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 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 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 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 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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 56 399 SDSS Quasars with kinematically offset broad
balmer emission lines
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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
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Bytes Format Units Label Explanations
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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
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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).
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History:
From electronic version of the journal
References:
Shen et al., Paper I, 2013ApJ...775...49S 2013ApJ...775...49S
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 10-Mar-2017