Ultraluminous X-ray sources ( ULXs ) are some of the brightest phenomena found outside of a galaxy ’ s nucleus , and their explanation typically invokes accretion of material onto a black hole .
Here , we perform the largest population study to date of ULXs in early-type galaxies , focusing on whether a galaxy ’ s large scale environment can affect its ULX content .
Using the AMUSE survey , which includes homogeneous X-ray coverage of 100 elliptical galaxies in the Virgo cluster and a similar number of elliptical galaxies in the field ( spanning stellar masses of 10 ^ { 8 } -10 ^ { 12 } ~ { } M _ { \odot } ) , we identify 37.9 \pm 10.1 ULXs in Virgo and 28.1 \pm 8.7 ULXs in the field .
Across both samples , we constrain the number of ULXs per unit stellar mass , i.e. , the ULX specific frequency , to be 0.062 \pm 0.013 ULXs per 10 ^ { 10 } ~ { } M _ { \odot } ( or about 1 ULX per 1.6 \times 10 ^ { 11 } ~ { } M _ { \odot } of galaxy stellar mass ) .
We find that the number of ULXs , the specific frequency of ULXs , and the average ULX spectral properties are all similar in both cluster and field environments .
Contrary to late-type galaxies , we do not see any trend between specific ULX frequency and host galaxy stellar mass , and we show that dwarf ellipticals host fewer ULXs than later-type dwarf galaxies at a statistically meaningful level .
Our results are consistent with ULXs in early-type galaxies probing the luminous tail of the low-mass X-ray binary population , and are briefly discussed in context of the influence of gravitational interactions on the long-term evolution of a galaxy ’ s ( older ) stellar population .