THE AGES OF HIGH-MASS X-RAY BINARIES IN NGC 2403 AND NGC 300.

We have examined resolved stellar photometry from HST imaging surrounding 18 high-mass ×-ray binary (HM×B) candidates in NGC 300 and NGC 2403 as determined from combined Chandra/HST analysis. We have fit the color-magnitude distribution of the surrounding stars with stellar evolution models. All but one region in NGC 300 and two in NGC 2403 contain a population with an age between 20 and 70 Myr. One of the candidates is the ultraluminous ×-ray source in NGC 2403, which we associate with a 60 ± 5 Myr old population. These age distributions provide additional evidence that 16 of these 18 candidates are HM×Bs. Furthermore, our results suggest that the most common HM×B age in these galaxies is 40-55 Myr. This preferred age is similar to observations of HM×Bs in the SmallMagellanic Cloud, providing new evidence of this formation timescale, but in higher metallicity populations. We suggest that this preferred HM×B age is the result of the fortuitous combination of two physical effects. First, this is the age of a population when the greatest rate of core-collapse events should be occurring, maximizing neutron star production. Second, this is the age when B stars are most likely to be actively losing mass. We also discuss our results in the context of HM×B feedback in galaxies, confirming HM×Bs as a potentially important source of energy for the interstellar medium in low-mass galaxies. [ABSTRACT FROM AUTHOR]