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The X-ray spectral properties of scuba galaxies

Alexander, D.M.; Bauer, F.E.; Chapman, S.C.; Smail, I.; Blain, A.W.; Brandt, W.N.; Ivison, R.J.

The X-ray spectral properties of scuba galaxies Thumbnail


F.E. Bauer

S.C. Chapman

A.W. Blain

W.N. Brandt

R.J. Ivison


Deep SCUBA surveys have uncovered a large population of massive submillimeter-emitting galaxies (SMGs; f850m 4 mJy) at z 1. Although it is generally believed that these galaxies host intense star formation activity, there is growing evidence that a substantial fraction also harbor an active galactic nucleus (AGN; i.e., an accreting super-massive black hole [SMBH]). We present here possibly the strongest evidence for this viewpoint to date: the combination of ultradeep X-ray observations (the 2 Ms Chandra Deep FieldNorth) and deep Keck spectroscopic data of SMGs with radio counterparts. We find that the majority (75%) of these radio-selected spectroscopically identified SMGs host AGN activity; the other 25% have X-ray properties consistent with star formation (X-rayderived star formation rates of 13002700 M yr-1). The AGNs have properties generally consistent with those of nearby luminous AGNs ( 1.8 ± 0.5, NH 10201024 cm-2, and LX 10431044.5 ergs s-1), and the majority (80%) are heavily obscured (NH 1023 cm-2). We construct composite rest-frame 220 keV spectra for three different obscuration classes [NH < 1023 cm-2, NH = (15) × 1023 cm-2, and NH > 5 × 1023 cm-2], which reveal features not seen in the individual X-ray spectra. An 1 keV equivalent width Fe K emission line is seen in the composite X-ray spectrum of the most heavily obscured AGNs, suggesting Compton-thick or near Compton-thick absorption. Even taking into account the effects of absorption, we find that the average X-ray to far-IR luminosity ratio of the AGN-classified SMGs (LX/LFIR = 0.004) is approximately 1 order of magnitude below that found for typical quasars. This result suggests that intense star formation activity (of order 1000 M yr-1) dominates the bolometric output of these SMGs. However, we also explore the possibility that the X-ray to far-IR luminosity ratio of the AGN components is intrinsically less than that found for typical quasars and postulate that some SMGs may be AGN dominated. We investigate the implications of our results for the growth of massive black holes, discuss the prospects for deeper X-ray observations, and explore the scientific potential offered by the next generation of X-ray observatories.

Journal Article Type Article
Publication Date Oct 1, 2005
Deposit Date Apr 23, 2008
Publicly Available Date Aug 12, 2014
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 632
Issue 2
Pages 736-750
Keywords Active galaxies, Starburst, Infrared, X-rays, Deep-field-North, Active galactic nuclei, Ultraluminous infrared galaxies, Star-forming galaxies, Seyfert 2, Luminous submillimeter galaxies, Formation rate indicator XM.
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Published Journal Article (586 Kb)

Copyright Statement
© 2005. The American Astronomical Society. All rights reserved.

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