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J/ApJ/789/140          SDSS quasars balmer emission lines           (Liu+, 2014)

Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy. II. The population with kinematically offset broad Balmer emission lines. Liu X., Shen Y., Bian F., Loeb A., Tremaine S. <Astrophys. J., 789, 140 (2014)> =2014ApJ...789..140L (SIMBAD/NED BibCode)
ADC_Keywords: QSOs ; Spectroscopy ; Balmer lines ; Redshifts Keywords: black hole physics - galaxies: active - galaxies: nuclei - line: profiles - quasars: general Abstract: A small fraction of quasars have long been known to show bulk velocity offsets (of a few hundred to thousands of km/s) in the broad Balmer lines with respect to the systemic redshift of the host galaxy. Models to explain these offsets usually invoke broad-line region gas kinematics/asymmetry around single black holes (BHs), orbital motion of massive (∼sub-parsec (sub-pc)) binary black holes (BBHs), or recoil BHs, but single-epoch spectra are unable to distinguish between these scenarios. The line-of-sight (LOS) radial velocity (RV) shifts from long-term spectroscopic monitoring can be used to test the BBH hypothesis. We have selected a sample of 399 quasars with kinematically offset broad Hβ lines from the Sloan Digital Sky Survey (SDSS) Seventh Data Release quasar catalog, and have conducted second-epoch optical spectroscopy for 50 of them. Combined with the existing SDSS spectra, the new observations enable us to constrain the LOS RV shifts of broad Hβ lines with a rest-frame baseline of a few years to nearly a decade. While previous work focused on objects with extreme velocity offset (>103 km/s), we explore the parameter space with smaller (a few hundred km/s) yet significant offsets (99.7% confidence). Using cross-correlation analysis, we detect significant (99% confidence) radial accelerations in the broad Hβ lines in 24 of the 50 objects, of ∼10-200 km/s/yr with a median measurement uncertainty of ∼10 km/s/yr, implying a high fraction of variability of the broad-line velocity on multi-year timescales. We suggest that 9 of the 24 detections are sub-pc BBH candidates, which show consistent velocity shifts independently measured from a second broad line (either Hα or Mg II) without significant changes in the broad-line profiles. Combining the results on the general quasar population studied in Paper I (Shen et al. 2013ApJ...775...49S), we find a tentative anti-correlation between the velocity offset in the first-epoch spectrum and the average acceleration between two epochs, which could be explained by orbital phase modulation when the time separation between two epochs is a non-negligible fraction of the orbital period of the motion causing the line displacement. We discuss the implications of our results for the identification of sub-pc BBH candidates in offset-line quasars and for the constraints on their frequency and orbital parameters. Description: We start with the SDSS DR7 quasar catalog (Schneider et al. 2010AJ....139.2360S, Cat. VII/260), adopting the spectral measurements of Shen et al. (2011, J/ApJS/194/45). Among the SDSS DR7 quasars, 20,774 are at z < 0.83, where SDSS spectra cover Hβ and [O III] λλ4959, 5007 (hereafter [O III] for short). From this parent sample of 20,774 objects we select a subset of 399 with offset broad Balmer emission lines, based on the spectral region around Hβ and [O III]. Our selection was a combination of automated spectral fitting (Shen et al. 2008, J/ApJ/680/169; 2011, J/ApJS/194/45) and visual examination. Here and throughout, we refer to the 399 objects as the "offset" sample. Using the spectral models, we measure the offset of the broad emission lines relative to the systemic velocity. The systemic redshift is estimated from the core component of [O III], which may be different (by a median offset of 32 km/s with a standard deviation of 125 km/s) from the nominal redshift listed by the DR7 catalog based on the SDSS spectroscopic pipeline (Stoughton et al. 2002AJ....123..485S). Our adopted systemic redshift agrees with the improved redshift for SDSS quasars from Hewett & Wild (2010, J/MNRAS/405/2302) within uncertainties. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table1.dat 56 399 SDSS Quasars with kinematically offset broad balmer emission lines
See also: VII/260 : The SDSS-DR7 quasar catalog (Schneider+, 2010) J/MNRAS/251/482 : A spectroscopic survey of faint QSOs (Boyle+ 1991) J/MNRAS/367/1261 : Redshift survey in 15 quasar fields (Morris+, 2006) J/ApJS/194/45 : QSO properties from SDSS-DR7 (Shen+, 2011) J/MNRAS/405/2302 : Improved redshifts for SDSS quasar spectra (Hewett+, 2010) J/ApJ/680/169 : SDSS DR5 virial black hole masses (Shen+, 2008) J/MNRAS/434/1411 : Variability of broad emission lines in QSOs (Woo+, 2013) J/ApJ/814/150 : Variability of SDSS broad absorption line QSOs (Wang+, 2015) J/ApJ/831/7 : SDSS-RM project: peak velocities of QSOs (Shen+, 2016) Byte-by-byte Description of file: table1.dat
Bytes Format Units Label Explanations
1- 18 A18 --- SDSS SDSS designation 20- 25 F6.4 --- z Redshift (1) 27- 30 I4 --- Plate Spectroscopic plate number 32- 34 I3 --- Fiber Fiber identification 36- 40 I5 d MJD Modified Julian Date 42- 46 I5 km/s VOFFP Broad Hβ peak velocity offset (2) 48- 50 I3 km/s e_VOFFP The 1σ uncertainty in VOFFP (3) 52- 56 I5 km/s VOFFC Broad Hβ centroid velocity offset
Note (1): Taken from Hewett & Wild (2010, J/MNRAS/405/2302). Note (2): Taken from Shen et al. (2011, J/ApJS/194/45). Note (3): Including both statistical and systematic errors estimated from Monte Carlo simulations (Shen et al. 2011, J/ApJS/194/45).
History: From electronic version of the journal References: Shen et al., Paper I, 2013ApJ...775...49S
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 10-Mar-2017
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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