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Anemic galaxies have less prominent star formation than normal galaxies of the same Hubble type. Previous studies showed they are deficient in total atomic hydrogen but not in molecular hydrogen. Here we compare the combined surface densities of HI and H2 at mid-disk radii with the Kennicutt threshold for star formation. The anemic galaxies are below threshold, which explains their lack of prominent star formation, but they are not much different than other early type galaxies, which also tend to be below threshold. The spiral wave amplitudes of anemic and normal galaxies were also compared, using images in B and J passbands from the OSU Bright Spiral Galaxy Survey. Anemic galaxies have normal spiral wave properties too, with the same amplitudes and radial dependencies as other galaxies of the same arm class. Because of the lack of gas, spiral waves in early type galaxies and anemics do not have a continuous supply of stars with low velocity dispersions to maintain a marginally stable disk. As a result, they are either short-lived, evolving toward lenticulars and S0 types in only a few rotations at mid-disk, or they are driven by the asymmetries associated with gas removal in the cluster environment.
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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.
Spiral arms are the most singular features in disc galaxies. These structures can exhibit different patterns, namely grand design and flocculent arms, with easily distinguishable characteristics. However, their origin and the mechanisms shaping them are unclear. The overall role of spirals in the chemical evolution of disc galaxies is another unsolved question. In particular, it has not been fully explored if the hii,regions of spiral arms present different properties from those located in the interarm regions. Here we analyse the radial oxygen abundance gradient of the arm and interarm star forming regions of 63 face-on spiral galaxies using CALIFA Integral Field Spectroscopy data. We focus the analysis on three characteristic parameters of the profile: slope, zero-point, and scatter. The sample is morphologically separated into flocculent versus grand design spirals and barred versus unbarred galaxies. We find subtle but statistically significant differences between the arm and interarm distributions for flocculent galaxies, suggesting that the mechanisms generating the spiral structure in these galaxies may be different to those producing grand design systems, for which no significant differences are found. We also find small differences in barred galaxies, not observed in unbarred systems, hinting that bars may affect the chemical distribution of these galaxies but not strongly enough as to be reflected in the overall abundance distribution. In light of these results, we propose bars and flocculent structure as two distinct mechanisms inducing differences in the abundance distribution between arm and interarm star forming regions.
In this work we determine the parameters of spiral structure for a sample of face-on spiral galaxies. In practice, the solution of this problem is a hard task because of the diversity of the observed characteristics of spiral structure, such as the arm number, their shape, arm contrast etc. In this work we study spiral structure in galaxies based on an analysis of photometric cuts perpendicular to the arm direction. The method is based on an approximation of these slices with an analytical function and derivation of the parameters of spiral structure (arm width, asymmetry, pitch angle) using the fitted parameters of this approximation. The algorithm has been applied to a sample of 155 galaxies selected from the Sloan Digital Sky Survey in different passbands. In this paper we only consider the results on the arm width: most spirals show an increase of their width with galactocentric distance. Only 14 per cent of galaxies in our sample show an opposite trend or have an almost constant arm width at all radii.
Fragmentation of a spiral arm is thought to drive the formation of giant clumps in galaxies. Using linear perturbation analysis for self-gravitating spiral arms, we derive an instability parameter and define the conditions for clump formation. We extend our analysis to multi-component systems that consist of gas and stars in an external potential. We then perform numerical simulations of isolated disc galaxies with isothermal gas, and compare the results with the prediction of our analytic model. Our model describes accurately the evolution of the spiral arms in our simulations, even when spiral arms dynamically interact with one another. We show that most of the giant clumps formed in the simulated disc galaxies satisfy the instability condition. The clump masses predicted by our model are in agreement with the simulation results, but the growth time-scale of unstable perturbations is overestimated by a factor of a few. We also apply our instability analysis to derive scaling relations of clump properties. The expected scaling relation between the clump size, velocity dispersion, and circular velocity is slightly different from that given by the Toomre instability analysis, but neither is inconsistent with currently available observations. We argue that the spiral-arm instability is a viable formation mechanism of giant clumps in gas-rich disc galaxies.
We investigate how the properties of spiral arms relate to other fundamental galaxy properties. To this end, we use previously published measurements of those properties, and our own measurements of arm-interarm luminosity contrasts for a large sample of galaxies, using 3.6$mu$m images from the Spitzer Survey of Stellar Structure in Galaxies. Flocculent galaxies are clearly distinguished from other spiral arm classes, especially by their lower stellar mass and surface density. Multi-armed and grand-design galaxies are similar in most of their fundamental parameters, excluding some bar properties and the bulge-to-total luminosity ratio. Based on these results, we discuss dense, classical bulges as a necessary condition for standing spiral wave modes in grand-design galaxies. We further find a strong correlation between bulge-to-total ratio and bar contrast, and a weaker correlation between arm and bar contrasts.
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