Thursday, June 6, 2013

Daily Paper #12 -- The I-Band TF relation for Cluster Galaxies

Title: The I Band Tully-Fisher Relation for Cluster Galaxies: a Template Relation, its Scatter and Bias Corrections.
Authors:Giovanelli, Haynes et al.
This is a 'classic' Tully Fisher paper, still being cited often. Their TF relation is also widely used as the reference.
It is the seventh paper from a series with the same topic, presenting the corrections done and the TF relation fitting procedure. They use velocity widths and I magnitudes of 784 field and cluster galaxies.
They discuss biases occuring and TF scatter extensively, and there are quite a few enlightening observations. For example, while discussing the total observed scatter, they name three main causes for it:
  • Raw measurement errors (inclinations, apparent magnitudes, velocities)
  • Uncertainties arising from corrections (extinction, inclination correction for velocities, asymmetric drift correction, turbulence, etc.)
  • 'Cosmic scatter' -- velocity disk distortions, disk distortions, photometric asymmetries, variations in mass-to-light ratio, variation in disk-to-bulge-ratio, etc.
I think that's a nice distinction to have in mind.
They discuss the error propagation and relative importance of various error sources. One important, and in retrospect quite obvious fact is that that the relative importance of internal extinction correction errors depends on the inclination and intrinsic rotation velocity of a given galaxy: they are the largest for highly inclined, fast-rotating galaxies. Conversely, velocity errors are the most significant for nearly face-on, slowly rotating galaxies. That might be an interesting plot to show.
They discuss the implications of TF scatter too, citing Franx and de Zeeuw 1992 who claim that TF scatter poses a strong constraint on the ellipticity of the potential in the disk plane. Rix and Zaritsky, 1995 find that departures from disk axisymmetry can contribute as much as ~0.15 mag to the TF scatter (optical observations). Eisenstein and Loeb, 1996 find that the TF scatter due to different formation histories of galaxes can be larger than 0.3 mag in some cosmological scenarios (I remember reading that one: the intrinsic scatter of TF is surely smaller than that, and they try to find out why it is so, there must be some regularising factor at work).
They report a slight TF offset dependence on morphology -- this is of course band dependent. They also discuss a number of bias present, such as incompleteness bias (which they correct by fitting the LF), Malmquist bias, edge-of-catalogue bias, effects of cluster environment (which they find to be negligible).
I do not understand some of the corrections done here, namely, the morphological type correction (they add a term to magnitudes of early type galaxies in order to have a straight line TF relation -- why do they do it? Extinction corrections are likely to be different, and why such a TF relation would be useful?).
The residuals of the TFR can show if there are uncorrected selection effects -- i.e. if there is a dependency of them on inclination. However, I would not want to try to cram all galaxies into the same line, if they are intrinsically different. That'd be losing information, in my opinion.

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