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 _ { \odot } yr ^ { -1 } and low foreground Galactic column densities ( N _ { H } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } { \raise 2. % 0 pt \hbox { $ < $ } } } } 5 \times 10 ^ { 20 } 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 \sim 2 \times 10 ^ { 39 } ~ { } erg~ { } s ^ { -1 } .
This may be a change in spectrum as 10 ~ { } M _ { \odot } black holes transit from a \sim Eddington to a super-Eddington accretion regime , and is supported by a plausible detection of partially-ionised absorption imprinted on the spectrum of the luminous ULX ( L _ { X } \approx 5 \times 10 ^ { 39 } ~ { } erg~ { } s ^ { -1 } ) CXOU J024238.9-000055 in NGC 1068 , consistent with the highly ionised 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 _ { \odot } ~ { } yr ^ { -1 } ) compared to the detection rate in nearby ( D _ { L } < 14.5 Mpc ) normal star forming galaxies .
This deficit also manifests itself as a lower differential X-ray luminosity function normalisation 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 N _ { ULX } / L _ { FIR } , 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 .