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We present results from a Chandra study of ultraluminous X-ray sources (ULXs)in a sample of 17 nearby (D_L<60 Mpc) luminous infrared galaxies (LIRGs),selected to have star formation rates (SFRs) in excess of 7 M_sun yr^-1 and lowforeground Galactic column densities (N_H < 5*10^20 cm^-2). A total of 53 ULXswere detected and we confirm that this is a complete catalogue of ULXs for theLIRG sample. We examine the evolution of ULX spectra with luminosity bystacking the spectra of individual objects in three luminosity bins, finding adistinct change in spectral index at luminosity ~2 *10^39 erg s^-1. This may bea change in spectrum as 10 M_sun black holes transit from a ~Eddington to asuper-Eddington accretion regime, and is supported by a plausible detection ofpartially-ionised absorption imprinted on the spectrum of the luminous ULX (L_X~5*10^39 erg s^-1) CXOU J024238.9-000055 in NGC 1068, consistent with thehighly ionised massive wind that we would expect to see driven by asuper-Eddington accretion flow. This sample shows a large deficit in the numberof ULXs detected per unit SFR (0.2 ULXs M_sun^-1 yr^-1). This deficit alsomanifests itself as a lower differential X-ray luminosity functionnormalisation for the LIRG sample than for samples of other star forminggalaxies. We show that it is unlikely that this deficit is a purelyobservational effect. Part of this deficit might be attributable to the highmetallicity of the LIRGs impeding the production efficiency of ULXs and/or alag between the star formation starting and the production of ULXs; however, weargue that the evidence -- including very low N_ULX/L_FIR, and an even lowerULX incidence in the central regions of the LIRGs -- shows that the mainculprit for this deficit is likely to be the high column of gas and dust inthese galaxies, that fuels the high SFR but also acts to obscure many ULXs fromour view.

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