6dFGS data quality: comparison of pipeline and IRAF redshifts

1
6dFGS data qualitycomparison of pipeline and
IRAF redshifts

• Lesa Moore
• Macquarie University
• AAO 6dF Workshop 2005

2
Outline

• Spectral reduction and S/N
• Wavelength calibration
• Cross-correlation redshift agreement
• Quality measures S/N, Q, r
• Repeatability
• Final uncertainties
• Based on comparisons between IRAF
  and 6dFDR/RUNZ processing

3
Data

• Three fields studied (A, B and C) with repeat
  observations of the B and C fields
• A Mar 16 2002, reflection gratings
• B1 Sep 9 2002, reflection gratings
• B2 Sep 29 2003, VPH gratings
• C1 Sep 29 2002, VPH gratings
• C2 Sep 18 2003, VPH gratings

4
Method

• 6dFDR on separate V and R spectra
• 6dFDR line lists
• RUNZ on spliced spectra
• IRAF dofibers on separate V and R spectra
• My own line lists
• IRAF xcsao on spliced spectra
• Batch mode processing, no heliocentric correction
  in either case

5
Spectral Reduction

• 6dFDR reduction
• Sometimes requires FIT rather than TRAM
  extraction (slower)
• Sometimes requires FLUX WEIGHTING option turned
  off

6
Signal to Noise
Cross-processed Pipeline and IRAF reductions
about equal
VPH data superior to reflection grating data
(figures from 6dFDR/RUNZ)
7
Line lists and l calibration

• Blue (V) data original line list had 14 lines
• 6dFDR typically locates 12
• Software throws away 2 worst-fitting (leaves
  10)
• For red (R) data
• Line list omitted strong Ne line at 7032.41 Å
• Arc spectra deficient of lines redward of 7500 Å
  (does not affect cross-correlation)
• Found 2nd order line from Hg at 8092 Å

8
l Calibration Test

• Field C1 VPH reduced without sky subtraction
• Sky lines measured
• 6dFDR results (0.4 Å) equal to or superior to
  IRAF

9
Cross-correlation

• An earlier version of RUNZ at Epping was applying
  the heliocentric correction incorrectly
• RUNZ confused by noise in low S/N spectra
• Spliced spectra much more reliable than separate
  R and V

10
Redshift Agreement

• Agreement if
• Dz 0.0005
• Dcz 150 km/s
• Overall agreement 81 for 294 galaxies in 3
  fields
• Could still both be wrong
• need to check by eye

11
Quality Measures

• Compared this redshift agreement with
• S/N
• RUNZ Q-ranking
• Cross-correlation r-values
• as obtained from pipeline processing

12
S/N

• No strong correspondence between redshift
  agreement and S/N in separate R and V spectra

13
Q-rankings

• Large scatter even with high Q

294
14
Q-rankings

• Around half the disagrees have Q
  of 3, 4, or 5

294
15
Q-rankings

• Around half the disagrees have Q of
  3, 4, or 5
• Q3 meant to imply 75 confidence (only 60
  agree)

294
16
Cross-correlation r-value

• R-value is a much more useful indicator of
  redshift reliability than Q-ranking
• 84 of disagrees have rlt6
• 27 of 33 disagrees with Q3,4,5 have rlt6
• 72 of agrees have rgt6

17
SIMBAD-RUNZ difference vs. Q-ranking

• Q rankings of 3, 4, 5 show large spread of
  error values (4, 14, 29 data points respectively)
• Have applied heliocentric correction this time
• Note bias towards high-q results

18
SIMBAD-RUNZ difference vs. R-value

• Differences scale inversely with r-values
• St dev sz 0.00012, skm/s 52 km/s
  (based on 34 galaxies whose redshift
  agreement meets criterion of Dcz 150 km/s)

19
Repeatability

• End columns are large discrepancies
• All rgt6 results lie within Dz 0.001
• Overall st.dev. sDz 0.00033
• sDkm/s 98 km/s

20
Final uncertainties

• Notes
• 1. Based on 0.4 Å at 4000 Å
• 2. Mean of verr from RUNZ (278 galaxies in
  total, 190 with rgt6, possibly over-stated)
• 3. 1/v2 st. dev.(Dz) of repeat 6dF observations
  (125 galaxies in total, 81 with rgt6)
• 4. Added in quadrature
• kms/s z 300,000 assumed in all cases

21
Final uncertainties

• Notes
• 1. Based on 0.4 Å at 4000 Å
• 2. Mean of verr from RUNZ (possibly
  over-stated)
• 3. 1/v2 st. dev.(Dz)
• 4. Added in quadrature, final results rounded to
  one significant figure
• kms/s z 300,000 assumed in all cases

22
Summary

• S/N superior with VPH gratings
• RUNZ q-ranking not reliable indicator of z
  quality
• Redshift agreement (IRAF-RUNZ) scales strongly
  with cross-correlation r-value
• Small uncertainty in l calibration test
• Larger uncertainties in cross-correlation and
  repeatability tests
• Total uncertainties
• 100 km/s general uncertainty
• 70 km/s uncertainty for rgt6 redshifts

23
Thanks

• Macquarie University
• Anglo-Australian Observatory
• Wide-Field Astronomy Unit, Edinburgh
• Supervisors Quentin Parker (MU/AAO), Will
  Saunders (AAO)
• 6df Galaxy Survey Team (37 members)
• References
• The 6dF Galaxy Survey samples, observational
  techniques and the first data release, MNRAS,
  355, 747-763 (2004)
• This research has made use of the SIMBAD
  database, operated at CDS, Strasbourg, France

24
(No Transcript)