Discovery of An Active Intermediate-Mass Black Hole Candidate in the Barred Bulgeless Galaxy NGC 3319

TL;DR

This paper reports the discovery of an active intermediate-mass black hole candidate in the center of the nearby barred bulgeless galaxy NGC 3319, providing insights into early black hole growth in such galaxies.

Contribution

It presents the first identification of an active IMBH in a barred bulgeless galaxy, using multi-wavelength observations and multiple estimation methods.

Findings

Black hole mass estimated between 300 and 300,000 solar masses.

The active nucleus shows spectral energy distribution similar to AGNs.

The IMBH resides in a nuclear star cluster with mass ~6 million solar masses.

Abstract

We report the discovery of an active intermediate-mass black hole (IMBH) candidate in the center of nearby barred bulgeless galaxy $\rm NGC~3319$. The point X-ray source revealed by archival Chandra and XMM-Newton observations is spatially coincident with the optical and UV galactic nuclei from Hubble Space Telescope observations. The spectral energy distribution derived from the unresolved X-ray and UV-optical flux is comparable with active galactic nuclei (AGNs) rather than ultra-luminous X-ray sources, although its bolometric luminosity is only $3.6\times10^{40}~\rm erg~s^{-1}$. Assuming an Eddington ratio range between 0.001 and 1, the black hole mass ($M_\rm{BH}$) will be located at $3\times10^2-3\times10^5~M_{\odot}$, placing it in the so-called IMBH regime and could be the one of the lowest reported so far. Estimates from other approaches (e.g., fundamental plane, X-ray variability) also suggest $M_\rm{BH}\lesssim10^5~M_{\odot}$.Similar to other BHs in bulgeless galaxies, the discovered IMBH resides in a nuclear star cluster with mass of $\sim6\times10^6~M_{\odot}$. The detection of such a low-mass BH offers us an ideal chance to study the formation and early growth of SMBH seeds, which may result from the bar-driven inflow in late-type galaxies with a prominent bar such as $\rm NGC~3319$.