Candidate intermediate-mass black hole discovered in an extremely young low-metallicity cluster in the tadpole galaxy KUG 1138+327

TL;DR

This paper reports the discovery of a candidate intermediate-mass black hole in a young, low-metallicity starburst cluster in the galaxy KUG 1138+327, using multiwavelength observations that suggest accretion activity and recent star formation.

Contribution

It presents the first evidence of an intermediate-mass black hole in an extremely young, low-metallicity starburst environment, combining X-ray, radio, and optical data.

Findings

Detection of a luminous, variable X-ray source consistent with an accreting black hole.

Identification of a nonthermal radio counterpart elongated over 200 pc.

Estimated starburst age of 2-4 Myr, correlating with the presence of the ULX.

Abstract

We explore what unusual products a starburst of about 6% solar metallicity and a mean estimated age of ~0.5 Myr can produce in KUG 1138 + 327 at a distance of 24.5 Mpc. Chandra X-ray observations show a dominant point-like source with an average 0.3-10 keV luminosity of 10^{40.3} erg/s and variability by a factor of ~2 over months. This extreme ultraluminous X-ray source (ULX) is apparently associated with the young central cluster. A multicolor disk modeling of the X-ray spectrum of the source suggests a standard accretion around a black hole. It also has a morphologically elongated nonthermal radio continuum counterpart on the scale of ~200 pc, probably the longest detected from such a source. The radio, optical, and X-ray findings suggest that it could well be an intermediate-mass black hole undergoing sub-Eddington accretion from a massive star companion. Accounting for the presence of the ULX and the prominent emission lines HeII\lambda4658 and [ArIV]\lambda4711 while lacking Wolf-Rayet spectral features, we estimate the true age of the starburst to be about 2-4 Myr. Only with such a moderate age can the starburst host this extraordinary ULX, probably triggered by a recent influx of extremely low-metallicity gas. This study demonstrates the potential of multiwavelength studies of low-metallicity starbursts to provide insights into what may commonly occur in high-redshift galaxies.