Distance Indicators and Peculiar Velocities

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Distance Indicators and Peculiar Velocities
Status of the 6dFGS V-survey
Lachlan Campbell, RSAA/AAO 6dFGS Workshop 26-27 A
pril 2005
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V-survey goals

• To map in detail the density and peculiar
  velocity fields over half the local volume out to
  15,000 km/s.
  
• To provide additional constraints on cosmological
  models, and better measurements of fundamental
  parameters, from statistics of these fields.
  
• To achieve these goals the aim was to calculate
  Dn-? distances for approximately 10-15000 bright
  early type galaxies.

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Distance Indicators

• Present day ellipticals obey scaling relations
  involving their structural and dynamical
  properties. Populate the fundamental plane in 3D
  space defined by re , e , ? .

• Dn-? is a projection of the FP, where Dn is
  defined as the diameter which encloses a chosen
  mean surface brightness level.

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Calibration Sample

• Sample with 272 galaxies in common with the SMAC
  survey, and 450 with ENEAR, both of which provide
  photometric parameters and velocity dispersions.
  
• Provide a basis for estimating external errors
  and understanding any systematic biases.
  

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2MASS photometry
K12.75 z0.05

• Corrections for extinction, K corrections
  surface brightness dimming, and most importantly
  seeing effects (using the prescription of Saglia
  et al. 1993).
• Work by Hogg suggests there should be a small
  statistically insignificant offset between 2MASS
  and literature K band photometry.

K9.5 z0.013
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Differences in FP parameter

• 2MASS appear to give a semi-major axis half-light
  radii.
  
• SMAC give circular half-light radii.
• 2MASS psf circularises small galaxies.
• Colour gradients between wavelengths.

K10.86 K11.58
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Summary of Photometry

• 2MASS photometry internally consistent to 0.01dex
  or 2
  
• External comparison with both Optical and NIR
  photometry gives errors of 0.03 dex or 7
  

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Velocity Dispersions

• Galaxy velocity dispersions are calculated using
  the two standard Fourier methods of
  cross-correlation (Tonry Davis 1979) and
  Fourier quotient (Sargent et al. 1979).
• Use synthetic SSP galaxy template spectra
  (Vazdekis 1999) to estimate the broadening of
  absorption lines, most significantly in the Mg II
  and Fe lines.
• The Fourier quotient pipeline is being managed at
  Durham University to provide an independent check
  on the pipeline at RSAA/AAO.

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Instrumental dispersion
580V (VPH Grating) 600V
(Reflection Grating)
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Summary of Velocity Dispersions

• 6dFGS sigmas internally consistent to better than
  6 based on comparisons between the pipelines.
  
• External comparison with both SMAC and ENEAR
  gives an error of 8.
  

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V-survey Size

• Initial estimates of final numbers based on the
  minimum cuts listed for each parameter.
  
• All of the cuts assume a minimum R value of 5,
  and an initial total of 35 of the galaxies
  passing the z-quality and distance cuts.

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Conclusions

• 2MASS photometry appears very reliable.
• 6dFGS velocity dispersions are as accurate as the
  best previous results.
  
• Therefore, we expect to at least match the
  standard previous distance accuracy of the FP of
  20, and for the hoped for numbers of galaxies.
  

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