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A deficit of ultraluminous X-ray sources in luminous infrared galaxies

Luangtip, W.; Roberts, T.P.; Mineo, S.; Lehmer, B.D.; Alexander, D.M.; Jackson, F.E.; Goulding, A.D.; Fischer, J.L.

A deficit of ultraluminous X-ray sources in luminous infrared galaxies Thumbnail


W. Luangtip

S. Mineo

B.D. Lehmer

F.E. Jackson

A.D. Goulding

J.L. Fischer


We present results from a Chandra study of ultraluminous X-ray sources (ULXs) in a sample of 17 nearby (DL < 60 Mpc) luminous infrared galaxies (LIRGs), selected to have star formation rates (SFRs) in excess of 7 M⊙ yr−1 and low foreground Galactic column densities (NH ≲ 5 × 1020 cm−2). A total of 53 ULXs were detected and we confirm that this is a complete catalogue of ULXs for the LIRG sample. We examine the evolution of ULX spectra with luminosity in these galaxies by stacking the spectra of individual objects in three luminosity bins, finding a distinct change in spectral index at luminosity ∼2 × 1039 erg s−1. This may be a change in spectrum as 10 M⊙ black holes transit from an ∼ Eddington to a super-Eddington accretion regime, and is supported by a plausible detection of partially ionized absorption imprinted on the spectrum of the luminous ULX (LX ≈ 5 × 1039 erg s−1) CXOU J024238.9-000055 in NGC 1068, consistent with the highly ionized massive wind that we would expect to see driven by a super-Eddington accretion flow. This sample shows a large deficit in the number of ULXs detected per unit SFR (0.2 versus 2 ULXs, per M⊙ yr−1) compared to the detection rate in nearby (DL < 14.5 Mpc) normal star-forming galaxies. This deficit also manifests itself as a lower differential X-ray luminosity function normalization for the LIRG sample than for samples of other star-forming galaxies. We show that it is unlikely that this deficit is a purely observational effect. Part of this deficit might be attributable to the high metallicity of the LIRGs impeding the production efficiency of ULXs and/or a lag between the star formation starting and the production of ULXs; however, we argue that the evidence – including very low NULX/LFIR, and an even lower ULX incidence in the central regions of the LIRGs – shows that the main culprit for this deficit is likely to be the high column of gas and dust in these galaxies, that fuels the high SFR but also acts to obscure many ULXs from our view.

Journal Article Type Article
Publication Date Jan 1, 2015
Deposit Date Dec 4, 2014
Publicly Available Date Dec 8, 2014
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 446
Issue 1
Pages 470-492
Keywords Accretion, Accretion discs, Black hole physics, Galaxies: starburst, Infrared: galaxies, X-rays: binaries.
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Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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