Chandra Observations of the Collisional Ring Galaxy NGC 922

TL;DR

This study uses Chandra X-ray observations to compare ULX populations in the starburst galaxy NGC 922 and the Cartwheel galaxy, finding that metallicity differences do not significantly affect ULX numbers.

Contribution

It provides the first detailed comparison of ULX populations in NGC 922 and the Cartwheel galaxy, highlighting the role of star formation rate over metallicity in ULX formation.

Findings

NGC 922 hosts 7 ULX, similar to the Cartwheel when scaled by star formation rate.

ULX populations in both galaxies are dominated by systems with wind accretion from supergiant donors.

Metallicity differences between the galaxies do not significantly impact ULX numbers.

Abstract

In this paper we report on Chandra observations of the starburst galaxy NGC 922. NGC 922 is a drop-through ring galaxy with an expanding ring of star formation, similar in many respects to the Cartwheel galaxy. The Cartwheel galaxy is famous for hosting 12 ULX, most of which are in the star forming ring. This is the largest number of ULX seen in a single system, and has led to speculation that the low metallicity of the Cartwheel (0.3 solar) may optimize the conditions for ULX formation. In contrast, NGC 922 has metallicity near solar. The Chandra observations reveal a population of bright X-ray sources, including 7 ULX. The number of ULX in NGC 922 and the Cartwheel scales with the star formation rate: we do not find any evidence for an excess of sources in the Cartwheel. Simulations of the binary population in these galaxies suggest that the ULX population in both systems is dominated by systems with strong wind accretion from supergiant donors onto direct-collapse BHs. The simulations correctly predict the ratio of the number of sources in NGC 922 and the Cartwheel. Thus it would appear that the the metallicity of the Cartwheel is not low enough to see a difference in the ULX population compared to NGC 922.