Chandra Observations of the Nuclear Star Cluster and Ultraluminous X-ray Sources in NGC 2139

TL;DR

Chandra observations of NGC 2139 reveal no active black hole in the nuclear star cluster but a significant population of ultraluminous X-ray sources, indicating recent starburst activity and providing insights into galaxy evolution.

Contribution

This study provides the first X-ray constraints on the nuclear star cluster of NGC 2139 and links X-ray source populations to recent star formation history.

Findings

Nuclear star cluster shows no significant X-ray emission, implying a non-accreting black hole.

Detected eight ultraluminous X-ray sources, exceeding expectations based on current star formation.

Galaxy's X-ray source population suggests a recent starburst event in NGC 2139.

Abstract

We report Chandra observations of the Scd galaxy NGC 2139, which is known to host a recently formed (10^7.6 yrs) nuclear star cluster. The star cluster is undetected in X-rays, with an upper bound on 0.5-7 keV luminosity of L_X < 7.1 x 10^37 erg/s. This bound implies a bolometric accretion luminosity <0.3 percent of the Eddington luminosity for a black hole with the mass (approximately 3400 M_sun) expected from extrapolation of the M-sigma relation. The lack of X-ray emission indicates that a black hole, if present, is not undergoing significant accretion at the current time. While the central cluster is undetected, the data reveal a substantial population of bright X-ray point sources elsewhere in this galaxy, with eight qualifying as ultraluminous X-ray sources with L_X > 10^39 erg/s. We use archival Hubble Space Telescope images to identify candidate optical counterparts for seven Chandra sources, which in most cases have optical luminosities and spatial profiles consistent with star clusters. Compared with other galaxies, the number of luminous X-ray sources in NGC 2139 is larger by a factor of 4 - 10 than expected based on its present star formation rate and stellar mass. This finding can be understood if NGC 2139 has concluded a burst of star formation in the recent past, and suggests that this galaxy could be important for testing the use of X-ray source populations as a chronometer of star formation history.