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PEARLS-09:
Spiral Arm Sub-Structure in Nearby Galaxies - 2009
Talks

Listing of Talk Abstracts

Spurs and Substructure in SPH Simulations of Spiral Galaxies
Dr.  Clare Dobbs (University of Exeter)
I will present calculations which show the formation of GMCs by the agglomeration of smaller clouds in spiral shocks, and the subsequent shearing of GMCs into interarm spurs. I will discuss how magnetic fields, thermodynamics and self gravity aid or suppress this process. I will also compare the GMCs from different simulations, and suggest how the properties of GMCs may give some insight on how they formed.
Radio Polarization Observations of Magnetic Fields in Spiral Arms
Dr.  Andrew Fletcher (Newcastle University)
Radio polarization intensity and orientation, Faraday rotation and even Faraday depolarization can be used to study the magnetic properties of nearby galaxies on scales down to a few hundred parsec. I will show how these different tracers can be interpreted, using recent work on the galaxies M31 and M51 as examples: on a first look both galaxies seem to host large-scale mean magnetic fields, but on closer examination one of them, M51, has a magnetic field that is mostly small-scale but anisotropic. Then I will look more closely at the magnetic properties of the spiral arms in the classic grand design galaxy M51, discussing the location of magnetic and gaseous arms and how the magnetic field changes as it passes through the spiral arm shock.
Dynamics and large scale star formation in disk galaxies.
Dr.  Rosa A. Gonzalez-Lopezlira (UNAM, Centro de Radioastronomia y Astrofisica)
Azimuthal color (age) gradients across spiral arms are one of the main predictions of density wave theory; gradients are the result of star formation triggering by the spiral waves. In a sample of 13 spiral galaxiesof types A and AB, we find that 10 of them present regions that match the theoretical predictions. By comparing the observed gradients with stellar population synthesis models, the pattern speed and the location of major resonances have been determined. The resonance positions inferred from this analysis indicate that 9 of the objects have spiral arms that extend to the outer Lindblad resonance (OLR); for one of the galaxies, the spiral arms reach the corotation radius. The effects of dust, and of stellar variable densities, velocities, and metallicities on the color gradients are also discussed. Notably, and further confirming the link between gradients and dynamics, the effects of non-circular stellar velocities are clearly discerned in the data.
The Flow-Driven Formation of Molecular Clouds: Insights from Numerical Models
Dr.  Fabian Heitsch (University of North Carolina at Chapel Hill)
Galactic star formation occurs at a surprisingly low rate. Yet, recent large-scale surveys of dark clouds in the Galaxy show that one rarely finds molecular clouds without young stellar objects, suggesting that star formation should occur rapidly upon molecular cloud formation. This rapid onset challenges the traditional concept of "slow" star formation in long-lived molecular clouds. It also imposes strong constraints on the physical properties of the parental clouds, mandating that a cloud's structure and dynamics controlling stellar birth must arise during its formation. This requires a new approach to study initial conditions of star formation, namely addressing the formation of molecular clouds. Taking into account the observational constraints, I will outline the physics of flow-driven molecular cloud formation. I will discuss the relevance and the limitations of this scenario for setting the star formation efficiency in our Galaxy and beyond.
The Properties of Giant Molecular Clouds in the Milky Way
Prof.  Mark Heyer (University of Massachusetts)
Giant Molecular Clouds (GMCs) and the subsequent stars that form within their domain are the products of large scale interstellar processes operating within the diffuse interstellar medium. The properties of GMCs provide important constraints to the theoretical descriptions of cloud formation. Yet, the standard references (Larson 1981; Solomon etal 1987) to those properties are based on data collected over 25 years ago. The properties of GMCs within the molecular ring of the Galaxy are examined with more recent imaging campaigns of molecular line emission that greatly improve on the older surveys with regards to angular resolution, sensitivity, and line opacity. The molecular gas surface density of clouds is smaller than values derived by Solomon etal (1987). I will discuss the physical consequences of such reduced surface densities with regards to cloud formation mechanisms and support against self-gravity. I will also discuss the near universality of turbulence within GMCs and the relationship to Larson's scaling relationships.
Formation of Giant Molecular Associations and Spurs Across Spiral Arms
Prof.  Jin Koda (Stony Brook University)
Based on the recent CO observations of M51 with CARMA and the Nobeyama 45m telescope, I will discuss the formation and evolution of Giant Molecular Associations (GMAs) and spurs.
Arm-Interarm Contrasts in HERACLES and THINGS
Dr.  Adam Leroy (Max Plack Institute for Astronomy)
HERACLES and THINGS are overlapping surveys of the molecular and atomic interstellar medium in nearby galaxies. I will give a short description of these surveys and a brief summary of our current results on what drives cloud and star formation. I will then present ongoing work on how the ratios of CO, HI, and star formation rate vary between the arms and interarm regions.
Spiral Structure in the Radio: What Do We See?
Dr.  Eva Schinnerer (MPI for Astronomy)
I will review recent results using high resolution radio continuum imaging data in spiral galaxies with a focus on the radio-infrared relation. In addition an outlook of possible new avenue using the EVLA will be given.
The Role of Magnetic Fields in Star Formation
Prof.  Frank Shu (UCSD)
We review modern theories of how dense rotating cores of molecular clouds that are partially supported against their own gravity by magnetic fields slowly slip past such fields and collapse to form a low-mass star plus surrounding disk into which infalling material from the molecular cloud core continues to rain for a few hundred thousand years. Because some of the original interstellar magnetic field is dragged into the disk, processes are induced in the disk that give rise, over millions of years, to the inward transport of mass and the outward transport of angular momentum. However, the material spiraling inwards onto the star is obstructed from directly joining onto the star by the star's own strong magnetic fields that are generated by dynamo action. The subsequent interaction between the accretion disk and the magnetosphere of the star leads to the X-wind ejection of some of the material, carrying away most of the remaining angular momentum, and the funneling of another part, which contains relatively little angular momentum, onto the slowly rotating, nearly spherical, central star. The basic features of the complete theory have now successfully passed numerous observational tests and seem also to apply to many aspects of high-mass star formation. A connection can therefore be made that links the magnetic fields threading giant molecular clouds (GMCs) with the large-scale compressional processes that create GMCs in the spiral arms of ordinary disk galaxies.
The (im)persistance of Spiral Structure(s) in NGC 5055
Dr.  Peter Teuben (University of Maryland)
The structure and kinematics of spirals in disk galaxies from interferometers (especially ones that mosaic) is susceptible to a variety of numeric effects due to the limiting spatial sensititivity of these interferometers. In this paper we will show a number of these effects, go over the methods to compute the kinematic and measure their effects on the derived kinematics.
Shock-induced formation of Giant Molecular Clouds
Dr.  Sven Van Loo (University of Leeds)
Observations of giant molecular clouds show that they are magnetically dominated with values of the ratio, beta, of thermal gas pressure to magnetic pressure of roughly 0.04. However, on scales larger than the distances between molecular clouds, the thermal and magnetic pressure are about equal. This means that the formation mechanism of the GMCs must reduce the value of beta. It is thought that the formation of GMCs on scales of many tens to hundreds of parsecs is regulated by the ram pressure from supersonic flows such as supernova remnants, superbubbles and winds of massive stars. Indeed, numerical simulations have shown that a process relying on pressure-driven compression and radiative cooling leads to the formation of highly magnetically dominated regions within a molecular cloud consistent with observations of GMCs. I will discuss one of these processes where the transition of a cloud from the warm atomic phase to the cold molecular one is triggered by a shock.
Distribution of Phases of the ISM in Galaxies
Dr.  Mark Wolfire (University of Maryland)
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