanced deprojected color images are shown which reveal that a secondary set of spiral arms commonly occurs in barred galaxies and also occasionally in ovally distorted galaxies. 6 refs.

On the relative frequency of flocculent and grand design spiral structures in barred galaxies

The bars in late-type spiral galaxies (SBbc to SBm and SABbc to SABm) show little correlation with grand design spiral structure, unlike the bars in early-type galaxies. This is further evidence for two different types of bars in the SB morphological class, and two different types of ovals in the SAB class. The weak, short, and exponential-shaped bars that tend to occur in late-type galaxies may end far inside the corotation resonance and be unable to produce large-scale wave patterns in the outer stellar disks. The strong, long, and flat-shaped bars that tend to occur in early-type galaxies may extend closer to the corotation resonance and be able to produce strong and symmetric arms. This difference in bar-spiral correlation is consistent with the theoretical prediction that short-wavelength trailing spirals propagate outward beyond corotation and inward inside corotation. 30 refs.

Molecules in galaxies. V - CO observations of flocculent and grand-design spirals

New carbon monoxide (1-0) observations of 29 spiral galaxies are presented and combined with previously existing data for 74 galaxies in order to study the dependence of the average CO surface brightness on the spiral-arm class. The total sample shows no significant correlation between the CO surface brightness and the presence or absence of grand-design spiral structure. The observations imply that most stellar density waves do not significantly influence the average surface brightness of CO. This suggests that, if density waves trigger molecular cloud formation, then they either inhibit the other mechanisms of cloud formation to keep the total formation rate constant or they increase the visible disk area of the Galaxy to keep the average surface density of molecules approximately constant. If density waves do not significantly trigger molecular cloud formation, then the giant CO complexes observed in density-wave spiral arms are temporary conglomerates of smaller molecular clouds that formed independently of the wave. 55 references.

Do density waves trigger star formation?

A comparison of B - V colors, blue and far-IR surface brightnesses, metallicities, and star formation rates calculated from H-alpha and UV fluxes reveals no difference in the average star formation rates for galaxies with and without grand design spiral structure. This implies that strong density waves do not trigger a significant excess of star formation compared to that in similar galaxies without a wave. Density waves organize the gas and young stars into spiral arms because of the flow pattern, and they may influence the formation and destruction of the largest cloud complexes, but they appear to contribute less than 50 percent to the overall star formation rate. 46 references.

Flocculent and grand design spiral galaxies in groups - Time scales for the persistence of grand design spiral structures

Spiral arm classifications were made for 261 low-inclination galaxies in groups listed by Huchra and Geller. The fractional occurrence of grand design spiral structure in nonbarred galaxies was found to increase from approx.0.1 to approx.0.6 and then level off as the group crossing rate or galaxy collision rate in a group increases. A simple model is discussed where the random encounters between galaxies of any type and flocculent galaxies induce transient grand design spirals in the flocculent galaxies. If this grand-design stimulation occurs for binary collisions with impact parameters less than ..cap alpha..R/sub 25/, were R/sub 25/ is the galactic radius at 25 mag arcsec/sup -2/, and if the induced grand design spirals persist for an average time equal to ..beta.. galactic rotations, then the quantity ..cap alpha../sup 2/..beta.. equals approximately 3 x 10/sup 4/. If binary collisions are responsible for grand design spirals, then this result implies either that the induced spirals last for many galactic rotations (..beta..>15), or that they can be stimulated by very remote encounters (..cap alpha..>45.) Alternatively, grand design spirals may be stimulated by multiple galaxy encounters, which would be the case for such large ..cap alpha.., or by interactions with the potential well ofmore » the associated group, rather than by simple binary encounters. Weak correlations between the grand design fraction and the galaxy size, or between this fraction and the total number of galaxies in a group, were also found. Spiral structures of barred galaxies show no correlations with group environment.« less

Flocculent and grand design spiral structure in field, binary and group galaxies

Flocculent and grand design spiral structure in field, binary and group galaxies

Flocculent and grand design spiral arm structure in cluster galaxies

Flocculent and grand design spiral arm structure in cluster galaxies