Caught in formation: the nuclear-cluster-to-be in NGC 2139

TL;DR

This study observes a young star cluster in NGC 2139, providing insights into nuclear cluster formation by analyzing its offset position and galaxy dynamics, suggesting nascent NC formation away from the galaxy's center.

Contribution

It offers new observational evidence of a forming nuclear cluster offset from the galaxy's center, challenging existing models of nuclear cluster formation.

Findings

The candidate NC is offset from the galaxy's kinematic and photometric centers.

The galaxy shows signs of being out of dynamic equilibrium.

A nascent NC is forming away from the galaxy's center, potentially migrating inward.

Abstract

Close to its center, the bulgeless galaxy NGC 2139 hosts a star cluster that is younger and less massive than any actual nuclear star cluster (NC) studied so far. We have measured the H-alpha velocity field around the photometric center of this galaxy using the VLT ARGUS integral field unit and GIRAFFE spectrograph in order to constrain different proposed theories of NC formation. We observe that the best-fit kinematic center and the candidate NC appear to be separated by 2.8 arcsec (320 pc). Indeed, the kinematic center also is offset from the galaxy's photometric center and a possible bar or extended region of star formation in which the young cluster resides, implying that this galaxy is not in dynamic equilibrium. The H-alpha flux map also reveals other regions of strong star formation in the possible bar. These observations suggest that a nascent NC is forming away from the kinematic center of NGC 2139 which may come to rest there on a time scale of a few 100 Myr.