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6dFGS data quality: comparison of pipeline and IRAF redshifts

6dFGS data quality: comparison of pipeline and IRAF redshifts. Lesa Moore Macquarie University AAO 6dF Workshop 2005. Outline. Spectral reduction and S/N Wavelength calibration Cross-correlation redshift agreement Quality measures – S/N, Q, r Repeatability Final uncertainties

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6dFGS data quality: comparison of pipeline and IRAF redshifts

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  1. 6dFGS data quality:comparison 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 • Q=3 meant to imply 75% confidence (only 60% agree) } 294

  16. Cross-correlation r-value • R-value is a much moreuseful indicator of redshift reliability than Q-ranking • 84% of disagrees have r<6 • 27 of 33 disagrees with Q=3,4,5 have r<6 • 72% of agrees have r>6

  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 r>6 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 r>6, possibly over-stated) • 3. 1/√2 * st. dev.(Dz) of repeat 6dF observations (125 galaxies in total, 81 with r>6) • 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/√2 * 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 r>6 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

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