The Connection Between X-ray Binaries and Star Clusters in NGC 4449

TL;DR

This study identifies and characterizes X-ray binaries in NGC 4449, linking their properties to star cluster ages and providing insights into their formation and evolution in a starburst galaxy.

Contribution

It presents a new catalog of star clusters and X-ray binaries, revealing their associations and ages, and highlights the connection between star cluster properties and X-ray binary populations.

Findings

Nearly 50% of X-ray binaries are very young, massive black-hole binaries.

About one-third of these binaries are still within their parent star clusters.

Older X-ray binaries are associated with star clusters aged 100-400 Myr.

Abstract

We present 23 candidate X-ray binaries with luminosities down to 1.8x10^36 erg/s, in the nearby starburst galaxy NGC 4449, from observations totaling 105 ksec taken with the ACIS-S instrument on the Chandra Space Telescope. We determine count rates, luminosities, and colors for each source, and perform spectral fits for sources with sufficient counts. We also compile a new catalog of 129 compact star clusters in NGC 4449 from high resolution, multi-band optical images taken with the Hubble Space Telescope, doubling the number of clusters known in this galaxy. The UBVI,Ha luminosities of each cluster are compared with predictions from stellar evolution models to estimate their ages and masses. We find strong evidence for a population of very young massive, black-hole binaries, which comprise nearly 50% of the detected X-ray binaries in NGC 4449. Approximately a third of these remain within their parent star clusters, which formed t < 6-8 Myr ago, while others have likely been ejected from their parent clusters. We also find evidence for a population of somewhat older X-ray binaries, including both supergiant and Be-binaries, which appear to be associated with somewhat older t ~ 100-400 Myr star clusters, and one X-ray binary in an ancient (t ~ 10 Gyr) globular cluster. Our results suggest that detailed information on star clusters can significantly improve constraints on X-ray binary populations in star-forming galaxies.