Improved redshifts for SDSS quasar spectra

By Paul Hewett, Vivienne Wild IoA Cambridge, Iap Paris. 2010

317 citations recorded by the source corpus.

A systematic investigation of the relationship between different redshift estimation schemes for more than 91000 quasars in the Sloan Digital Sky Survey (SDSS) Data Release 6 (DR6) is presented. The publicly available SDSS quasar redshifts are shown to possess systematic biases of Dz/(1+z)>=0.002 (600km/s) over both small (dz~0.1) and large (dz~1) redshift intervals. Empirical relationships between redshifts based on i) CaII H & K host galaxy absorption, ii) quasar [OII] 3728, iii) [OIII] 4960,5008 emission, and iv) cross-correlation (with a master quasar template) that includes, at increasing quasar redshift, the prominent MgII 2799, CIII] 1908 and CIV 1549 emission lines, are established as a function of quasar redshift and luminosity. New redshifts in the resulting catalogue possess systematic biases a factor of ~20 lower compared to the SDSS redshift values; systematic effects are reduced to the level of Dz/(1+z)<10^-4 (30km/s) per unit redshift, or <2.5x10^-5 per unit absolute magnitude. Redshift errors, including components due both to internal reproducibility and the intrinsic quasar-to-quasar variation among the population, are available for all quasars in the catalogue. The improved redshifts and their associated errors have wide applicability in areas such as quasar absorption outflows, quasar clustering, quasar-galaxy clustering and proximity-effect determinations.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/1548748f069ee00a55c55e11dbf4a6475bb3190f

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Citations in the corpus

A refined measurement of the mean transmitted flux in the Lyα forest over 2 < z < 5 using composite quasar spectra (2012, methodology): “The redshift uncertainties for the SDSS quasar spectra also increase with redshift (Table 3 of Hewett & Wild 2010) as the C iv emission line has to be used in the redshift determination.”
H0LiCOW – I. H0 Lenses in COSMOGRAIL's wellspring: program overview (2016, background): “2 The source redshift of zs = 0.654 is based on the narrow emission lines, which is considered more accurate than the Hα and MgII lines (Hewett & Wild 2010) that yield zs = 0.657 (Sluse et al. 2007).”
The Sloan Digital Sky Survey Quasar Catalog: Sixteenth Data Release (2020, methodology): “We also include the redshifts from Hewett & Wild (2010), where available, in the field Z_DR6Q_HW, as they are formally in the Hewett and Wild paper and have been used in some of our companion papers.”
SPECTRAL CLASSIFICATION AND REDSHIFT MEASUREMENT FOR THE SDSS-III BARYON OSCILLATION SPECTROSCOPIC SURVEY (2012, methodology): “We do not employ the redshift estimates of Hewett & Wild (2010) for the quasar-template training sample because the current BOSS pipeline is not configured to incorporate the absolute-magnitude information that would be necessary to take advantage of the increased precision afforded by these…”
THE NINTH DATA RELEASE OF THE SLOAN DIGITAL SKY SURVEY: FIRST SPECTROSCOPIC DATA FROM THE SDSS-III BARYON OSCILLATION SPECTROSCOPIC SURVEY (2012, methodology): “The limitations of the quasar redshifts in previous data releases were highlighted by Hewett & Wild (2010) in a reanalysis of DR6 quasar redshifts.”
BLACK HOLE MASS ESTIMATES AND EMISSION-LINE PROPERTIES OF A SAMPLE OF REDSHIFT z > 6.5 QUASARS (2013, background): “…for J1120+0641 and measured a systemic redshift z[C,ii] = 7.0842 ± 0.0004, which is in agreement with the redshift estimate obtained by Mortlock et al. (2011, z = 7.085 ± 0.003) from the crosscorrelation of the observed Mg ii emission line with the quasar composite spectrum of Hewett & Wild (2010).”
The Giant Gemini GMOS survey of zem > 4.4 quasars – I. Measuring the mean free path across cosmic time (2014, methodology): “A comparison to the Hewett & Wild (2010) redshifts reveals a similar tail.”
A strong redshift dependence of the broad absorption line quasar fraction (2010, methodology): “…spectrum pairs allows an empirical check on the reproducibility of the BI determinations as a function of spectrum S/N. Improved quasar redshifts were taken from a preliminary implementation of the Hewett & Wild (2010) scheme for generating self-consistent redshifts for quasars with SDSS spectra.”
THE SDSS-IV EXTENDED BARYON OSCILLATION SPECTROSCOPIC SURVEY: QUASAR TARGET SELECTION (2015, background): “2 see the eBOSS overview paper (Dawson et al. 2015) for a discussion of this requirement and Hewett & Wild (2010) for details of the precision of SDSS quasar redshifts.”
The Sloan Digital Sky Survey Quasar Catalog: Twelfth data release (2016, methodology): “Hewett & Wild (2010) carefully studied the systematic emission-lines shifts in the DR6 quasar sample and computed accurate quasar redshifts by correcting for these shifts.”
The Sloan Digital Sky Survey quasar catalog: ninth data release (2012, methodology): “We first describe the use of Hewett & Wild (2010) redshifts and then an improved approach using the peak of the Mgii emission line in individual spectra.”
The clustering of the SDSS-IV extended Baryon Oscillation Spectroscopic Survey DR14 quasar sample: measurement of the growth rate of structure from the anisotropic correlation function between redshift 0.8 and 2.2 (2018, background): “Hewett & Wild (2010) and Shen et al. (2016) showed that the Mg II feature is the quasar broad emission line that has the smallest velocity shifts (∼200 km s−1) because as it is a lower ionization species it presumably lies at a larger distance from the central black hole.”
The Sloan Digital Sky Survey Quasar Catalog: Fourteenth data release (2017, methodology): “Various studies have shown that the Mg II emission line is the quasar broad emission line that is the least affected by systematic shifts (e.g. Hewett & Wild 2010; Shen et al. 2016).”
THE SLOAN DIGITAL SKY SURVEY QUASAR CATALOG. V. SEVENTH DATA RELEASE (2010, background): “Hewett & Wild (2010) present evidence for systematic errors of ≈0.003 in SDSS quasar redshift measurements; these errors are small for most applications, but can be significant for some investigations.”
UBIQUITOUS GIANT Lyα NEBULAE AROUND THE BRIGHTEST QUASARS AT z ∼ 3.5 REVEALED WITH MUSE (2016, background): “…are available (e.g. for the Slug Nebula, Cantalupo et al. 2014), the majority of quasar redshifts (e.g. from SDSS) are typically estimated from broad emission lines such Mg II that have an error in velocity comparable to the central part of the NB filters themselves (Hewett & Wild 2010).”