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Bars kill spiral galaxies

With the help of the army of volunteers working on the Galaxy Zoo 2 'citizen science' project, an international team of scientists have discovered that the bars found in many spiral galaxies could be helping to kill them off. The researchers present their results in the journal Monthly Notices of the Royal Astronomical Society.
Bars are important for the evolution of galaxies as they provide a way to move material in and out in the disk and possibly help to spark star formation in the central regions. They may even help feed the central massive black hole that seems to be present in almost all galaxies. But bars provide us with a great puzzle because we still don't understand why some galaxies have bars and others do not.

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Title: Galaxy Zoo: Bars in Disk Galaxies
Authors: Karen L. Masters (ICG, Portsmouth), Robert C. Nichol (ICG, Portsmouth), Ben Hoyle (ICG, Portsmouth/Barcelona), Chris Lintott (Oxford/Adler Planetarium), Steven Bamford (Nottingham), Edward M. Edmondson (ICG, Portsmouth), Lucy Fortson (Adler Planetarium/Minnesota), William C. Keel (Alabama), Kevin Schawinski (Yale), Arfon Smith (Oxford), Daniel Thomas (ICG, Portsmouth)
(Version v2)

We present first results from Galaxy Zoo 2, the second phase of the highly successful Galaxy Zoo project. Using a volume-limited sample of 13665 disk galaxies (0.01< z < 0.06 and M_r<-19.38), we study the fraction of galaxies with bars as a function of global galaxy properties like colour, luminosity and bulge prominence. Overall, 29.4±0.5% of galaxies in our sample have a bar, in excellent agreement with previous visually classified samples of galaxies (although this overall fraction is lower than measured by automated bar-finding methods). We see a clear increase in the bar fraction with redder (g-r) colours, decreased luminosity and in galaxies with more prominent bulges, to the extent that over half of the red, bulge-dominated, disk galaxies in our sample possess a bar. We see evidence for a colour bi-modality for our sample of disk galaxies, with a "red sequence" that is both bulge and bar-dominated, and a "blue cloud" which has little, or no, evidence for a (classical) bulge or bar. These results are consistent with similar trends for barred galaxies seen recently both locally and at higher redshift, and with early studies using the RC3. We discuss these results in the context of internal (secular) galaxy evolution scenarios and the possible links to the formation of bars and bulges in disk galaxies.

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Spiral Galaxies

Six spectacular spiral galaxies are seen in a clear new light in images from ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile. The pictures were taken in infrared light, using the impressive power of the HAWK-I camera, and will help astronomers understand how the remarkable spiral patterns in galaxies form and evolve.
HAWK-I is one of the newest and most powerful cameras on ESO's Very Large Telescope (VLT). It is sensitive to infrared light, which means that much of the obscuring dust in the galaxies' spiral arms becomes transparent to its detectors. Compared to the earlier, and still much-used, VLT infrared camera ISAAC, HAWK-I has sixteen times as many pixels to cover a much larger area of sky in one shot and, by using newer technology than ISAAC, it has a greater sensitivity to faint infrared radiation. Because HAWK-I can study galaxies stripped bare of the confusing effects of dust and glowing gas it is ideal for studying the vast numbers of stars that make up spiral arms.

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Six spiral galaxies unveiled

European astronomers unveiled gorgeous images of six spiral galaxies Wednesday, views of islands of stars resembling our own Milky Way.
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Hubble shows that the beautiful spirals galaxies of the modern Universe were the ugly ducklings of six billion years ago.
If confirmed, the finding highlights the importance to many galaxies of collisions and mergers in the recent past. It also provides clues for the unique status of our own galaxy, the Milky Way.

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Forming the present-day spiral galaxies

Using data from the NASA/ESA Hubble Space Telescope, astronomers have, for the first time, created a demographic census of galaxy types and shapes from a time before the Earth and the Sun existed, to the present day. The results show that, contrary to contemporary thought, more than half of the present-day spiral galaxies had so-called peculiar shapes only 6 billion years ago, which, if confirmed, highlights the importance of collisions and mergers in the recent past of many galaxies. It also provides clues for the unique status of our own galaxy, the Milky Way.
Galaxy morphology, or the study of the shapes and formation of galaxies, is a critical and much-debated topic in astronomy. An important tool for this is the Hubble sequence or Hubble tuning-fork diagram, a classification scheme invented in 1926 by the same Edwin Hubble in whose honour the space telescope is named.

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Title: New Developments in Spiral Structure Theory
Authors: J. A. Sellwood (Rutgers University)

After a short review of the principal theories of spiral structure in galaxies, I describe two new developments. First, it now seems clear that linear theory cannot yield a full description for the development of spiral patterns because N-body simulations suggest that non-linear effects manifest themselves at a relative overdensity of ~2%, which is well below the believed spiral amplitudes in galaxies. Second, I summarize the evidence that some stars in the solar neighbourhood have been scattered at an inner Lindblad resonance. This evidence strongly supports a picture of spirals as a recurring cycle of transient instabilities, each caused by resonant scattering by a previous wave.

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Title: Galactic Spiral Structure
Authors: Charles Francis, Erik Anderson
(Version v2)

We describe the structure and composition of six major stellar streams in a population of 20 574 local stars in the New Hipparcos Reduction with known radial velocities. We find that, once fast moving stars are excluded, almost all stars belong to one of these streams. The results of our investigation have lead us to re-examine the hydrogen maps of the Milky Way, from which we identify the possibility of a symmetric two-armed spiral with half the conventionally accepted pitch angle. We describe a model of spiral arm motions which matches the observed velocities and composition of the six major streams, as well as the observed velocities of the Hyades and Praesepe clusters at the extreme of the Hyades stream. We model stellar orbits as perturbed ellipses aligned at a focus in coordinates rotating at the rate of precession of apocentre. Stars join a spiral arm just before apocentre, follow the arm for more than half an orbit, and leave the arm soon after pericentre. Spiral pattern speed equals the mean rate of precession of apocentre. Spiral arms are shown to be stable configurations of stellar orbits, up to the formation of a bar and/or ring. Pitch angle is directly related to the distribution of orbital eccentricities in a given spiral galaxy. We show how spiral galaxies can evolve to form bars and rings. We show that orbits of gas clouds are stable only in bisymmetric spirals. We conclude that spiral galaxies evolve toward grand design two-armed spirals. We infer from the velocity distributions that the Milky Way evolved into this form about 9 Gyrs ago.

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Preferred Handedness of Spiral Galaxie
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Title: Evidence for a Preferred Handedness of Spiral Galaxies
Authors: Michael J. Longo

In this article I extend an earlier study of spiral galaxies in the Sloan Digital Sky Survey (SDSS) to investigate whether the universe has an overall handedness. A preference for spiral galaxies in one sector of the sky to be left-handed or right-handed spirals would indicate a parity-violating asymmetry in the overall universe and a preferred axis. The previous study used 2616 spiral galaxies with redshifts <0.04 and identified handedness. The new study uses 15158 with redshifts <0.085 and obtains very similar results to the first with a signal exceeding 5 sigma, corresponding to a probability ~2.5x10-7 for occurring by chance. A similar asymmetry is seen in the Southern Galaxy spin catalogue of Iye and Sugai. The axis of the dipole asymmetry lies at approx. (l, b) =(52 d, 68.5 d), roughly along that of our Galaxy and close to alignments observed in the WMAP cosmic microwave background distributions.

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Barred galaxies
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Title: Rings and spirals in barred galaxies. I Building blocks
Authors: E. Athanassoula, M. Romero-Gomez, J. J. Masdemont
(Version v2)

In this paper we present building blocks which can explain the formation and properties both of spirals and of inner and outer rings in barred galaxies. We first briefly summarise the main results of the full theoretical description we have given elsewhere, presenting them in a more physical way, aimed to an understanding without the requirement of extended knowledge of dynamical systems or of orbital structure. We introduce in this manner the notion of manifolds, which can be thought of as tubes guiding the orbits. The dynamics of these manifolds can govern the properties of spirals and of inner and outer rings in barred galaxies. We find that the bar strength affects how unstable the L1 and L2 Lagrangian points are, the motion within the 5A5A5Amanifold tubes and the time necessary for particles in a manifold to make a complete turn around the galactic centre. We also show that the strength of the bar, or, to be more precise, of the non-axisymmetric forcing at and somewhat beyond the corotation region, determines the resulting morphology. Thus, less strong bars give rise to R1 rings or pseudorings, while stronger bars drive R2, R1R2 and spiral morphologies. We examine the morphology as a function of the main parameters of the bar and present descriptive two dimensional plots to that avail. We also derive how the manifold morphologies and properties are modified if the L1 and L2 Lagrangian points become stable. Finally, we discuss how dissipation affects the manifold properties and compare the manifolds in gas-like and in stellar cases. Comparison with observations, as well as clear predictions to be tested by observations will be given in an accompanying paper.

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