A Close Nuclear Black Hole Pair in the Spiral Galaxy NGC 3393

TL;DR

This paper reports the discovery of a close pair of active massive black holes in the spiral galaxy NGC 3393, providing evidence for minor merger evolution in galaxy development.

Contribution

It presents the first observation of a ~430 light-year separated MBH pair in a spiral galaxy, indicating minor merger processes.

Findings

Two active MBHs separated by ~430 light-years identified in NGC 3393

The galaxy shows a regular spiral morphology with old stellar populations

Evidence supports minor merger as a pathway for MBH pairing in spirals

Abstract

The current picture of galaxy evolution advocates co-evolution of galaxies and their nuclear massive black holes (MBHs), through accretion and merging. Quasar pairs (6,000-300,000 light-years separation) exemplify the first stages of this gravitational interaction. The final stages, through binary MBHs and final collapse with gravitational wave emission, are consistent with the sub-light-year separation MBHs inferred from optical spectra and light-variability of two quasars. The double active nuclei of few nearby galaxies with disrupted morphology and intense star formation (e.g., NGC 6240 and Mkn 463; ~2,400 and ~12,000 light-years separation respectively) demonstrate the importance of major mergers of equal mass spirals in this evolution, leading to an elliptical galaxy, as in the case of the double radio nucleus (~15 light-years separation) elliptical 0402+379. Minor mergers of galaxies with a smaller companion should be a more common occurrence, evolving into spiral galaxies with active MBH pairs, but have hitherto not been seen. Here we report the presence of two active MBHs, separated by ~430 light-years, in the Seyfert galaxy NGC 3393. The regular spiral morphology and predominantly old circum-nuclear stellar population of this galaxy, and the closeness of the MBHs embedded in the bulge, suggest the result of minor merger evolution.