Do Disk Galaxies Have Different Central Velocity Dispersions At A Given Rotation Velocity?

Details Do Disk Galaxies Have Different Central Velocity Dispersions At A Given Rotation Velocity? Do Disk Galaxies Have Different Central Velocity Dispersions At A Given Rotation Velocity?

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21840802d&link_type=abstract

American Astronomical Society, AAS Meeting #218, #408.02; Bulletin of the American Astronomical Society, Vol. 43, 2011

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Hubble's classification of spiral galaxies was one dimensional. Actually it was 1.5 dimensional, as he distinguished barred spirals. Van den Bergh's was two dimensional: spirals had luminosity classes too. Other schemes are summarized at http://www.daviddarling.info/encyclopedia/G/galaxyclassification.html
A more quantitative approach is to classify spiral galaxies by rotation velocity. Their central velocity dispersion (bulge) tends to be roughly one half of their rotation velocity (disk). There is a trend from σ/W = 0.8 to σ/W = 0.2 as one goes from W = 100 to 500 km/s, where W is twice the rotation velocity. But some fraction of spirals have a velocity dispersion up to a factor of two larger than that.
In hierarchical galaxy formation models, the relative contributions of σ and W depend on the mass accretion history of the galaxy, which determines the mass distribution of the dynamical components such as disk, bulge and dark matter halo. The wide variety of histories that originate in the hierarchical mass assembly produce at any value of W a wide range of σ/W, that reaches high values in more bulge- dominated systems.
In a sense the two classifiers were both right: spirals are mostly one dimensional, but σ/W (bulge to disk ratio) is often larger than average. Is this a signature of merger history?

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