Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy. II. The population with kinematically offset broad Balmer emission lines
Abstract
A small fraction of quasars have long been known to show bulk velocity offsets (of a few hundred to thousands of km s{sup –1}) 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 (>10{sup 3} km s{sup –1}), we explore the parameter space with smaller (a few hundred km s{sup –1}) yet significant offsets (99.7% confidence). Usingmore »
- Authors:
-
- Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States)
- Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)
- Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611 (Australia)
- Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
- Institute for Advanced Study, Princeton, NJ 08540 (United States)
- Publication Date:
- OSTI Identifier:
- 22365676
- Resource Type:
- Journal Article
- Journal Name:
- Astrophysical Journal
- Additional Journal Information:
- Journal Volume: 789; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; ASYMMETRY; BALMER LINES; BINARY STARS; BLACK HOLES; CATALOGS; CORRELATIONS; DETECTION; EMISSION; GALAXIES; GALAXY NUCLEI; HYPOTHESIS; QUASARS; RADIAL VELOCITY; RED SHIFT; SKY; SPECTRA; SPECTROSCOPY
Citation Formats
Liu, Xin, Shen, Yue, Bian, Fuyan, Loeb, Abraham, and Tremaine, Scott. Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy. II. The population with kinematically offset broad Balmer emission lines. United States: N. p., 2014.
Web. doi:10.1088/0004-637X/789/2/140.
Liu, Xin, Shen, Yue, Bian, Fuyan, Loeb, Abraham, & Tremaine, Scott. Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy. II. The population with kinematically offset broad Balmer emission lines. United States. https://doi.org/10.1088/0004-637X/789/2/140
Liu, Xin, Shen, Yue, Bian, Fuyan, Loeb, Abraham, and Tremaine, Scott. 2014.
"Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy. II. The population with kinematically offset broad Balmer emission lines". United States. https://doi.org/10.1088/0004-637X/789/2/140.
@article{osti_22365676,
title = {Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy. II. The population with kinematically offset broad Balmer emission lines},
author = {Liu, Xin and Shen, Yue and Bian, Fuyan and Loeb, Abraham and Tremaine, Scott},
abstractNote = {A small fraction of quasars have long been known to show bulk velocity offsets (of a few hundred to thousands of km s{sup –1}) 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 (>10{sup 3} km s{sup –1}), we explore the parameter space with smaller (a few hundred km s{sup –1}) 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{sup –1} yr{sup –1} with a median measurement uncertainty of ∼10 km s{sup –1} yr{sup –1}, 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, 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.},
doi = {10.1088/0004-637X/789/2/140},
url = {https://www.osti.gov/biblio/22365676},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 2,
volume = 789,
place = {United States},
year = {Thu Jul 10 00:00:00 EDT 2014},
month = {Thu Jul 10 00:00:00 EDT 2014}
}