Tuesday, August 15, 2006

B. Vollmer on "Galaxy Evolution in the Virgo Cluster"

How do Spiral galaxies evolve in clusters? Looking at the close-by (17 Mpc) Virgo with its huge central galaxy M87 reveals much detail. Clusters have many galaxies, so they are more likely to collide, but apart from this galaxy-galaxy interaction, there is the hot gas that fills the cluster potential and that exerts pressure onto the ISM of the galaxies moving through it - so called ram-pressure-stripping takes place.

Which of these interactions is more important? Polarized radio continuum emission gets stronger when the ISM is compressed, because the magnetic field is locked into it. This emission therefore traces compressions and shear. What Vollmer finds in a sample of galaxies, is that only one shows the normal signatures of field-spirals, while the others show distortions and excess emission on the leading side of the movement, thereby confirming that compression indeed takes place and that ram-pressure-stripping is efficient.

He also presents some simulations from different groups and they seem to agree (at least within a factor of two) with observations when plotting the stripping radius over the density of the medium. The general outcome thus seems to be a truncated disk, i.e. the galaxy can retain their gas inside a certain radius, depending on the strength of ram-pressure-stripping.

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