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Published July 1, 2010 | Published
Journal Article Open

A Runaway Black Hole in COSMOS: Gravitational Wave or Slingshot Recoil?


We present a detailed study of a peculiar source detected in the COSMOS survey at z = 0.359. Source CXOC J100043.1+020637, also known as CID-42, has two compact optical sources embedded in the same galaxy. The distance between the two, measured in the HST/ACS image, is 0."495 ± 0."005 that, at the redshift of the source, corresponds to a projected separation of 2.46 ± 0.02 kpc. A large (~1200 km s^(−1)) velocity offset between the narrow and broad components of Hβ has been measured in three different optical spectra from the VLT/VIMOS and Magellan/IMACS instruments. CID-42 is also the only X-ray source in COSMOS, having in its X-ray spectra a strong redshifted broad absorption iron line and an iron emission line, drawing an inverted P-Cygni profile. The Chandra and XMM-Newton data show that the absorption line is variable in energy by ΔE = 500 eV over four years and that the absorber has to be highly ionized in order not to leave a signature in the soft X-ray spectrum. That these features—the morphology, the velocity offset, and the inverted P-Cygni profile—occur in the same source is unlikely to be a coincidence. We envisage two possible explanations, both exceptional, for this system: (1) a gravitational wave (GW) recoiling black hole (BH), caught 1–10 Myr after merging; or (2) a Type 1/Type 2 system in the same galaxy where the Type 1 is recoiling due to the slingshot effect produced by a triple BH system. The first possibility gives us a candidate GW recoiling BH with both spectroscopic and imaging signatures. In the second case, the X-ray absorption line can be explained as a BAL-like outflow from the foreground nucleus (a Type 2 AGN) at the rearer one (a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us the first opportunity to show that fast winds are present in obscured active galactic nuclei (AGNs), and possibly universal in AGNs.

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© 2010 The American Astronomical Society. Received 2009 August 3; accepted 2010 May 9; published 2010 June 9. F.C. thanks E. Costantini, T. J. Cox, M. Dotti, L. Hernquist, and A. Sesana for useful discussion. The authors thank Chien Y. Peng for useful discussions about GALFIT. The authors thank the anonymous referee, whose critical analysis helped improve this paper, making it more interesting. This work was supported in part by NASA Chandra grant number GO7-8136A (F.C., M.E., and A.F.), NASA contract NAS8-39073 (Chandra X-ray Center). K.J. acknowledges support from the Emmy Noether Programme of the German Science Foundation (DFG) through grant number JA 1114/3-1. In Italy, this work is supported by ASI/INAF contracts I/023/05/0, I/024/05/0, and I/088/06, and by PRIN/MUR grant 2006-02-5203. In Germany, this project is supported by the Bundesministerium f¨ur Bildung und Forschung/Deutsches Zentrum f¨ur Luft und Raumfahrt and the Max Planck Society. Facilities: CXO (ACIS), HST (ACS), XMM (pn)

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